Carbon Nanotube and Nanofiber Exposure Assessments: An Analysis of 14 Site Visits

Characterizing applications, exposure risks, and hazard communication for engineered nanomaterials in construction

BACKGROUND

Engineered nanomaterials (ENMs) may pose health risks to workers. Objectives were to characterize ENM applications in construction, identify exposure scenarios, and evaluate the quality of safety data sheets (SDSs) for nano-enabled construction products.

METHODS

SDSs and product data were obtained from a public database of nano-enabled construction products. Descriptive statistics were calculated for affected trades, product categories, and types of ENMs. A sample of SDSs (n = 33) was evaluated using modified criteria developed by NIOSH researchers. Bulk analysis via transmission electron microscopy characterized nanoparticles in a subset of products.

RESULTS

Companies report using >50 ENMs in construction products. ENM composition could not be determined via SDSs for 38.1% of the 907 products examined. Polymers and metal oxides tied for most frequently reported ENMs (n = 87, 9.6%). Nano silica, graphene, carbon nanotubes, and silver nanoparticles were also frequently reported. Most of the products were paints and coatings (n = 483, 53.3%), followed by pre-market additives, cementitious materials, insulation, and lubricants. Workers in twenty construction trades are likely to handle nano-enabled products, these particularly encompass cement and brick masons, painters, laborers, carpenters, glaziers, and insulators. A wide range of exposure scenarios were identified. SDSs were classified as satisfactory (18%), in need of improvement (12%), or in need of significant improvement (70%). Bulk analyses revealed discrepancies between actual ENM composition and those in SDSs.

DISCUSSION AND CONCLUSION

There has been significant progress investigating risks to construction workers posed by ENMs, but SDSs need major improvements. This study provides new insights on the use of ENMs in construction, exposure risks, and hazard communication.

Immunomodulation of susceptibility to pneumococcal pneumonia infection in mouse lungs exposed to carbon nanoparticles via dysregulation of innate and adaptive immune responses

Carbon nanotubes (CNTs) are emerging pollutants of occupational and environmental health concern. While toxicological mechanisms of CNTs are emerging, there is paucity of information on their modulatory effects on susceptibility to infections. Here, we investigated cellular and molecular events underlying the effect of multi-walled CNT (MWCNT) exposure on susceptibility to Streptococcus pneumoniae infection in our 28-day sub-chronic exposure mouse model. Data indicated reduced phagocytic function in alveolar macrophages (AMs) from MWCNT-exposed lungs evidenced by lower pathogen uptake in 1-h infection assay. At 24-h post-infection, intracellular pathogen count in exposed AMs showed 2.5 times higher net increase (2-fold in vehicle- versus 5-fold in MWCNT-treated), indicating a greater rate of intracellular multiplication and/or survival due to MWCNT exposure. AMs from MWCNT-exposed lungs exhibited downregulation of pathogen-uptake receptors CD163, Phosphatidyl-serine receptor (Ptdsr), and Macrophage scavenger receptors class A type 1 (Msr1) and type 2 (MSr2). In whole lung, MWCNT exposure shifted the macrophage polarization state towards the immunosuppressive phenotype M2b and increased the CD11c+ dendritic cell population required to activate the adaptive immune response. Notably, the MWCNT pre-exposure dysregulated T-cell immunity, evidenced by diminished CD4 and Th17 response, and exacerbated Th1 and Treg responses (skewed Th17/Treg ratio), thereby favoring the pneumococcal infection. Overall, these findings indicated that MWCNT exposure compromises both innate and adaptive immunity leading to diminished host lung defense against pneumonia infection. To our knowledge, this is the first report on an immunomodulatory role of CNT pre-exposure on pneumococcal infection susceptibility due to dysregulation of both innate and adaptive immunity targets.

Occupational exposure to nanomaterials: A bibliometric study of publications over the last decade

Owing to the increased use of nanomaterials, the number of employees and professionals who are exposed to these chemicals is on the rise, despite the paucity of organized data on the possible dangers associated with exposure to these compounds. Multiple studies reveal that the lack of nanosafety awareness among employees and businesses is a serious problem that must be addressed. This shortage of information may result from insufficient knowledge generation or transmission. Academic publications play a significant role in producing new knowledge. This study presents a comprehensive bibliometric analysis of published research on workplace nanosafety which this bibliometric analysis aims to establish the permeability and significance of nanosafety themes from an occupational safety and health viewpoint in academia, to gain a better understanding of the knowledge generation phase in the area, and detected advantages and disadvantages of the topic. Scopus was used as the data source in this study. A total of 1170 publications were gathered and analyzed. The results indicated that the United States is a leader in several aspects of nanosafety at the workplace. The synthesis of co-citation and author-keyword phrases provided insight into determining the focal points of the current study. Analysis of meta data indicates that the number of writers is rising. Nanomaterial toxicity, risk assessment, and occupational exposure are the three hottest topics in this field, according to the result. In addition, the findings included worldwide growth rate, collaborative research fields, keywords, journals, and funding agencies. The advantages and disadvantages of the knowledge creation phase of nanosafety at the workplace were examined in the conclusion.

Understanding toxicity associated with boron nitride nanotubes: Review of toxicity studies, exposure assessment at manufacturing facilities, and read-across

Boron nitride nanotubes (BNNT) are produced by many different methods leading to variances in physicochemical characteristics and impurities in the final product. These differences can alter the toxicity profile. The importance of understanding the potential pathological implications of this high aspect ratio nanomaterial is increasing as new approaches to synthesize and purify in large scale are being developed. In this review, we discuss the various factors of BNNT production that can influence its toxicity followed by summarizing the toxicity findings from in vitro and in vivo studies conducted to date, including a review of particle clearance observed with various exposure routes. To understand the risk to workers and interpret relevance of toxicological findings, exposure assessment at manufacturing facilities was discussed. Workplace exposure assessment of BNNT from two manufacturing facilities measured boron concentrations in personal breathing zones from non-detectable to 0.95 μg/m3 and TEM structure counts of 0.0123 ± 0.0094 structures/cm3, concentrations well below what was found with other engineered high aspect ratio nanomaterials like carbon nanotubes and nanofibers. Finally, using a purified BNNT, a "read-across" toxicity assessment was performed to demonstrate how known hazard data and physicochemical characteristics can be utilized to evaluate potential inhalation toxicity concerns.

Prospects of Polymeric Nanocomposite Membranes for Water Purification and Scalability and their Health and Environmental Impacts: A Review

Water shortage is a major worldwide issue. Filtration using genuine polymeric membranes demonstrates excellent pollutant separation capabilities; however, polymeric membranes have restricted uses. Nanocomposite membranes, which are produced by integrating nanofillers into polymeric membrane matrices, may increase filtration. Carbon-based nanoparticles and metal/metal oxide nanoparticles have received the greatest attention. We evaluate the antifouling and permeability performance of nanocomposite membranes and their physical and chemical characteristics and compare nanocomposite membranes to bare membranes. Because of the antibacterial characteristics of nanoparticles and the decreased roughness of the membrane, nanocomposite membranes often have greater antifouling properties. They also have better permeability because of the increased porosity and narrower pore size distribution caused by nanofillers. The concentration of nanofillers affects membrane performance, and the appropriate concentration is determined by both the nanoparticles' characteristics and the membrane's composition. Higher nanofiller concentrations than the recommended value result in deficient performance owing to nanoparticle aggregation. Despite substantial studies into nanocomposite membrane manufacturing, most past efforts have been restricted to the laboratory scale, and the long-term membrane durability after nanofiller leakage has not been thoroughly examined.

Results of the 2019 Survey of Engineered Nanomaterial Occupational Health and Safety Practices

In collaboration with RTI International, the U.S. National Institute for Occupational Safety and Health (NIOSH) administered a survey to North American companies working with nanomaterials to assess health and safety practices. The results would contribute to understanding the impact of the efforts made by the NIOSH Nanotechnology Research Center (NTRC) in communicating occupational health and safety (OHS) considerations for workers when handling these materials. The survey, developed by RAND Corporation, was conducted online from September 2019–December 2019. Forty-five companies or organizations in the U.S. and Canada that fabricate, manufacture, handle, dispose, or otherwise use nanomaterials completed the survey. The survey was designed to answer research questions regarding the nanomaterials in use, which resources the companies have consulted for OHS guidance, and the overall OHS culture at the companies. Other questions specifically addressed whether the companies interacted with NIOSH or NIOSH resources to inform OHS policies and practices. Among participating companies, 57.8% had a maximum of 50 employees. Gold nanoparticles and polymers were most common (n = 20; 45.5% each), followed by graphene (36.4%), carbon nanotubes and nanofibers (34.1%), and zinc oxide nanoparticles (31.8%). Environmental monitoring was performed by 31.8% of the companies. While 88.9% of the companies had laminar flow cabinets, only 67.5% required it to be used with ENMs. Information and training programs were indicated by 90% of the sample, and only 29.6% performed specific health surveillance for ENM workers. Personal protective equipment primarily included gloves (100%) and eye/face protection (97.7%). More than a third (37.8%) of the respondents reported using at least one NIOSH resource to acquire information about safe handling of ENMs. The small number of companies that responded to and completed the survey is a considerable limitation to this study. However, the survey data are valuable for gauging the reach and influence of the NIOSH NTRC on nano OHS and for informing future outreach, particularly to small businesses.

Screening Risk Assessment at the Production and Use Stage of Carbon Nanomaterials Generated in Hydrogen Manufacture by Methane Decomposition

We performed a screening evaluation of the human health risk posed by nanocarbon materials at the lifecycle stages of manufacturing and the use of the solid carbon generated in hydrogen manufacture by methane decomposition. We first estimated the atmospheric emission volumes of the solid carbon produced by small-, medium-, and large-scale hydrogen manufacturing plants. We then estimated the atmospheric emission due to tire wear, which largely contributes to the emission of solid carbon usage. Next, we estimated the atmospheric concentration of solid carbon in an atmospheric simulation using the METI–LIS model, which estimates the atmospheric distribution of a pollutant’s concentration near methane decomposition factories. We also used the AIST–ADMER model that estimated the regional atmospheric distributions around central Tokyo, where the traffic volume is the highest nationally. Finally, we performed a screening evaluation of human health risk in the surrounding areas, considering the permissible exposure concentrations of solid carbon. Our study identified no risk concerns at small- and medium-scale factory locations equipped with high-efficiency particulate air (HEPA) filtration facilities. At large-scale factories installed with HEPA filters, these emissions likely remain within the factory site. Furthermore, we determined that emissions from tire wear pose no risk to human health. The surroundings of small- and medium-scale factory sites installed with HEPA filters posed no risk to human health.

Crosstalk between gut microbiota and lung inflammation in murine toxicity models of respiratory exposure or co-exposure to carbon nanotube particles and cigarette smoke extract

Carbon nanotubes (CNTs) are emerging environmental and occupational toxicants known to induce lung immunotoxicity. While the underlying mechanisms are evolving, it is yet unknown whether inhaled CNTs would cause abnormalities in gut microbiota (dysbiosis), and if such microbiota alteration plays a role in the modulation of CNT-induced lung immunotoxicity. It is also unknown whether co-exposure to tobacco smoke will modulate CNT effects. We compared the effects of lung exposure to multi-wall CNT, cigarette smoke extract (CSE), and their combination (CNT + CSE) in a 4-week chronic toxicity mouse model. The exposures induced differential perturbations in gut microbiome as evidenced by altered microbial α- and β- diversity, indicating a lung-to-gut communication. The gut dysbiosis due to CNTs, unlike CSE, was characterized by an increase in Firmicutes/Bacteroidetes ratio typically associated with proinflammatory condition. Notably, while all three exposures reduced Proteobacteria, the CNT exposure and co-exposure induced appearance of Tenericutes and Cyanobacteria, respectively, implicating them as potential biomarkers of exposure. CNTs differentially induced certain lung proinflammatory mediators (TNF-α, IL-1β, CCL2, CXCL5) whereas CNTs and CSE commonly induced other mediators (CXCL1 and TGF-β). The co-exposure showed either a component-dominant effect or a summative effect for both dysbiosis and lung inflammation. Depletion of gut microbiota attenuated both the differentially-induced and commonly-induced (TGF-β) lung inflammatory mediators as well as granulomas implying gut-to-lung communication and a modulatory role of gut dysbiosis. Taken together, the results demonstrated gut dysbiosis as a systemic effect of inhaled CNTs and provided the first evidence of a bidirectional gut-lung crosstalk modulating CNT lung immunotoxicity.

Assessment of the toxicity and carcinogenicity of double-walled carbon nanotubes in the rat lung after intratracheal instillation: a two-year study

Background

Considering the expanding industrial applications of carbon nanotubes (CNTs), safety assessment of these materials is far less than needed. Very few long-term in vivo studies have been carried out. This is the first 2-year in vivo study to assess the effects of double walled carbon nanotubes (DWCNTs) in the lung and pleura of rats after pulmonary exposure.

Methods

Rats were divided into six groups: untreated, Vehicle, 3 DWCNT groups (0.12 mg/rat, 0.25 mg/rat and 0.5 mg/rat), and MWCNT-7 (0.5 mg/rat). The test materials were administrated by intratracheal-intrapulmonary spraying (TIPS) every other day for 15 days. Rats were observed without further treatment until sacrifice.

Results

DWCNT were biopersistent in the rat lung and induced marked pulmonary inflammation with a significant increase in macrophage count and levels of the chemotactic cytokines CCL2 and CCL3. In addition, the 0.5 mg DWCNT treated rats had significantly higher pulmonary collagen deposition compared to the vehicle controls. The development of carcinomas in the lungs of rats treated with 0.5 mg DWCNT (4/24) was not quite statistically higher (p = 0.0502) than the vehicle control group (0/25), however, the overall incidence of lung tumor development, bronchiolo-alveolar adenoma and bronchiolo-alveolar carcinoma combined, in the lungs of rats treated with 0.5 mg DWCNT (7/24) was statistically higher (p < 0.05) than the vehicle control group (1/25). Notably, two of the rats treated with DWCNT, one in the 0.25 mg group and one in the 0.5 mg group, developed pleural mesotheliomas. However, both of these lesions developed in the visceral pleura, and unlike the rats administered MWCNT-7, rats administered DWCNT did not have elevated levels of HMGB1 in their pleural lavage fluids. This indicates that the mechanism by which the mesotheliomas that developed in the DWCNT treated rats is not relevant to humans.

Conclusions

Our results demonstrate that the DWCNT fibers we tested are biopersistent in the rat lung and induce chronic inflammation. Rats treated with 0.5 mg DWCNT developed pleural fibrosis and lung tumors. These findings demonstrate that the possibility that at least some types of DWCNTs are fibrogenic and tumorigenic cannot be ignored.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-022-00469-8.

Revealing the structural and chemical properties of copper-based nanoparticles released from copper treated wood

Copper-based preservatives consisting of micronized and nanoscale copper particles have been widely used in applications for wood protection. The widespread use of these preservatives along with the potential release of copper-containing nanoparticles (Cu NPs) during the life cycle of treated wood, has raised concerns over the impacts on the environment and occupational exposure. Along with assessing the potential hazards of these materials, a critical step is determining the chemical and morphological characteristics of the copper species released from copper-treated wood. Therefore, a combination of scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) was utilized to characterize and differentiate the released copper-containing particles based on their structures, sizing, and chemical properties. Airborne wood dust samples were collected during the abrasion and sawing of micronized copper (MC) treated wood in a laboratory testing system. Based on the signature Cu L_2,3 edge of EEL spectra, three different copper species (i.e., basic copper carbonate, copper, and copper–wood complex) were identified as major components of the embedded particles in wood dust. In addition, two types of individual Cu NPs consisting of basic copper carbonate and copper were identified. The variation of morphologies and chemical properties of copper-containing particles indicates the importance of copper–wood interactions to determine the formation and distribution of copper species in wood components. Our findings will advance the fundamental understanding of their released forms, potential transformation, and environmental fate during the life cycle.

A combination of analytical electron microscopy and electron energy loss spectroscopy enables effective speciation and characterization of airborne copper nanoparticles released from copper-treated wood.

Histopathology of the broad class of carbon nanotubes and nanofibers used or produced in U.S. facilities in a murine model

BACKGROUND

Multi-walled carbon nanotubes and nanofibers (CNT/F) have been previously investigated for their potential toxicities; however, comparative studies of the broad material class are lacking, especially those with a larger diameter. Additionally, computational modeling correlating physicochemical characteristics and toxicity outcomes have been infrequently employed, and it is unclear if all CNT/F confer similar toxicity, including histopathology changes such as pulmonary fibrosis. Male C57BL/6 mice were exposed to 40 µg of one of nine CNT/F (MW #1-7 and CNF #1-2) commonly found in exposure assessment studies of U.S. facilities with diameters ranging from 6 to 150 nm. Human fibroblasts (0-20 µg/ml) were used to assess the predictive value of in vitro to in vivo modeling systems.

RESULTS

All materials induced histopathology changes, although the types and magnitude of the changes varied. In general, the larger diameter MWs (MW #5-7, including Mitsui-7) and CNF #1 induced greater histopathology changes compared to MW #1 and #3 while MW #4 and CNF #2 were intermediate in effect. Differences in individual alveolar or bronchiolar outcomes and severity correlated with physical dimensions and how the materials agglomerated. Human fibroblast monocultures were found to be insufficient to fully replicate in vivo fibrosis outcomes suggesting in vitro predictive potential depends upon more advanced cell culture in vitro models. Pleural penetrations were observed more consistently in CNT/F with larger lengths and diameters.

CONCLUSION

Physicochemical characteristics, notably nominal CNT/F dimension and agglomerate size, predicted histopathologic changes and enabled grouping of materials by their toxicity profiles. Particles of greater nominal tube length were generally associated with increased severity of histopathology outcomes. Larger particle lengths and agglomerates were associated with more severe bronchi/bronchiolar outcomes. Spherical agglomerated particles of smaller nominal tube dimension were linked to granulomatous inflammation while a mixture of smaller and larger dimensional CNT/F resulted in more severe alveolar injury.

Fibre electronics: towards scaled-up manufacturing of integrated e-textile systems

The quest for a close human interaction with electronic devices for healthcare, safety, energy and security has driven giant leaps in portable and wearable technologies in recent years. Electronic textiles (e-textiles) are emerging as key enablers of wearable devices. Unlike conventional heavy, rigid, and hard-to-wear gadgets, e-textiles can lead to lightweight, flexible, soft, and breathable devices, which can be worn like everyday clothes. A new generation of fibre-based electronics is emerging which can be made into wearable e-textiles. A suite of start-of-the-art functional materials have been used to develop novel fibre-based devices (FBDs), which have shown excellent potential in creating wearable e-textiles. Recent research in this area has led to the development of fibre-based electronic, optoelectronic, energy harvesting, energy storage, and sensing devices, which have also been integrated into multifunctional e-textile systems. Here we review the key technological advancements in FBDs and provide an updated critical evaluation of the status of the research in this field. Focusing on various aspects of materials development, device fabrication, fibre processing, textile integration, and scaled-up manufacturing we discuss current limitations and present an outlook on how to address the future development of this field. The critical analysis of key challenges and existing opportunities in fibre electronics aims to define a roadmap for future applications in this area.

Occupational Exposure to Carbon Nanotubes and Carbon Nanofibres: More Than a Cobweb

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are erroneously considered as singular material entities. Instead, they should be regarded as a heterogeneous class of materials bearing different properties eliciting particular biological outcomes both in vitro and in vivo. Given the pace at which the industrial production of CNTs/CNFs is increasing, it is becoming of utmost importance to acquire comprehensive knowledge regarding their biological activity and their hazardous effects in humans. Animal studies carried out by inhalation showed that some CNTs/CNFs species can cause deleterious effects such as inflammation and lung tissue remodeling. Their physico-chemical properties, biological behavior and biopersistence make them similar to asbestos fibers. Human studies suggest some mild effects in workers handling CNTs/CNFs. However, owing to their cross-sectional design, researchers have been as yet unable to firmly demonstrate a causal relationship between such an exposure and the observed effects. Estimation of acceptable exposure levels should warrant a proper risk management. The aim of this review is to challenge the conception of CNTs/CNFs as a single, unified material entity and prompt the establishment of standardized hazard and exposure assessment methodologies able to properly feed risk assessment and management frameworks.

A transfer learning approach for improved classification of carbon nanomaterials from TEM images

The extensive use of carbon nanomaterials such as carbon nanotubes/nanofibers (CNTs/CNFs) in industrial settings has raised concerns over the potential health risks associated with occupational exposure to these materials. These exposures are commonly in the form of CNT/CNF-containing aerosols, resulting in a need for a reliable structure classification protocol to perform meaningful exposure assessments. However, airborne carbonaceous nanomaterials are very likely to form mixtures of individual nano-sized particles and micron-sized agglomerates with complex structures and irregular shapes, making structure identification and classification extremely difficult. While manual classification from transmission electron microscopy (TEM) images is widely used, it is time-consuming due to the lack of automation tools for structure identification. In the present study, we applied a convolutional neural network (CNN) based machine learning and computer vision method to recognize and classify airborne CNT/CNF particles from TEM images. We introduced a transfer learning approach to represent images by hypercolumn vectors, which were clustered via K-means and processed into a Vector of Locally Aggregated Descriptors (VLAD) representation to train a softmax classifier with the gradient boosting algorithm. This method achieved 90.9% accuracy on the classification of a 4-class dataset and 84.5% accuracy on a more complex 8-class dataset. The developed model established a framework to automatically detect and classify complex carbon nanostructures with potential applications that extend to the automated structural classification for other nanomaterials.

Physicochemical characterization and genotoxicity of the broad class of carbon nanotubes and nanofibers used or produced in U.S. facilities

BACKGROUND

Carbon nanotubes and nanofibers (CNT/F) have known toxicity but simultaneous comparative studies of the broad material class, especially those with a larger diameter, with computational analyses linking toxicity to their fundamental material characteristics was lacking. It was unclear if all CNT/F confer similar toxicity, in particular, genotoxicity. Nine CNT/F (MW #1-7 and CNF #1-2), commonly found in exposure assessment studies of U.S. facilities, were evaluated with reported diameters ranging from 6 to 150 nm. All materials were extensively characterized to include distributions of physical dimensions and prevalence of bundled agglomerates. Human bronchial epithelial cells were exposed to the nine CNT/F (0-24 μg/ml) to determine cell viability, inflammation, cellular oxidative stress, micronuclei formation, and DNA double-strand breakage. Computational modeling was used to understand various permutations of physicochemical characteristics and toxicity outcomes.

RESULTS

Analyses of the CNT/F physicochemical characteristics illustrate that using detailed distributions of physical dimensions provided a more consistent grouping of CNT/F compared to using particle dimension means alone. In fact, analysis of binning of nominal tube physical dimensions alone produced a similar grouping as all characterization parameters together. All materials induced epithelial cell toxicity and micronuclei formation within the dose range tested. Cellular oxidative stress, DNA double strand breaks, and micronuclei formation consistently clustered together and with larger physical CNT/F dimensions and agglomerate characteristics but were distinct from inflammatory protein changes. Larger nominal tube diameters, greater lengths, and bundled agglomerate characteristics were associated with greater severity of effect. The portion of tubes with greater nominal length and larger diameters within a sample was not the majority in number, meaning a smaller percentage of tubes with these characteristics was sufficient to increase toxicity. Many of the traditional physicochemical characteristics including surface area, density, impurities, and dustiness did not cluster with the toxicity outcomes.

CONCLUSION

Distributions of physical dimensions provided more consistent grouping of CNT/F with respect to toxicity outcomes compared to means only. All CNT/F induced some level of genotoxicity in human epithelial cells. The severity of toxicity was dependent on the sample containing a proportion of tubes with greater nominal lengths and diameters.

The pulmonary toxicity of carboxylated or aminated multi-walled carbon nanotubes in mice is determined by the prior purification method

Background

Inhalation of multi-walled carbon nanotubes (MWCNTs) poses a potential risk to human health. In order to safeguard workers and consumers, the toxic properties of MWCNTs need to be identified. Functionalization has been shown to either decrease or increase MWCNT-related pulmonary injury, depending on the type of modification. We, therefore, investigated both acute and chronic pulmonary toxicity of a library of MWCNTs derived from a common pristine parent compound (NC7000).

Methods

MWCNTs were thermally or chemically purified and subsequently surface functionalized by carboxylation or amination. To evaluate pulmonary toxicity, male C57BL6 mice were dosed via oropharyngeal aspiration with either 1.6 or 4 mg/kg of each MWCNT type. Mitsui-7 MWCNT was used as a positive control. Necropsy was performed at days 3 and 60 post-exposure to collect bronchoalveolar lavage fluid (BALF) and lungs.

Results

At day 3 all MWCNTs increased the number of neutrophils in BALF. Chemical purification had a greater effect on pro-inflammatory cytokines (IL-1β, IL-6, CXCL1) in BALF, while thermal purification had a greater effect on pro-fibrotic cytokines (CCL2, OPN, TGF-β1). At day 60, thermally purified, carboxylated MWCNTs had the strongest effect on lymphocyte numbers in BALF. Thermally purified MWCNTs caused the greatest increase in LDH and total protein in BALF. Furthermore, the thermally purified and carboxyl- or amine-functionalized MWCNTs caused the greatest number of granulomatous lesions in the lungs. The physicochemical characteristics mainly associated with increased toxicity of the thermally purified derivatives were decreased surface defects and decreased amorphous content as indicated by Raman spectroscopy.

Conclusions

These data demonstrate that the purification method is an important determinant of lung toxicity induced by carboxyl- and amine-functionalized MWCNTs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-020-00390-y.

Exposures during wet production and use processes of nanomaterials: a summary of 11 worksite evaluations

From 2011-2015, the National Institute for Occupational Safety and Health Nanotechnology Field Studies Team conducted 11 evaluations at worksites that either produced engineered nanomaterials (ENMs) via a wet process or used ENMs in a wetted, suspended, or slurry form. Wet handling or processing of ENMs reduces potential exposure compared to dry handling or processing; however, air sampling data indicated exposures may still occur. Information was gathered about each company, production processes, ENMs of interest, and control measures. Exposure assessments included air sampling using filter media, surface wipe sampling, and real-time particle counting by direct-reading instruments. Electron microscopy analysis of air filters confirmed the presence of ENMs of interest (10 of 11 sites). When a method was available, chemical analysis of filters was also used to detect the presence of ENMs (nine of 11 sites). Wipe samples were collected at four of the 11 sites, and, in each case, confirmed the presence of ENMs on surfaces. Direct-reading data showed potential nanomaterial emissions (nine of 11 sites). Engineering controls included fume hoods, cleanrooms, and enclosed processes. Personal protective equipment was required during all 11 evaluations. Recommendations to address potential exposures were provided to each company following the hierarchy of controls.

Estimates of carbon nanotube deposition in the lung: improving quality and robustness

Carbon nanotube (CNT) in vivo inhalation studies are increasingly providing estimates of the quantity of material deposited in the lung, generally estimated using standard formulae and pulmonary deposition models. These models have typically been developed and validated using data from studies using sphere-like particles. Given the importance of particle morphology to pulmonary deposition, the appropriateness of such an approach was explored to identify any potential limitations. Aerosolized CNT particles typically form 'fiber-like' and/or 'broadly spherical' agglomerates. A review of currently used deposition models indicates that none have been directly validated against results for CNT, however, models for spherical particles have been extensively validated against a wide range of particle sizes and materials and are thus expected to provide reasonable estimates for most 'broadly spherical' CNT particles, although experimental confirmation of this would be of benefit, especially given their low density. The validation of fiber deposition models is significantly less extensive and, in general, focused on larger particles, e.g. asbestos. This raises concerns about the accuracy of deposition estimates for 'fiber-like' CNT particles and recommendations are made for future research to address this. An appreciation of the uncertainties on CNT deposition estimates is important for their interpretation and thus it is recommended that model sensitivity and uncertainty assessments be undertaken. Issues surrounding the measurement and derivation of model input data are also addressed, including instrument responses and particle density assessment options. Recommendations are also made for aerosol characterization to 'future-proof' CNT inhalation studies regarding advances in deposition modeling and toxicological understanding.

Laboratory evaluation of a personal aethalometer for assessing airborne carbon nanotube exposures

Aethalometers are direct-reading instruments primarily used for measuring black carbon (BC) concentrations in workplace and ambient atmospheres. Aethalometer BC measurements of carbon nanotubes (CNTs) were compared to measurements made by other methods when subjected to high (>30 µg/m3) and low (1-30 µg/m3) CNT aerosol concentrations representing worst-case and typical workplace concentrations, respectively. A laboratory-based system was developed to generate carbon black, as an example of a nearly pure carbon, micron-sized aerosol, and two forms of multi-walled carbon nanotubes (CNTs): small-diameter (<8 nm) and large-diameter (50-80 nm). High-concentration trials were conducted during which a scanning mobility particle sizer (SMPS) was used to track particle count concentrations over time. Relative to the behavior of the SMPS counts over time, aethalometer readings exhibited a downward drift, which is indicative of aethalometer response subjected to high BC loading on the receiving filter of the instrument. A post-sample mathematical method was applied that adequately corrected for the drift. Low-concentration trials, during which concentration drift did not occur, were conducted to test aethalometer accuracy. The average BC concentration during a trial was compared to elemental carbon (EC) concentration sampled with a quartz-fiber filter and quantified by NIOSH Method 5040. The CB and large-diameter CNT concentrations measured with the aethalometer produced slopes when regressed on EC that were not significantly different from unity, whereas the small-diameter CNTs were under-sampled by the aethalometer relative to EC. These results indicate that aethalometer response may drift when evaluating CNT exposure scenarios, such as cleaning and powder handling, that produce concentrations >30 µg/m3. However, aethalometer accuracy remains consistent over time when sampling general work zones in which CNT concentrations are expected to be <30 µg/m3. A calibration check of aethalometer response relative to EC measured with Method 5040 is recommended to ensure that the aethalometer readings are not under sampling CNT concentrations as occurred with one of the CNTs evaluated in this study.

Predicting Occupational Exposures to Carbon Nanotubes and Nanofibers Based on Workplace Determinants Modeling

BACKGROUND

Recent cross-sectional epidemiologic studies have examined the association between human health effects and carbon nanotube and nanofiber (CNT/F) workplace exposures. However, due to the latency of many health effects of interest, cohort studies with sufficient follow-up will likely be needed. The objective of this study was to identify workplace determinants that contribute to exposure and develop predictive models to estimate CNT/F exposures for future use in epidemiologic studies.

METHODS

Exposure measurements were compiled from 15 unique facilities for the metrics of elemental carbon (EC) mass at both the respirable and inhalable aerosol size fractions as well as a quantitative analysis performed by transmission electron microscopy (TEM). These metrics served as the dependent variables in model development. Repeated personal samples were collected from most of the 127 CNT/F worker participants for 252 total observations. Determinants were categorized as company-level or worker-level and used to describe the exposure relationship within the dependent variables. The influence of determinants on variance components was explored using mixed linear models that utilized a backwards stepwise selection process with a lowering of the AIC for model determinant selection. Additional ridge regression models were created that examined predictive performance with and without all two-way interactions. Cross-validation was performed on each model to evaluate the generalizability of its predictive capabilities while predictive performance was evaluated according to the corresponding R2 value and root mean square error (RMSE).

RESULTS

Determinants at the company-level that increased exposure included an inadequate or semi-adequate engineering control rating, increasing average CNT/F diameter/length, daily quantities of material handled from 101 g to >1 kg and >1 kg, the use of CNF materials, the industry type of hybrid producer/user, and the expert assessment of a high exposure potential. Worker-level determinants associated with higher exposure included handling the dry-powdered form of CNT/F, handling daily quantities of material >1 kg, direct/indirect exposure, having the job title of engineer, using a respirator, using a ventilated or unventilated enclosure, and the job task of powder handling. The mixed linear models explained >60% of the total variance when using all company- and worker-level determinants to create the three exposure models. The cross-validated RMSE values for each of the three mixed models ranged from 2.50 to 4.23. Meanwhile, the ridge regression models, without all two-way interactions, estimated cross-validated RMSE values of 2.85, 2.23, and 2.76 for the predictive models of inhalable EC, respirable EC, and TEM, respectively.

CONCLUSIONS

The ridge regression models demonstrated the best performance for predicting exposures to CNT/F for each exposure metric, although they only provided a modest predictive capability. Therefore, it was concluded that the models alone would not be adequate in predicting workplace exposures and would need to be integrated with other methods.

State of knowledge on the occupational exposure to carbon nanotubes

Carbon nanotubes (CNT) trigger fascination as well as anxiety, given their unique physical and chemical properties, and continuing concerns around their possible health effects. CNT exposure assessment is an integral component of occupational and environmental epidemiology, risk assessment, and management. We conducted a systematic review to analyze the quality of CNT occupational exposure assessments in field studies and to assess the relevance of available quantitative data from occupational hygiene and epidemiological perspectives. PubMed and Scopus databases were searched for the period 2000-2018. To grade the quality of each study, we used a standardized grid of seven criteria. The first criterion addressed 12 items deemed most relevant CNT physical-chemical properties with respect to their in vitro and in vivo toxicity. We included 27 studies from 11 countries in the review and graded them high (n = 2), moderate (n = 15) and low quality (n = 10). Half of the studies measured elemental carbon mass concentration (EC) using different methods and aerosol fractions. In 85% of studies, the observed values exceed the US National Institute for Occupational Safety and Health Recommended Exposure Limit. The quantification of CNT agglomerates and/or CNT contained fibers becomes increasingly common although lacking methodological standardization. Work activities with the greatest mean CNT mass concentrations were non-enclosed and included sieving, harvesting, packaging, reactor cleaning, extrusion and pelletizing. Some of the large studies defined standardized job titles according to exposure estimates at corresponding workstations and classified them by decreasing CNT exposure level: technicians > engineers > chemists. The already initiated harmonization of CNT exposure assessment and result reporting need to continue to favor not only studies in the field, but also to identify companies and workers using CNTs to characterize their exposures as well as monitor their health. This will enable an objective and realistic evaluation of risks associated with CNT applications and an appropriate risk management.

Comparison of four nanoparticle monitoring instruments relevant for occupational hygiene applications

Background

The aim of this study is to make a comparison of a new small sized nanoparticle monitoring instrument, Nanoscan  SMPS, with more traditional large size instruments, known to be precise and accurate [Scanning Mobility Particle Sampler (SMPS) and Fast Mobility Particle Sizer (FMPS)], and with an older small size instrument with bulk measurements of 10-1000 nm particles (CPC3007). The comparisons are made during simulated exposure scenarios relevant to occupational hygiene studies.

Methods

Four scenarios were investigated: metal inert gas (MIG) welding, polyvinyl chloride (PVC) welding, cooking, and candle-burning. Ratios between results are compaed and Pearsson correlations analysis was performed.

Results

The highest correlation between the results is found between Nanoscan and SMPS, with Pearsson correlation coefficients above 0.9 for all scenarios. However, Nanoscan tended to overestimate the results from the SMPS; the ratio between the UFP concentrations vary between 1.44 and 2.01, and ratios of total concentrations between 1.18 and 2.33. CPC 3007 did not show comparable results with the remaining instruments.

Conclusion

Based on the results of this study, the choice of measurement equipment may be crucial when evaluating measurement results against a reference value or a limit value for nanoparticle exposure. This stresses the need for method development, standardisation, and harmonisation of particle sampling protocols before reference values are introduced. Until this is established, the SMPS instruments are the most reliable for quantification of the concentrations of UFP, but in a more practical occupational hygiene context, the Nanoscan SMPS should be further tested.

Real-Time Measurement of Airborne Carbon Nanotubes in Workplace Atmospheres

With growing applications of carbon nanomaterials, there is a concern over health risks presented by inhalation of carbon nanotube (CNT) aerosol in workplace atmospheres. Current methods used for CNT aerosol measurement lack selectivity to specific form of carbonaceous component or allotrope of interest. Moreover, the detection limits of these methods are also inadequate for short-term monitoring. Here, we describe, for the first time, a near real-time, field-portable instrument for selective quantification of airborne CNT concentration. The approach uses an automated cyclical scheme involving collect-analyze-ablate steps to obtain continuous near real-time measurement using Raman spectroscopy. The method achieves significantly lower detection limits by employing corona-assisted particle microconcentration for efficient coupling with laser Raman spectroscopy. A combination of techniques involving (i) use of characteristic Raman peaks, (ii) distinct ratio of disordered and graphitic peaks, and (iii) principal component classification and regression is employed to identify and quantify the specific form of the aerosolized carbonaceous nanomaterial. We show that the approach is capable of selectively quantifying trace single-walled CNT in the presence of interfering agents such as diesel particulate matter. The detection limit of the method for the single-walled CNT studied in this work was 60 ng m^-3, corresponding to a 10 min aerosol collection period, which is significantly lower than that for the NIOSH Method 5040 (∼0.15 μg m^-3 for an 8-h collection on a 25 mm filter at 4 L min^-1), a commonly used method for elemental carbon. We demonstrate the automated real-time capability of this field-portable method by continuously measuring a transient single-walled CNT aerosol.

Mitsui-7, heat-treated, and nitrogen-doped multi-walled carbon nanotubes elicit genotoxicity in human lung epithelial cells

Background

The unique physicochemical properties of multi-walled carbon nanotubes (MWCNT) have led to many industrial applications. Due to their low density and small size, MWCNT are easily aerosolized in the workplace making respiratory exposures likely in workers. The International Agency for Research on Cancer designated the pristine Mitsui-7 MWCNT (MWCNT-7) as a Group 2B carcinogen, but there was insufficient data to classify all other MWCNT. Previously, MWCNT exposed to high temperature (MWCNT-HT) or synthesized with nitrogen (MWCNT-ND) have been found to elicit attenuated toxicity; however, their genotoxic and carcinogenic potential are not known. Our aim was to measure the genotoxicity of MWCNT-7 compared to these two physicochemically-altered MWCNTs in human lung epithelial cells (BEAS-2B & SAEC).

Results

Dose-dependent partitioning of individual nanotubes in the cell nuclei was observed for each MWCNT material and was greatest for MWCNT-7. Exposure to each MWCNT led to significantly increased mitotic aberrations with multi- and monopolar spindle morphologies and fragmented centrosomes. Quantitative analysis of the spindle pole demonstrated significantly increased centrosome fragmentation from 0.024–2.4 μg/mL of each MWCNT. Significant aneuploidy was measured in a dose-response from each MWCNT-7, HT, and ND; the highest dose of 24 μg/mL produced 67, 61, and 55%, respectively. Chromosome analysis demonstrated significantly increased centromere fragmentation and translocations from each MWCNT at each dose. Following 24 h of exposure to MWCNT-7, ND and/or HT in BEAS-2B a significant arrest in the G1/S phase in the cell cycle occurred, whereas the MWCNT-ND also induced a G2 arrest. Primary SAEC exposed for 24 h to each MWCNT elicited a significantly greater arrest in the G1 and G2 phases. However, SAEC arrested in the G1/S phase after 72 h of exposure. Lastly, a significant increase in clonal growth was observed one month after exposure to 0.024 μg/mL MWCNT-HT & ND.

Conclusions

Although MWCNT-HT & ND cause a lower incidence of genotoxicity, all three MWCNTs cause the same type of mitotic and chromosomal disruptions. Chromosomal fragmentation and translocations have not been observed with other nanomaterials. Because in vitro genotoxicity is correlated with in vivo genotoxic response, these studies in primary human lung cells may predict the genotoxic potency in exposed human populations.

Electronic supplementary material

The online version of this article (10.1186/s12989-019-0318-0) contains supplementary material, which is available to authorized users.

Quantification of Carbon Nanotubes by Raman Analysis

The increasing prevalence of carbon nanotubes (CNTs) in manufacturing and research environments, together with the potential exposure risks, necessitates development of reliable and accurate monitoring methods for these materials. We examined quantification of CNTs by two distinct methods based on Raman spectroscopy. First, as measured by the Raman peak intensity of aqueous CNT suspensions, and second, by Raman mapping of air filter surfaces onto which CNTs were collected as aerosols or applied as small-area (0.05 cm2) deposits. Correlation (R2 = 0.97) between CNT concentration and Raman scattering intensity for suspensions in cuvettes was found over a concentration range from about 2 to 10 µg/ml, but measurement variance precludes practical determination of a calibration curve. Raman mapping of aerosol sample filter surfaces shows correlation with CNT mass when the surface density is relatively high (R2 = 0.83 and 0.95 above about 5 µg total mass on filter), while heterogeneity of CNT deposition makes obtaining representative maps of lower density samples difficult. This difficulty can be mitigated by increasing the area mapped relative to the total sample area, improving both precision and the limit of detection (LOD). For small-area deposits, detection of low masses relevant to occupational monitoring can be achieved, with an estimated LOD of about 50 ng.

Evaluation of total and inhalable samplers for the collection of carbon nanotube and carbon nanofiber aerosols

A growing number of carbon nanotubes and nanofibers (CNT/F) exposure and epidemiologic studies have utilized 25-mm and 37-mm open-faced cassettes (OFC) to assess the inhalable aerosol fraction. It has been previously established that the 37-mm OFC under-samples particles greater than 20 μm in diameter, but the size-selective characteristics of the 25-mm OFC have not yet been fully evaluated. This article describes an experimental study conducted to determine if the 25- and 37-mm OFCs performed with relative equivalence to a reference inhalable aerosol sampler when challenged with CNT/F particles. Side-by-side paired samples were collected within a small Venturi chamber using a 25-mm styrene OFC, 37-mm styrene OFC, 25-mm aluminum OFC, and Button Inhalable Aerosol Sampler. Three types of CNT/F materials and an Arizona road dust were used as challenge aerosols for the various sampler configurations. Repeated experiments were conducted for each sampler configuration and material. The OFC samplers operated at flow rates of 2 and 5 liters per minute. Results showed that the 25-mm OFC performed comparably to the Button Sampler when challenged with CNT/F aerosols, which was demonstrated in five of the six experimental scenarios with an average error of 20%. Overall, the results of this study indicate that the sampling efficiency of the 25- and 37-mm OFCs adequately followed the ISO/ACGIH/CEN inhalable sampling convention when challenged with CNT/F aerosols. Past exposure and epidemiologic studies that used these OFC samplers can directly compare their results to studies that have used other validated inhalable aerosol samplers.

Sampling Techniques on Collecting Fine Carbon Nanotube Fibers for Exposure Assessment

Carbon nanotube (CNT) sampling using an open-faced 25 mm cassette fiber sampling method and a newly developed direct sampling device was evaluated for the size fractioned analysis of collected airborne CNT fibers to improve the sampling and analytical methods. The open-faced 25 mm cassette fiber sampling method primarily collected large agglomerates, with the majority of collected particles being larger than two micrometer in size. Most of CNT structures collected by the new direct sampling device were individual fibers and clusters smaller than one micrometer with a high particle number concentration discrepancy compared to the open-faced 25 mm cassette method raising the concern of this sampling method to representatively characterize the respirable size fraction of CNT aerosols. This work demonstrates that a specialized technique is needed for collecting small fibers to provide a more representative estimate of exposure. It is recommended that an additional sampler be used to directly collect and analyze small fibers in addition to the widely accepted sampling method which utilizes an open-faced 25 mm cassette.

Quantitative evaluation of carbon nanomaterial releases during electric heating wire cutting and sawing machine cutting of expanded polystyrene-based composites using thermal carbon analysis

Field measurements were conducted at a facility where expanded polystyrene-based carbon nanomaterial composites, namely, carbon nanotube and carbon black composites, were cut with an electric heating wire cutter or a circular sawing machine. The aerosol particles released during the cutting of the composites were measured using real-time aerosol monitoring, gravimetric analysis, thermal carbon analysis, and scanning electron microscopic observations. This study had two major goals: (1) to quantitatively evaluate the concentrations of airborne carbon nanomaterials during the cutting of their composites; (2) to evaluate the capability of thermal carbon analysis to quantify airborne carbon nanomaterials in the presence of expanded polystyrene-derived particles. The results of thermal carbon analysis showed that the concentrations of elemental carbon (an indicator of carbon nanomaterials) for all the respirable dust samples in both cutting processes were less than the limit of detection (∼2 µg/m³), which is nearly equivalent to or lower than the occupational exposure limits for carbon nanotubes (1 to 50 µg/m³). For total dust, which includes particles larger than respirable size, although the elemental carbon concentrations during heating wire cutting were low (<3 µg/m³), those during sawing machine cutting were up to 58 µg/m³. In scanning electron microscopic observations, micron-sized particles composed of or including carbon nanotubes were detected only in aerosol particles collected during the sawing machine cutting. Therefore, heating wire cutting is considered preferable. This study demonstrated that thermal carbon analysis can quantify airborne carbon nanomaterials in the presence of expanded polystyrene-derived particles.

Multi-walled carbon nanotube oxidation dependent keratinocyte cytotoxicity and skin inflammation

BACKGROUND

The effects of carbon nanotubes on skin toxicity have not been extensively studied; however, our lab has previously shown that a carboxylated multi-walled carbon nanotube (MWCNT) exacerbates the 2, 4-dinitrofluorobenzene induced contact hypersensitivity response in mice. Here we examine the role of carboxylation in MWCNT skin toxicity.

RESULTS

MWCNTs were analyzed by transmission electron microscopy, zetasizer, and x-ray photoelectron spectroscopy to fully characterize the physical properties. Two MWCNTs with different levels of surface carboxylation were chosen for further testing. The MWCNTs with a high level of carboxylation displayed increased cytotoxicity in a HaCaT keratinocyte cell line, compared to the MWCNTs with intermediate levels of carboxylation. However, neither functionalized MWCNT increased the level of in vitro reactive oxygen species suggesting an alternative mechanism of cytotoxicity. Each MWCNT was tested in the contact hypersensitivity model, and only the MWCNTs with greater than 20% surface carboxylation exacerbated the ear swelling responses. Analysis of the skin after MWCNT exposure reveals that the same MWCNTs with a high level of carboxylation increase epidermal thickness, mast cell and basophil degranulation, and lead to increases in polymorphonuclear cell recruitment when co-administered with 2, 4-dinitrofluorobenzene.

CONCLUSIONS

The data presented here suggest that acute, topical application of low doses of MWCNTs can induce keratinocyte cytotoxicity and exacerbation of allergic skin conditions in a carboxylation dependent manner.

The asbestos-carbon nanotube analogy: An update

Nanotechnology is an emerging industry based on commercialization of materials with one or more dimensions of 100 nm or less. Engineered nanomaterials are currently incorporated into thin films, porous materials, liquid suspensions, or filler/matrix nanocomposites with future applications predicted in energy and catalysis, microelectronics, environmental sensing and remediation, and nanomedicine. Carbon nanotubes are one-dimensional fibrous nanomaterials that physically resemble asbestos fibers. Toxicologic studies in rodents demonstrated that some types of carbon nanotubes can induce mesothelioma, and the World Health Organization evaluated long, rigid multiwall carbon nanotubes as possibly carcinogenic for humans in 2014. This review summarizes key physicochemical similarities and differences between asbestos fibers and carbon nanotubes. The "fiber pathogenicity paradigm" has been extended to include carbon nanotubes as well as other high-aspect-ratio fibrous nanomaterials including metallic nanowires. This paradigm identifies width, length, and biopersistence of high-aspect-ratio fibrous nanomaterials as critical determinants of lung disease, including mesothelioma, following inhalation. Based on recent theoretical modeling studies, a fourth factor, mechanical bending stiffness, will be considered as predictive of potential carcinogenicity. Novel three-dimensional lung tissue platforms provide an opportunity for in vitro screening of a wide range of high aspect ratio fibrous nanomaterials for potential lung toxicity prior to commercialization.

Three-dimensional printing with nano-enabled filaments releases polymer particles containing carbon nanotubes into air

Fused deposition modeling (FDM™) 3-dimensional printing uses polymer filament to build objects. Some polymer filaments are formulated with additives, though it is unknown if they are released during printing. Three commercially available filaments that contained carbon nanotubes (CNTs) were printed with a desktop FDM™ 3-D printer in a chamber while monitoring total particle number concentration and size distribution. Airborne particles were collected on filters and analyzed using electron microscopy. Carbonyl compounds were identified by mass spectrometry. The elemental carbon content of the bulk CNT-containing filaments was 1.5 to 5.2 wt%. CNT-containing filaments released up to 1010 ultrafine (d < 100 nm) particles/g printed and 106 to 108 respirable (d ~0.5 to 2 μm) particles/g printed. From microscopy, 1% of the emitted respirable polymer particles contained visible CNTs. Carbonyl emissions were observed above the limit of detection (LOD) but were below the limit of quantitation (LOQ). Modeling indicated that, for all filaments, the average proportional lung deposition of CNT-containing polymer particles was 6.5%, 5.7%, and 7.2% for the head airways, tracheobronchiolar, and pulmonary regions, respectively. If CNT-containing polymer particles are hazardous, it would be prudent to control emissions during use of these filaments.

A New Approach Combining Analytical Methods for Workplace Exposure Assessment of Inhalable Multi-Walled Carbon Nanotubes

To date there is no consensus about the most appropriate analytical method for measuring carbon nanotubes (CNTs), hampering the assessment and limiting the comparison of data. The goal of this study is to develop an approach for the assessment of the level and nature of inhalable multi-wall CNTs (MWCNTs) in an actual workplace setting by optimizing and evaluating existing analytical methods. In a company commercially producing MWCNTs, personal breathing zone samples were collected for the inhalable size fraction with IOM samplers; which were analyzed with carbon analysis, inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX). Analytical methods were optimized for carbon analysis and SEM/EDX. More specifically, methods were applied and evaluated for background correction using carbon analyses and SEM/EDX, CNT structure count with SEM/EDX and subsequent mass conversion based on both carbon analyses and SEM/EDX. A moderate-to-high concordance correlation coefficient (RC) between carbon analyses and SEM/EDX was observed [RC = 0.81, 95% confidence interval (CI): 0.59-0.92] with an absolute mean difference of 59 µg m-3. A low RC between carbon analyses and ICP-MS (RC = 0.41, 95% CI: 0.07-0.67) with an absolute mean difference of 570 µg m-3 was observed. The large absolute difference between EC and metals is due to the presence of non-embedded inhalable catalyst particles, as a result of which MWCNT concentrations were overestimated. Combining carbon analysis and SEM/EDX is the most suitable for quantitative exposure assessment of MWCNTs in an actual workplace situation.

Carbon nanotube and nanofiber exposure and sputum and blood biomarkers of early effect among U.S. workers

BACKGROUND

Carbon nanotubes and nanofibers (CNT/F) are increasingly used for diverse applications. Although animal studies suggest CNT/F exposure may cause deleterious health effects, human epidemiological studies have typically been small, confined to single workplaces, and limited in exposure assessment.

OBJECTIVES

We conducted an industrywide cross-sectional epidemiological study of 108 workers from 12 U.S. sites to evaluate associations between occupational CNT/F exposure and sputum and blood biomarkers of early effect.

METHODS

We assessed CNT/F exposure via personal breathing zone, filter-based air sampling to measure background-corrected elemental carbon (EC) (a CNT/F marker) mass and microscopy-based CNT/F structure count concentrations. We measured 36 sputum and 37 blood biomarkers. We used factor analyses with varimax rotation to derive factors among sputum and blood biomarkers separately. We used linear, Tobit, and unconditional logistic regression models to adjust for potential confounders and evaluate associations between CNT/F exposure and individual biomarkers and derived factors.

RESULTS

We derived three sputum and nine blood biomarker factors that explained 78% and 67%, respectively, of the variation. After adjusting for potential confounders, inhalable EC and total inhalable CNT/F structures were associated with the most sputum and blood biomarkers, respectively. Biomarkers associated with at least three CNT/F metrics were 72 kDa type IV collagenase/matrix metalloproteinase-2 (MMP-2), interleukin-18, glutathione peroxidase (GPx), myeloperoxidase, and superoxide dismutase (SOD) in sputum and MMP-2, matrix metalloproteinase-9, metalloproteinase inhibitor 1/tissue inhibitor of metalloproteinases 1, 8-hydroxy-2'-deoxyguanosine, GPx, SOD, endothelin-1, fibrinogen, intercellular adhesion molecule 1, vascular cell adhesion protein 1, and von Willebrand factor in blood, although directions of associations were not always as expected.

CONCLUSIONS

Inhalable rather than respirable CNT/F was more consistently associated with fibrosis, inflammation, oxidative stress, and cardiovascular biomarkers.

Association of pulmonary, cardiovascular, and hematologic metrics with carbon nanotube and nanofiber exposure among U.S. workers: a cross-sectional study

BACKGROUND

Commercial use of carbon nanotubes and nanofibers (CNT/F) in composites and electronics is increasing; however, little is known about health effects among workers. We conducted a cross-sectional study among 108 workers at 12 U.S. CNT/F facilities. We evaluated chest symptoms or respiratory allergies since starting work with CNT/F, lung function, resting blood pressure (BP), resting heart rate (RHR), and complete blood count (CBC) components.

METHODS

We conducted multi-day, full-shift sampling to measure background-corrected elemental carbon (EC) and CNT/F structure count concentrations, and collected induced sputum to measure CNT/F in the respiratory tract. We measured (nonspecific) fine and ultrafine particulate matter mass and count concentrations. Concurrently, we conducted physical examinations, BP measurement, and spirometry, and collected whole blood. We evaluated associations between exposures and health measures, adjusting for confounders related to lifestyle and other occupational exposures.

RESULTS

CNT/F air concentrations were generally low, while 18% of participants had evidence of CNT/F in sputum. Respiratory allergy development was positively associated with inhalable EC (p=0.040) and number of years worked with CNT/F (p=0.008). No exposures were associated with spirometry-based metrics or pulmonary symptoms, nor were CNT/F-specific metrics related to BP or most CBC components. Systolic BP was positively associated with fine particulate matter (p-values: 0.015-0.054). RHR was positively associated with EC, at both the respirable (p=0.0074) and inhalable (p=0.0026) size fractions. Hematocrit was positively associated with the log of CNT/F structure counts (p=0.043).

CONCLUSIONS

Most health measures were not associated with CNT/F. The positive associations between CNT/F exposure and respiratory allergies, RHR, and hematocrit counts may not be causal and require examination in other studies.

Review of techniques and studies characterizing the release of carbon nanotubes from nanocomposites: Implications for exposure and human health risk assessment

Composites made with engineered nanomaterials (nanocomposites) have a wide range of applications, from use in basic consumer goods to critical national defense technologies. Carbon nanotubes (CNTs) are a popular addition in nanocomposites because of their enhanced mechanical, thermal, and electrical properties. Concerns have been raised, though, regarding potential exposure and health risks from nanocomposites containing CNTs because of comparisons to other high aspect ratio fibers. Assessing the factors affecting CNT release from composites is therefore paramount for understanding potential exposure scenarios that may occur during product handling and manipulation. Standardized methods for detecting and quantifying released CNTs, however, have not yet been developed. We therefore evaluated experimental approaches deployed by various researchers, with an emphasis on characterizing free versus composite bound CNTs. From our analysis of published studies characterizing CNT releases from nanocomposites, we found that the qualitative and quantitative methods used across studies varied greatly, thus limiting the ability for objective comparison and evaluation of various release factors. Nonetheless, qualitative results indicated that factors such as composite type, CNT functionalization, and energy input during manipulation (i.e., grinding) may affect CNT release. Based on our findings, we offer several recommendations for future product testing and assessment of potential exposure and health risks associated with CNT nanocomposites.

Carbon Nanotubes and Other Engineered Nanoparticles Induced Pathophysiology on Mesothelial Cells and Mesothelial Membranes

Nanoparticles have great potential for numerous applications due to their unique physicochemical properties. However, concerns have been raised that they may induce deleterious effects on biological systems. There is accumulating evidence that, like asbestos, inhaled nanomaterials of >5 μm and high aspect ratio (3:1), particularly rod-like carbon nanotubes, may inflict pleural disease including mesothelioma. Additionally, a recent set of case reports suggests that inhalation of polyacrylate/nanosilica could in part be associated with inflammation and fibrosis of the pleura of factory workers. However, the adverse outcomes of nanoparticle exposure to mesothelial tissues are still largely unexplored. In that context, the present review aims to provide an overview of the relevant pathophysiological implications involving toxicological studies describing effects of engineered nanoparticles on mesothelial cells and membranes. In vitro studies primarily emphasize on simulating cellular uptake and toxicity of nanotubes on benign or malignant cell lines. On the other hand, in vivo studies focus on illustrating endpoints of serosal pathology in rodent animal models. From a molecular aspect, some nanoparticle categories are shown to be cytotoxic and genotoxic after acute treatment, whereas chronic incubation may lead to malignant-like transformation. At an organism level, a number of fibrous shaped nanotubes are related with features of chronic inflammation and MWCNT-7 is the only type to consistently inflict mesothelioma.

The Effects of Varying Degree of MWCNT Carboxylation on Bioactivity in Various In Vivo and In Vitro Exposure Models

Functionalization has been shown to alter toxicity of multi-walled carbon nanotube (MWCNT) in several studies. This study varied the degree of functionalization (viz., amount of MWCNT surface carboxylation) to define the relationship between the extent of carboxylation and effects in a variety of in vitro cell models and short-term ex vivo/in vivo particle exposures. Studies with vitamin D₃ plus phorbol ester transformed THP-1 macrophages demonstrated that functionalization, regardless of amount, corresponded with profoundly decreased NLRP3 inflammasome activation. However, all MWCNT variants were slightly toxic in this model. Alternatively, studies with A549 epithelial cells showed some varied effects. For example, IL-33 and TNF-α release were related to varying amounts of functionalization. For in vivo particle exposures, autophagy of alveolar macrophages, measured using green fluorescent protein (GFP)- fused-LC3 transgenic mice, increased for all MWCNT tested three days after exposure, but, by Day 7, autophagy was clearly dependent on the amount of carboxylation. The instilled source MWCNT continued to produce cellular injury in alveolar macrophages over seven days. In contrast, the more functionalized MWCNT initially showed similar effects, but reduced over time. Dark-field imaging showed the more functionalized MWCNTs were distributed more uniformly throughout the lung and not isolated to macrophages. Taken together, the results indicated that in vitro and in vivo bioactivity of MWCNT decreased with increased carboxylation. Functionalization by carboxylation eliminated the bioactive potential of the MWCNT in the exposure models tested. The observation that maximally functionalized MWCNT distribute more freely throughout the lung with the absence of cellular damage, and extended deposition, may establish a practical use for these particles as a safer alternative for unmodified MWCNT.

Exposure assessments for a cross-sectional epidemiologic study of US carbon nanotube and nanofiber workers

BACKGROUND

Recent animal studies have suggested the potential for wide-ranging health effects resulting from exposure to carbon nanotubes and nanofibers (CNT/F). To date, no studies in the US have directly examined the relationship between occupational exposure and potential human health effects.

OBJECTIVES

Our goal was to measure CNT/F exposures among US workers with representative job types, from non-exposed to highly exposed, for an epidemiologic study relating exposure to early biologic effects.

METHODS

108 participants were enrolled from 12 facilities across the US. Personal, full-shift exposures were assessed based on the mass of elemental carbon (EC) at the respirable and inhalable aerosol particle size fractions, along with quantitatively characterizing CNT/F and estimating particle size via transmission electron microscopy (TEM). Additionally, sputum and dermal samples were collected and analyzed to determine internal exposures and exposures to the hands/wrists.

RESULTS

The mean exposure to EC was 1.00 μg/m³ at the respirable size fraction and 6.22 μg/m³ at the inhalable fraction. Analysis by TEM found a mean exposure of 0.1275 CNT/F structures/cm³, generally to agglomerated materials between 2 and 10 μm. Internal exposures to CNT/F via sputum analysis were confirmed in 18% of participants while ∼70% had positive dermal exposures.

CONCLUSIONS

We demonstrated the occurrence of a broad range of exposures to CNT/F within 12 facilities across the US. Analysis of collected sputum indicated internal exposures are currently occurring within the workplace. This is an important first step in determining if exposures in the workforce have any acute or lasting health effects.

Characterizing workforces exposed to current and emerging non-carbonaceous nanomaterials in the U.S

OBJECTIVE

Toxicology studies suggest that exposure to certain types of engineered nanomaterials (ENMs) may cause adverse health effects, but little is known about the workforce in the United States that produces or uses these materials. In addition, occupational exposure control strategies in this industry are not well characterized. This study identified U.S. ENM manufacturers and users (other than carbon nanotubes and nanofibers, which have been characterized elsewhere), determined workforce size, characterized types and quantities of materials used, occupational exposure control strategies, and the feasibility of occupational ENM exposure studies.

METHODS

Eligible companies were identified and information was collected through phone surveys on nanomaterials produced or used, workforce size, location, work practices, and exposure control strategies. The companies were classified into groups for additional examinations.

RESULTS

Forty-nine companies producing or using ENMs in the U.S. were identified. These companies employed at least 1,500 workers. Most companies produced or used some form of nanoscale metal. More than half of the eligible companies were suppliers for the coatings, composite materials, or general industries. Each company provided information about worker exposure reduction strategies through engineering controls, administrative controls, or personal protective equipment. Production-scale companies reported greater use of specific exposure control strategies for ENMs than laboratory-scale companies.

CONCLUSIONS

Workplaces producing or using ENMs report using engineering and administrative controls as well as personal protective equipment to control worker exposure. Industrywide exposure assessment studies appear feasible due to workforce size. However, more effort must be taken to target industries using specific ENMs based on known toxicological effects and health risks.

Methodological, political and legal issues in the assessment of the effects of nanotechnology on human health

Engineered nanomaterials (ENMs) raise questions among the scientific community and public health authorities about their potential risks to human health. Studying a prospective cohort of workers exposed to ENMs would be considered the gold standard for identifying potential health effects of nanotechnology and confirming the 'no effect' levels derived from cellular and animal models. However, because only small, cross-sectional studies have been conducted in the past 5 years, questions remain about the health risks of ENMs. This essay addresses the scientific, methodological, political and regulatory issues that make epidemiological research in nanotechnology-exposed communities particularly complex. Scientific challenges include the array of physicochemical parameters and ENM production conditions, the lack of universally accepted definitions of ENMs and nanotechnology workers, and the lack of information about modes of action, target organs and likely dose-response functions of ENMs. Standardisation of data collection and harmonisation of research protocols are needed to eliminate misclassification of exposures and health effects. Forming ENM worker cohorts from a combination of smaller cohorts and overcoming selection bias are also challenges. National or international registries for monitoring the exposures and health of ENM workers would be helpful for epidemiological studies, but the creation of such a registry and ENM worker cohorts will require political support and dedicated funding at the national and international levels. Public authorities and health agencies should consider carrying out an ENM awareness campaign to educate and engage all stakeholders and concerned communities in discussion of such a project.

Changes in DNA methylation induced by multi-walled carbon nanotube exposure in the workplace

This study was designed to assess the epigenetic alterations in blood cells, induced by occupational exposure to multi-wall carbon nanotubes (MWCNT). The study population comprised of MWCNT-exposed workers (n=24) and unexposed controls (n=43) from the same workplace. We measured global DNA methylation/hydroxymethylation levels on the 5th cytosine residues using a validated liquid chromatography tandem-mass spectrometry (LC-MS/MS) method. Sequence-specific methylation of LINE1 retrotransposable element 1 (L1RE1) elements, and promoter regions of functionally important genes associated with epigenetic regulation [DNA methyltransferase-1 (DNMT1) and histone deacetylase 4 (HDAC4)], DNA damage/repair and cell cycle pathways [nuclear protein, coactivator of histone transcription/ATM serine/threonine kinase (NPAT/ATM)], and a potential transforming growth factor beta (TGF-β) repressor [SKI proto-oncogene (SKI)] were studied using bisulfite pyrosequencing. Analysis of global DNA methylation levels and hydroxymethylation did not reveal significant difference between the MWCNT-exposed and control groups. No significant changes in Cytosine-phosphate-Guanine (CpG) site methylation were observed for the LINE1 (L1RE1) elements. Further analysis of gene-specific DNA methylation showed a significant change in methylation for DNMT1, ATM, SKI, and HDAC4 promoter CpGs in MWCNT-exposed workers. Since DNA methylation plays an important role in silencing/regulation of the genes, and many of these genes have been associated with occupational and smoking-induced diseases and cancer (risk), aberrant methylation of these genes might have a potential effect in MWCNT-exposed workers.

In Vivo Toxicity Assessment of Occupational Components of the Carbon Nanotube Life Cycle To Provide Context to Potential Health Effects

Pulmonary toxicity studies on carbon nanotubes focus primarily on as-produced materials and rarely are guided by a life cycle perspective or integration with exposure assessment. Understanding toxicity beyond the as-produced, or pure native material, is critical, due to modifications needed to overcome barriers to commercialization of applications. In the first series of studies, the toxicity of as-produced carbon nanotubes and their polymer-coated counterparts was evaluated in reference to exposure assessment, material characterization, and stability of the polymer coating in biological fluids. The second series of studies examined the toxicity of aerosols generated from sanding polymer-coated carbon-nanotube-embedded or neat composites. Postproduction modification by polymer coating did not enhance pulmonary injury, inflammation, and pathology or in vitro genotoxicity of as-produced carbon nanotubes, and for a particular coating, toxicity was significantly attenuated. The aerosols generated from sanding composites embedded with polymer-coated carbon nanotubes contained no evidence of free nanotubes. The percent weight incorporation of polymer-coated carbon nanotubes, 0.15% or 3% by mass, and composite matrix utilized altered the particle size distribution and, in certain circumstances, influenced acute in vivo toxicity. Our study provides perspective that, while the number of workers and consumers increases along the life cycle, toxicity and/or potential for exposure to the as-produced material may greatly diminish.

Sparse Supervised Classification Methods Predict and Characterize Nanomaterial Exposures: Independent Markers of MWCNT Exposures

Recent experimental evidence indicates significant pulmonary toxicity of multiwalled carbon nanotubes (MWCNTs), such as inflammation, interstitial fibrosis, granuloma formation, and carcinogenicity. Although numerous studies explored the adverse potential of various CNTs, their comparability is often limited. This is due to differences in administered dose, physicochemical characteristics, exposure methods, and end points monitored. Here, we addressed the problem through sparse classification method, a supervised machine learning approach that can reduce the noise contained in redundant variables for discriminating among MWCNT-exposed and MWCNT-unexposed groups. A panel of proteins measured from bronchoalveolar lavage fluid (BAL) samples was used to predict exposure to various MWCNT and determine markers that are attributable to MWCNT exposure and toxicity in mice. Using sparse support vector machine-based classification technique, we identified a small subset of proteins clearly distinguishing each exposure. Macrophage-derived chemokine (MDC/CCL22), in particular, was associated with various MWCNT exposures and was independent of exposure method employed, that is, oropharyngeal aspiration versus inhalation exposure. Sustained expression of some of the selected protein markers identified also suggests their potential role in MWCNT-induced toxicity and proposes hypotheses for future mechanistic studies. Such approaches can be used more broadly for nanomaterial risk profiling studies to evaluate decisions related to dose/time-response relationships that could delineate experimental variables from exposure markers.

Evaluating the mechanistic evidence and key data gaps in assessing the potential carcinogenicity of carbon nanotubes and nanofibers in humans

In an evaluation of carbon nanotubes (CNTs) for the IARC Monograph 111, the Mechanisms Subgroup was tasked with assessing the strength of evidence on the potential carcinogenicity of CNTs in humans. The mechanistic evidence was considered to be not strong enough to alter the evaluations based on the animal data. In this paper, we provide an extended, in-depth examination of the in vivo and in vitro experimental studies according to current hypotheses on the carcinogenicity of inhaled particles and fibers. We cite additional studies of CNTs that were not available at the time of the IARC meeting in October 2014, and extend our evaluation to include carbon nanofibers (CNFs). Finally, we identify key data gaps and suggest research needs to reduce uncertainty. The focus of this review is on the cancer risk to workers exposed to airborne CNT or CNF during the production and use of these materials. The findings of this review, in general, affirm those of the original evaluation on the inadequate or limited evidence of carcinogenicity for most types of CNTs and CNFs at this time, and possible carcinogenicity of one type of CNT (MWCNT-7). The key evidence gaps to be filled by research include: investigation of possible associations between in vitro and early-stage in vivo events that may be predictive of lung cancer or mesothelioma, and systematic analysis of dose-response relationships across materials, including evaluation of the influence of physico-chemical properties and experimental factors on the observation of nonmalignant and malignant endpoints.

Lung carcinogenicity of inhaled multi-walled carbon nanotube in rats

BACKGROUND

Multi-walled carbon nanotubes (MWCNTs) constitute one of the most promising types of nanomaterials in industry today. With their increasing use, the potential toxicity and carcinogenicity of MWCNT needs to be evaluated in bioassay studies using rodents. Since humans are mainly exposed to MWCNT by inhalation, we performed a 104-week carcinogenicity study using whole-body inhalation exposure chambers with a fibrous straight type of MWCNT at concentrations of 0, 0.02, 0.2, and 2 mg/m3 using male and female F344 rats.

RESULTS

Lung carcinomas, mainly bronchiolo-alveolar carcinoma, and combined carcinomas and adenomas were significantly increased in males exposed to 0.2 and 2 mg/m3 MWNT-7 and in females exposed to 2 mg/m3 MWNT-7 compared to the clean air control group. However, no development of pleural mesothelioma was observed. Concentration-dependent toxic effects in the lung such as epithelial hyperplasia, granulomatous change, localized fibrosis, and alteration in BALF parameters were found in MWNT-7 treatment groups of both sexes. There were no MWNT-7-specific macroscopic findings in the other organs, including the pleura and peritoneum. Absolute and relative lung weights were significantly elevated in male rats exposed to 0.2 and 2 mg/m3 MWNT-7 and in all exposed female groups. The lung burdens of MWNT-7 were clearly increased in a concentration-dependent as well as a duration-dependent manner.

CONCLUSION

There is clear evidence that MWNT-7 is carcinogenic to the lungs of male and female F344 rats, however no plural mesothelioma was observed.

Development of Portable Aerosol Mobility Spectrometer for Personal and Mobile Aerosol Measurement

We describe development of a Portable Aerosol Mobility Spectrometer (PAMS) for size distribution measurement of submicrometer aerosol. The spectrometer is designed for use in personal or mobile aerosol characterization studies and measures approximately 22.5 × 22.5 × 15 cm and weighs about 4.5 kg including the battery. PAMS uses electrical mobility technique to measure number-weighted particle size distribution of aerosol in the 10-855 nm range. Aerosol particles are electrically charged using a dual-corona bipolar corona charger, followed by classification in a cylindrical miniature differential mobility analyzer. A condensation particle counter is used to detect and count particles. The mobility classifier was operated at an aerosol flow rate of 0.05 L/min, and at two different user-selectable sheath flows of 0.2 L/min (for wider size range 15-855 nm) and 0.4 L/min (for higher size resolution over the size range of 10.6-436 nm). The instrument was operated in voltage stepping mode to retrieve the size distribution, which took approximately 1-2 minutes, depending on the configuration. Sizing accuracy and resolution were probed and found to be within the 25% limit of NIOSH criterion for direct-reading instruments (NIOSH 2012). Comparison of size distribution measurements from PAMS and other commercial mobility spectrometers showed good agreement. The instrument offers unique measurement capability for on-person or mobile size distribution measurements of ultrafine and nanoparticle aerosol.

Refinement of the Nanoparticle Emission Assessment Technique into the Nanomaterial Exposure Assessment Technique (NEAT 2.0)

Engineered nanomaterial emission and exposure characterization studies have been completed at more than 60 different facilities by the National Institute for Occupational Safety and Health (NIOSH). These experiences have provided NIOSH the opportunity to refine an earlier published technique, the Nanoparticle Emission Assessment Technique (NEAT 1.0), into a more comprehensive technique for assessing worker and workplace exposures to engineered nanomaterials. This change is reflected in the new name Nanomaterial Exposure Assessment Technique (NEAT 2.0) which distinguishes it from NEAT 1.0. NEAT 2.0 places a stronger emphasis on time-integrated, filter-based sampling (i.e., elemental mass analysis and particle morphology) in the worker's breathing zone (full shift and task specific) and area samples to develop job exposure matrices. NEAT 2.0 includes a comprehensive assessment of emissions at processes and job tasks, using direct-reading instruments (i.e., particle counters) in data-logging mode to better understand peak emission periods. Evaluation of worker practices, ventilation efficacy, and other engineering exposure control systems and risk management strategies serve to allow for a comprehensive exposure assessment.

Bridging the gap between exposure assessment and inhalation toxicology: Some insights from the carbon nanotube experience

The early incorporation of exposure assessment can be invaluable to help design, prioritize, and interpret toxicological studies or outcomes. The sum total of the exposure assessment findings combined with preliminary toxicology results allows for exposure-informed toxicological study design and the findings can then be integrated, together with available epidemiologic data, to provide health effect relevance. With regard to engineered nanomaterial inhalation toxicology in particular, a single type of material (e.g. carbon nanotube, graphene) can have a vast array of physicochemical characteristics resulting in the potential for varying toxicities. To compound the matter, the methodologies necessary to establish a material adequate for in vivo exposure testing raises questions on the applicability of the outcomes. From insights gained from evaluating carbon nanotubes, we recommend the following integrated approach involving exposure-informed hazard assessment and hazard-informed exposure assessment especially for materials as diverse as engineered nanomaterials: 1) market-informed identification of potential hazards and potentially exposed populations, 2) initial toxicity screening to drive prioritized assessments of exposure, 3) development of exposure assessment-informed chronic and sub-chronic in vivo studies, and 4) conduct of exposure- and hazard-informed epidemiological studies.