Nanomaterials induce different levels of oxidative stress, depending on the used model system: Comparison of in vitro and in vivo effects

Neurological Outcomes of Joint Exposure to Polystyrene Micro/Nanospheres and Silver Nanoparticles in Zebrafish

BACKGROUND

Micro/nanoplastics and silver nanoparticles (AgNPs) are emerging environmental contaminants widely detected in aquatic environments. However, previous research has primarily focused on the interactions between micro/nanoplastics and organic substances or heavy metals, whereas the interactions and combined toxic effects of micro/nanoplastics with AgNPs remain unclear.

OBJECTIVE

Our study aimed to investigate the effects and mechanisms of coexposure to AgNPs and polystyrene micro/nanospheres (PS M/NPs) on the nervous system, comparing the toxicity of AgNPs alone and in combination with PS M/NPs in larval zebrafish.

METHODS

We investigated the dynamics of AgNPs' (5  nm ) adsorption onto PS M/NPs (5 μ m / 100  nm ) using inductively coupled plasma-mass spectrometry. Zebrafish larvae were coexposed to PS M/NPs (200 μ g / L ) and AgNPs (10 μ g / L ) from 6 h post fertilization (hpf) to 72  hpf to ∼ 120  hpf to evaluate neuroinflammatory effects from multiple perspectives, including developmental abnormalities, oxidative stress, neurobehavioral differences, vascular development, immune responses, differences in gene expression, and differences upon neuroinflammation inhibitor addition.

RESULTS

Adsorption experiments showed PS M/NPs could stably adsorb AgNPs, with higher adsorption in smaller particles. Zebrafish larvae exposed to combined PS M/NPs and AgNPs demonstrated neurodevelopmental abnormalities, including developmental malformations, lower levels of locomotor activity, delayed response, and abnormal neuronal development. In addition, exposed zebrafish also exhibited disrupted neurodevelopmental markers, including vascular and apoptotic indicators, and oxidative stress and neuroimmune responses. Quantitative real-time polymerase chain reaction analysis showed differences in gene expression within neurotoxic pathways in PS M/NPs and AgNPs-exposed zebrafish, focusing on key genes in immunity, apoptosis, vascular, and neural development. Furthermore, these neurotoxic effects induced by combined exposure were alleviated following the introduction of the neuroinflammation inhibitor curcumin.

DISCUSSION

Our findings demonstrate that polystyrene nanospheres (PSNPs) intensified AgNPs-induced neurotoxicity in larval zebrafish, whereas polystyrene microspheres (PSMPs) had a lesser effect, indicating distinct gene regulation roles when combined with AgNPs. These findings enhance the assessment of environmental risks in settings with coexisting nanomaterials and microplastics, offering important insights for evaluating combined exposure risks. https://doi.org/10.1289/EHP14873.

Novel nanomaterials-based combating strategies against drug-resistant bacteria

Numerous types of contemporary antibiotic treatment regimens have become ineffective with the increasing incidence of drug tolerance. As a result, it is pertinent to seek novel and innovative solutions such as antibacterial nanomaterials (NMs) for the prohibition and treatment of hazardous microbial infections. Unlike traditional antibiotics (e.g., penicillin and tetracycline), the unique physicochemical characteristics (e.g., size dependency) of NMs endow them with bacteriostatic and bactericidal potential. However, it is yet difficult to mechanistically predict or decipher the networks of molecular interaction (e.g., between NMs and the biological systems) and the subsequent immune responses. In light of such research gap, this review outlines various mechanisms accountable for the inception of drug tolerance in bacteria. It also delineates the primary factors governing the NMs-induced molecular mechanisms against microbes, specifically drug-resistant bacteria along with the various NM-based mechanisms of antibacterial activity. The review also explores future directions and prospects for NMs in combating drug-resistant bacteria, while addressing challenges to their commercial viability within the healthcare industry.

Biocompatibility and photoinactivation evaluation of zinc hydroxide chloride nanosheets against Streptococcus mutans

BACKGROUND

Considering the efficacy of antimicrobial photodynamic therapy (PDT) in inactivating bacteria, this study reports that zinc hydroxide chloride nanosheets (ZHC-NSs) are useful for this purpose.

MATERIALS & METHODS

The characterization of ZHC-NSs was performed using microscopic and spectroscopic techniques. The irritation test, acute toxicity test, and genotoxicity test of ZHC-NSs were evaluated and their effects on human pulp fibroblast cells (HPFC) viability, intracellular reactive oxygen species (ROS) levels, and antibacterial activity of ZHC-NSs (1-8 mg ml-1) alone or in light conditions were investigated.

RESULTS

The ZHC-NSs structure showed a crystalline form and their sheets' thickness had an average size of 129.6 ± 19.50 nm. ZHC-NSs did not severely damage internal organs and were not genotoxic. The cytotoxic effect of ZHC-NSs on HPFC was concentration-dependent so that ZHC-NSs at higher concentrations (4 and 8 mg ml-1) killed half of the HPFC cells. When ZHC-NSs were used in combination with a 980 nm diode laser, namely ZHC-NS©, ROS production increased and led to enhanced antibacterial activity against Streptococcus mutans in planktonic and biofilm form. A statistically significant difference was found between ZHC-NSs without laser irradiation and photoexcited ZHC-NSs.

CONCLUSION

ZHC-NSs© with the potential ability to produce ROS could be effective in complementary treatment against S. mutans.

Mechanistic insights for efficient removal of intracellular and extracellular antibiotic resistance genes by iron-based nanocopper: Intracellular oxidative stress and internalization of nanocopper

The widespread use of antibiotics has led to a severe pollution issue with antibiotic resistance genes (ARGs), which poses a significant threat to both ecological environments and human health. In this study, we developed an iron-based nanocopper bimetallic material (Fe-nCu) for the efficient removal of ARGs. Our results indicate that nCu can attach to the surface of iron, forming aggregated copper nanoclusters resembling wheat ears. The composition of Fe-nCu particles consists of 75.90 % iron and 20.95 % copper. Fe-nCu demonstrates a unique capability in eliminating ARGs, achieving removal efficiencies of 3.75 and 4.36 logs for intracellular and extracellular ARGs, respectively. Furthermore, Fe-nCu remains stable in complex water environments and is unaffected by organic substances in the water. This material induces oxidative stress in cells within a short period, leading to an imbalance in intracellular redox levels and resulting in cell membrane damage. nCu causes severe membrane damage to E. coli, penetrating the cell due to its size advantage, which leads to the encapsulation and internalization of E. coli by the copper nanoparticles. Once inside, the nCu particles cleave DNA and disrupt the function of ARGs. This study not only provides a cost-effective material for the removal of ARGs but also offers an in-depth understanding of the action mechanism of Fe-nCu, presenting a novel pathway for inhibiting the propagation of ARGs.

Maternal exposure to fullerenols impairs placental development in mice by inhibiting estriol synthesis and reducing ERα

Fullerenols, a water-soluble polyhydroxy derivative of fullerene, hold promise in medical and materials science due to their unique properties. However, concerns about their potential embryotoxicity remain. Using a pregnancy mouse model and metabolomics analysis, our findings reveal that fullerenols exposure during pregnancy not only significantly reduced mice placental weight and villi thickness, but also altered the classes and concentrations of metabolites in the mouse placenta. Furthermore, we found that fullerenols exposure reduced the levels of CYP3A4, ERα and estriol (E3), while increasing the levels of estradiol (E2) and oxidative stress both in mouse placenta and placental trophoblast cells, and exogenous supplementation with E3 and ER agonists was effective in restoring these changes in vitro. Moreover, CYP3A4 inhibition was effective in decreasing intracellular E3 levels, whereas overexpression of CYP3A4 resisted the fullerenols-induced decrease in E3 expression Additionally, we synthesized glutathione-modified fullerenols (C60-(OH)n-GSH), which demonstrated improved biocompatibility and reduced embryotoxicity by enhancing intracellular glutathione levels and mitigating oxidative stress. In summary, our results demonstrated that fullerenols exposure decreased E3 synthesis by inhibiting CYP3A4 and exacerbated oxidative stress through downregulation of estrogen receptor activation and decreased glutathione levels. These findings highlight the risks of fullerenols exposure during pregnancy and offer strategies for safer nanomaterial development.

Effects of engineered nanomaterials on the cardiovascular system

With the explosive development of nanotechnology, engineered nanomaterials are currently being used in various industries, including food and medicine. Concern about the health effects of nanomaterials has been raised, and available research indicates that the relative surface area of nanomaterials seems to correlate with the severity of their toxicity. With regard to engineered nanomaterials, the scope of their acute and chronic toxicities and their mechanisms are not fully understood. Studies suggest that exposure to certain nanomaterials can generate reactive oxidant species and enhance permeability of the phagolysosomal membrane, which leads to inflammasome activation, causing oxidative stress and inflammation. Since the latter 2 are implicated in the development of cardiovascular diseases, such as hypertension and atherosclerosis, it can be presumed that exposure to engineered nanomaterials could significantly impact cardiovascular function. In this review, I raise issues that should be considered in the assessment of the effects of engineered nanomaterials on cardiovascular function, and evaluate their cardiovascular toxicity as described in various in vitro and/or in vivo toxicological studies and industrial investigations.

Ferroptosis participated in inhaled polystyrene nanoplastics-induced liver injury and fibrosis

The emerging contaminant nanoplastics (NPs) have received considerable attention. Due to their tiny size and unique colloidal properties, NPs could more easily enter the body and cross biological barriers with inhalation exposure. While NPs-induced hepatotoxicity has been reported, the hepatic impact of inhaled NPs was still unknown. To close this gap, a 40 nm polystyrene NPs (PS-NPs) inhalation exposure mice model was developed to explore the hepatotoxicity during acute (1 week), subacute (4 weeks), and subchronic period (12 weeks), with four exposure doses (0, 16, 40, and 100 μg/day). Results showed that inhaled PS-NPs caused a remarkable increase of ALT, AST, and ALP with a decrease of CHE, indicating liver dysfunction. Various histological abnormalities and significantly higher levels of inflammation in a dose- and time-dependent manner were observed. Moreover, after 4 weeks and 12 weeks of exposure, Masson staining and upregulated expression of TGF-β, α-SMA, and Col1a1 identified that inhaled PS-NPs exposure triggered the progression of liver fibrosis with the exposure time prolonged. From the mechanistic perspective, transcriptome analysis revealed that ferroptosis was involved in PS-NPs-induced liver hepatotoxicity, and key features of ferroptosis were detected, including persistent oxidative stress, iron overload, increased LPO, mitochondria damage, and the expression changes of GPX4, TFRC, and Ferritin. And in vitro and in vivo recovery tests showed that ferroptosis inhibitor Fer-1 treatment alleviated liver injury and fibrosis. The above results confirmed the critical role of ferroptosis in PS-NPs-induced hepatotoxicity. Furthermore, to better conclude our findings and understand the mechanistic causality within it, an adverse outcome pathway (AOP) framework was established. In total, this present study conducted the first experimental assessment of inhalation exposure to PS-NPs on the liver, identified that continuous inhaled PS-NPs could cause liver injury and fibrosis, and PS-NPs- ferroptosis provided a novel mechanistic explanation.

The hepatotoxicity assessment of micro/nanoplastics: A preliminary study to apply the adverse outcome pathways

Plastic pollution has become a significant global problem over the years, leading to the continuous decomposition and accumulation of micro/nanoplastics (MNPLs) in the environment. As a result, human exposure to these MNPLs is inevitable. The liver, in particular, is highly susceptible to potential MNPL toxicity. In this study, we systematically reviewed the current literature on MNPLs-induced hepatotoxicity and collected data on toxic events occurring at different biological levels. Then, to better understand the cause-mechanism causality, we developed an Adverse Outcome Pathway (AOP) framework for MNPLs-induced hepatotoxicity. The AOP framework provided insights into the mechanism of MNPL-induced hepatotoxicity and highlighted potential health risks such as liver dysfunction and inflammation, metabolism disorders and liver fibrosis. Moreover, we discussed the potential application of emerging toxicological models in the hepatotoxicity study. Liver organoids and liver-on-chips, which can simulate the structure and function of the liver in vitro, offer a promising alternative platform for toxicity testing and risk assessment. We proposed combining the AOP framework with these emerging toxicological models to improve our understanding of the hepatotoxic effects of MNPLs. Overall, this study performed a preliminary exploration of novel toxicological methodologies to assess the hepatotoxicity of MNPLs, providing a deeper understanding of environmental toxicology.

Polymer-based nanocomposite adsorbents for resource recovery from wastewater

Developing mitigation mechanisms for eutrophication caused by the uncontrolled release of nutrients is in the interest of the scientific community. Adsorption, being operationally simple and economical with no significant secondary pollution, has proven to be a feasible technology for resource recovery. However, the utility of adsorption often lies in the availability of effective adsorbents. In this regard, polymer-based nanocomposite (PNC) adsorbents have been highly acclaimed by researchers because of their high surface area, multiple functional groups, biodegradability, and ease of large-scale production. This review paper elaborates on the functionality, adsorption mechanisms, and factors that affect the adsorption and adsorption-desorption cycles of PNC adsorbents toward nutrient resources. Moreover, this review gives insight into the application of recovered nutrient resources in soil amendment.

Multiparametric in vitro genotoxicity assessment of different variants of amorphous silica nanomaterials in rat alveolar epithelial cells

The hazard posed to human health by inhaled amorphous silica nanomaterials (aSiO2 NM) remains uncertain. Herein, we assessed the cyto- and genotoxicity of aSiO2 NM variants covering different sizes (7, 15, and 40 nm) and surface modifications (unmodified, phosphonate-, amino- and trimethylsilyl-modified) on rat alveolar epithelial (RLE-6TN) cells. Cytotoxicity was evaluated at 24 h after exposure to the aSiO2 NM variants by the lactate dehydrogenase (LDH) release and WST-1 reduction assays, while genotoxicity was assessed using different endpoints: DNA damage (single- and double-strand breaks [SSB and DSB]) by the comet assay for all aSiO2 NM variants; cell cycle progression and γ-H2AX levels (DSB) by flow cytometry for those variants that presented higher cytotoxic and DNA damaging potential. The variants with higher surface area demonstrated a higher cytotoxic potential (SiO2_7, SiO2_15_Unmod, SiO2_15_Amino, and SiO2_15_Phospho). SiO2_40 was the only variant that induced significant DNA damage on RLE-6TN cells. On the other hand, all tested variants (SiO2_7, SiO2_15_Unmod, SiO2_15_Amino, and SiO2_40) significantly increased total γ-H2AX levels. At high concentrations (28 µg/cm2), a decrease in G0/G1 subpopulation was accompanied by a significant increase in S and G2/M sub-populations after exposure to all tested materials except for SiO2_40 which did not affect cell cycle progression. Based on the obtained data, the tested variants can be ranked for its genotoxic DNA damage potential as follows: SiO2_7 = SiO2_40 = SiO2_15_Unmod > SiO2_15_Amino. Our study supports the usefulness of multiparametric approaches to improve the understanding on NM mechanisms of action and hazard prediction.

The toxicity of nanoparticles and their interaction with cells: an in vitro metabolomic perspective

Nowadays, nanomaterials (NMs) are widely present in daily life due to their significant benefits, as demonstrated by their application in many fields such as biomedicine, engineering, food, cosmetics, sensing, and energy. However, the increasing production of NMs multiplies the chances of their release into the surrounding environment, making human exposure to NMs inevitable. Currently, nanotoxicology is a crucial field, which focuses on studying the toxicity of NMs. The toxicity or effects of nanoparticles (NPs) on the environment and humans can be preliminary assessed in vitro using cell models. However, the conventional cytotoxicity assays, such as the MTT assay, have some drawbacks including the possibility of interference with the studied NPs. Therefore, it is necessary to employ more advanced techniques that provide high throughput analysis and avoid interferences. In this case, metabolomics is one of the most powerful bioanalytical strategies to assess the toxicity of different materials. By measuring the metabolic change upon the introduction of a stimulus, this technique can reveal the molecular information of the toxicity induced by NPs. This provides the opportunity to design novel and efficient nanodrugs and minimizes the risks of NPs used in industry and other fields. Initially, this review summarizes the ways that NPs and cells interact and the NP parameters that play a role in this interaction, and then the assessment of these interactions using conventional assays and the challenges encountered are discussed. Subsequently, in the main part, we introduce the recent studies employing metabolomics for the assessment of these interactions in vitro.

Comparing species-different responses in pulmonary fibrosis research: Current understanding of in vitro lung cell models and nanomaterials

Pulmonary fibrosis (PF) is a chronic, irreversible lung disease that is typically fatal and characterized by an abnormal fibrotic response. As a result, vast areas of the lungs are gradually affected, and gas exchange is impaired, making it one of the world's leading causes of death. This can be attributed to a lack of understanding of the onset and progression of the disease, as well as a poor understanding of the mechanism of adverse responses to various factors, such as exposure to allergens, nanomaterials, environmental pollutants, etc. So far, the most frequently used preclinical evaluation paradigm for PF is still animal testing. Nonetheless, there is an urgent need to understand the factors that induce PF and find novel therapeutic targets for PF in humans. In this regard, robust and realistic in vitro fibrosis models are required to understand the mechanism of adverse responses. Over the years, several in vitro and ex vivo models have been developed with the goal of mimicking the biological barriers of the lung as closely as possible. This review summarizes recent progress towards the development of experimental models suitable for predicting fibrotic responses, with an emphasis on cell culture methods, nanomaterials, and a comparison of results from studies using cells from various species.

Polymer-based nanocomposite adsorbents for resource recovery from wastewater

Adsorption is alternative technique for recovery of nutrient resources with no/less secondary pollution. PNC adsorbents are effective for removal and recovery of nutrient resources, and reusing nutrients as fertilizer could prevent eutrophication.

Predicting nanomaterials pulmonary toxicity in animals by cell culture models: Achievements and perspectives

Animal experiments are highly relevant models for the assessment of toxicological effects of engineered nanomaterials (ENMs), due to lack of biomonitoring and epidemiological studies. However, the expanding number of ENMs with different physico-chemical properties strains this approach, as there are ethical concerns and economical challenges with the use of animals in toxicology. There is an urgent need for cell culture models that predict the level of toxicological responses in vivo, consequently reducing or replacing the use of animals in nanotoxicology. However, there is still a limited number of studies on in vitro-in vivo correlation of toxicological responses following ENMs exposure. In this review, we collected studies that have compared in vitro and in vivo toxic effects caused by ENMs. We discuss the influence of cell culture models and exposure systems on the predictability of in vitro models to equivalent toxic effects in animal lungs after pulmonary exposure to ENMs. In addition, we discuss approaches to qualitatively or quantitatively compare the effects in vitro and in vivo. The magnitude of toxicological responses in cells that are exposed in submerged condition is not systematically different from the response in cells exposed in air-liquid interface systems, and there appears to be similar ENMs hazard ranking between the two exposure systems. Overall, we show that simple in vitro models with cells exposed to ENMs in submerged condition can be used to predict toxic effects in vivo, and identify future strategies to improve the associations between in vitro and in vivo ENMs-induced pulmonary toxicity. This article is categorized under: Toxicology and Regulatory Issues in Nanomedicine > Toxicology of Nanomaterials.

A new source of representative secondary PET nanoplastics. Obtention, characterization, and hazard evaluation

Micro and nanoplastics (MNPLs) are emergent environmental pollutants requiring urgent information on their potential risks to human health. One of the problems associated with the evaluation of their undesirable effects is the lack of representative samples, matching those resulting from the environmental degradation of plastic wastes. To such end, we propose an easy method to obtain polyethylene terephthalate nanoplastics from water plastic bottles (PET-NPLs) but, in principle, applicable to any other plastic goods sources. An extensive characterization indicates that the proposed process produces uniform samples of PET-NPLs of around 100 nm, as determined by using AF4 and multi-angle and dynamic light scattering methodologies. An important point to be highlighted is that to avoid the metal contamination resulting from methods using metal blades/burrs for milling, trituration, or sanding, we propose to use diamond burrs to produce metal-free samples. To visualize the toxicological profile of the produced PET-NPLs we have evaluated their ability to be internalized by cells, their cytotoxicity, their ability to induce oxidative stress, and induce DNA damage. In this preliminary approach, we have detected their cellular uptake, but without the induction of significant biological effects. Thus, no relevant increases in toxicity, reactive oxygen species (ROS) induction, or DNA damage -as detected with the comet assay- have been observed. The use of representative samples, as produced in this study, will generate relevant data in the discussion about the potential health risks associated with MNPLs exposures.

Study on the biological effects of ZnO nanosheets on EBL cells

In this study, the biological effects of ZnO nanosheets were initially investigated using embryonic bovine lung (EBL) cells cultured in vitro as a model. ZnO nanosheets were prepared by a hydrothermal method, and their structure and morphology were characterized, and their effects on EBL cell viability, oxidative stress, cell proliferation, and apoptosis were investigated. The results showed that 12.5 μg ml⁻¹ ZnO nanosheets can cause morphological changes in EBL cells. The toxic effects of ZnO nanosheets on EBL cells were time-dependent. Caspase-3 activity in EBL cells changed under certain conditions with the introduction of 25 μg ml⁻¹ ZnO nanomaterials, and EBL cell apoptosis was promoted. Under different concentration and time effects, ZnO nanosheets induced an increase in ROS levels in EBL cells, indicating that they have an oxidative damage effect on cells. The toxic effects of ZnO nanosheets on EBL cells were discussed, including concentration effect, time effect, and cytotoxic effect, which eventually led to cell oxidative damage.

Phytochemical Composition and Protective Effect of Vernonanthura polyanthes Leaf against In Vivo Doxorubicin-Mediated Toxicity

Vernonanthura polyanthes (Spreng.) A.J. Vega & Dematt. (syn.: Vernonia polyanthes Less) is popularly known as “assa-peixe” and its leaves are used in folk medicine mainly to treat respiratory diseases. In this study, we evaluated the cytogenotoxic and anticytogenotoxic potential of the V. polyanthes leaf aqueous extract (VpLAE) and its n-butanol fraction (n-BF) in the presence or absence of doxorubicin (DXR) (pre-, co-, and post-treatments) on a murine model for 24 h or 120 h. The micronucleus test (MN) and the comet assay were used to assess the cytogenotoxic and anticytogenotoxic potential of VpLAE and n-BF (250, 500, and 1000 mg/kg) administered via gavage to Swiss Webster mice. The chemical profiles of VpLAE and n-BF were assessed by liquid chromatography coupled to mass spectrometry, and their metabolites were putatively identified. Lastly, the possible biological activities related to the (anti) cytogenotoxicity of the compounds were predicted using the PASS online webserver. The in vivo results showed that different doses of VpLAE and n-BF did not present cytotoxic activity; however, the MN test revealed a slight mutagenic activity for the 24 h treatments. Moderate genotoxic effects were demonstrated for all treatments in the comet assay. Regarding anticytotoxicity and antimutagenicity, VpLAE and n-BF presented a high cytoprotective potential against DXR toxic effects. In the co-treatment, VpLAE reduced the DXR genotoxicity by ~27%, and n-BF did not demonstrate antigenotoxic potential. In contrast, an antigenotoxic effect was observed for both VpLAE and n-BF in the pre- and post-treatments, reducing DXR genotoxicity by ~41% and ~47%, respectively. Chemical analysis of VpLAE and n-BF showed the presence of eight phenolic compounds, including seven chlorogenic acids and a flavonoid. The PASS online tool predicted antimutagenic, anticancer, antineoplastic, chemoprotective, antioxidant, and radical scavenging activities for all constituents identified in VpLAE and n-BF. V. polyanthes leaves presented a protective effect against DXR cytogenotoxicity. In general, VpLAE and n-BF showed a greater antigenotoxic potential in the pre- and post-treatments. The metabolites putatively identified in VpLAE and n-BF exhibited antioxidant and chemoprotective potential according to computational prediction analysis. Altogether, our results highlight the potential application of V. polyanthes to protect against toxic manifestations induced by DXR.

Possibilities to group nanomaterials across different substances - A case study on organic pigments

Grouping concepts to reduce the testing of NFs have been developed for regulatory purposes for different forms of the same substance. Here we explore possibilities to group nanomaterials across different substances for non-regulatory applications, using the example of 16 organic pigments from six chemical classes. Organic pigments are particles consisting of low-molar-mass organic molecules, and rank by tonnage among the most important substances manufactured in nanoform (NF). Tiered testing strategies relevant to the inhalation route included Tier 1 (deposition, dissolution, reactivity, inflammation) and if available Tier 3 data (in vivo). A similarity assessment of the pigment NF data was conducted in a quantitative (Tier 1 and Tier 3 in vivo potency) or qualitative (Tier 3 in vivo effects) manner. We observed that chemical similarity of organic pigments was predictive for their similarity of reactivity and dissolution, but that additional NF descriptors such as surface area or size, modulate the similarity in inflammation or cytotoxicity. We applied the concept of biologically relevant ranges to crop the values of the Tier 1 data matrix before applying similarity algorithms. The Tier 3 assessment by in vivo inhalation confirmed the IATA methodical choices and IATA assessment criteria as consistent and conservative. We suggested limits of acceptable similarity for Tier 1 data and demonstrated their application to support the grouping of some candidate NFs (subsequently confirmed by Tier 3 data). Four candidate NFs exceeded the limits of acceptability for Tier 1 and were escalated from Tier 1 to Tier 3, but were then included in the group, demonstrating the conservative Tier 1 criteria. The resulting group of low-solubility, low-reactivity materials included both NFs and non-NFs of various substances, and could find use for risk management purposes in the occupational handling of pigment powders.