Cutting-edge computational chemical exposure research at the U.S. Environmental Protection Agency

Exposure science is evolving from its traditional "after the fact" and "one chemical at a time" approach to forecasting chemical exposures rapidly enough to keep pace with the constantly expanding landscape of chemicals and exposures. In this article, we provide an overview of the approaches, accomplishments, and plans for advancing computational exposure science within the U.S. Environmental Protection Agency's Office of Research and Development (EPA/ORD). First, to characterize the universe of chemicals in commerce and the environment, a carefully curated, web-accessible chemical resource has been created. This DSSTox database unambiguously identifies >1.2 million unique substances reflecting potential environmental and human exposures and includes computationally accessible links to each compound's corresponding data resources. Next, EPA is developing, applying, and evaluating predictive exposure models. These models increasingly rely on data, computational tools like quantitative structure activity relationship (QSAR) models, and machine learning/artificial intelligence to provide timely and efficient prediction of chemical exposure (and associated uncertainty) for thousands of chemicals at a time. Integral to this modeling effort, EPA is developing data resources across the exposure continuum that includes application of high-resolution mass spectrometry (HRMS) non-targeted analysis (NTA) methods providing measurement capability at scale with the number of chemicals in commerce. These research efforts are integrated and well-tailored to support population exposure assessment to prioritize chemicals for exposure as a critical input to risk management. In addition, the exposure forecasts will allow a wide variety of stakeholders to explore sustainable initiatives like green chemistry to achieve economic, social, and environmental prosperity and protection of future generations.

Computational estimates of daily aggregate exposure to PFOA/PFOS from 2011 to 2017 using a basic intake model

BACKGROUND

Human exposure to per- and polyfluoroalkyl substances has been modeled to estimate serum concentrations. Given that the production and use of these compounds have decreased in recent years, especially PFOA and PFOS, and that additional concentration data have become available from the US and other industrialized countries over the past decade, aggregate median intakes of these two compounds were estimated using more recent data.

METHODS

Summary statistics from secondary sources were collected, averaged, and mapped for indoor and outdoor air, water, dust, and soil for PFOA and PFOS to estimate exposures for adults and children. European dietary intake estimates were used to estimate daily intake from food.

RESULTS

In accordance with decreased concentrations in media, daily intake estimates among adults, i.e., 40 ng/day PFOA and 40 ng/day PFOS, are substantially lower than those reported previously, as are children's estimates of 14 ng/day PFOA and 17 ng/day PFOS. Using a first-order pharmacokinetic model, these results compare favorably to the National Health and Nutrition Examination Survey serum concentration measurements.

CONCLUSION

Concomitant blood concentrations support this enhanced estimation approach that captures the decline of PFOA/PFOS serum concentration over a decade.

Exposure Prioritization (Ex Priori): A Screening-Level High-Throughput Chemical Prioritization Tool

To estimate potential chemical risk, tools are needed to prioritize potential exposures for chemicals with minimal data. Consumer product exposures are a key pathway, and variability in consumer use patterns is an important factor. We designed Ex Priori, a flexible dashboard-type screening-level exposure model, to rapidly visualize exposure rankings from consumer product use. Ex Priori is Excel-based. Currently, it is parameterized for seven routes of exposure for 1108 chemicals present in 228 consumer product types. It includes toxicokinetics considerations to estimate body burden. It includes a simple framework for rapid modeling of broad changes in consumer use patterns by product category. Ex Priori rapidly models changes in consumer user patterns during the COVID-19 pandemic and instantly shows resulting changes in chemical exposure rankings by body burden. Sensitivity analysis indicates that the model is sensitive to the air emissions rate of chemicals from products. Ex Priori's simple dashboard facilitates dynamic exploration of the effects of varying consumer product use patterns on prioritization of chemicals based on potential exposures. Ex Priori can be a useful modeling and visualization tool to both novice and experienced exposure modelers and complement more computationally intensive population-based exposure models.

Integrating Personal Air Sensor and GPS to Determine Microenvironment-Specific Exposures to Volatile Organic Compounds

Personal exposure to volatile organic compounds (VOCs) from indoor sources including consumer products is an understudied public health concern. To develop and evaluate methods for monitoring personal VOC exposures, we performed a pilot study and examined time-resolved sensor-based measurements of geocoded total VOC (TVOC) exposures across individuals and microenvironments (MEs). We integrated continuous (1 min) data from a personal TVOC sensor and a global positioning system (GPS) logger, with a GPS-based ME classification model, to determine TVOC exposures in four MEs, including indoors at home (Home-In), indoors at other buildings (Other-In), inside vehicles (In-Vehicle), and outdoors (Out), across 45 participant-days for five participants. To help identify places with large emission sources, we identified high-exposure events (HEEs; TVOC > 500 ppb) using geocoded TVOC time-course data overlaid on Google Earth maps. Across the 45 participant-days, the MEs ranked from highest to lowest median TVOC were: Home-In (165 ppb), Other-In (86 ppb), In-Vehicle (52 ppb), and Out (46 ppb). For the two participants living in single-family houses with attached garages, the median exposures for Home-In were substantially higher (209, 416 ppb) than the three participant homes without attached garages: one living in a single-family house (129 ppb), and two living in apartments (38, 60 ppb). The daily average Home-In exposures exceeded the estimated Leadership in Energy and Environmental Design (LEED) building guideline of 108 ppb for 60% of the participant-days. We identified 94 HEEs across all participant-days, and 67% of the corresponding peak levels exceeded 1000 ppb. The MEs ranked from the highest to the lowest number of HEEs were: Home-In (60), Other-In (13), In-Vehicle (12), and Out (9). For Other-In and Out, most HEEs occurred indoors at fast food restaurants and retail stores, and outdoors in parking lots, respectively. For Home-In HEEs, the median TVOC emission and removal rates were 5.4 g h^-1 and 1.1 h^-1, respectively. Our study demonstrates the ability to determine individual sensor-based time-resolved TVOC exposures in different MEs, in support of identifying potential sources and exposure factors that can inform exposure mitigation strategies.

Enhancing life cycle chemical exposure assessment through ontology modeling

In its 2014 report, A Framework Guide for the Selection of Chemical Alternatives, the National Academy of Sciences placed increased emphasis on comparative exposure assessment throughout the life cycle (i.e., from manufacturing to end-of-life) of a chemical. The inclusion of the full life cycle greatly increases the data demands for exposure assessments, including both the quantity and type of data. High throughput tools for exposure estimation add to this challenge by requiring rapid accessibility to data. In this work, ontology modeling was used to bridge the domains of exposure modeling and life cycle inventory modeling to facilitate data sharing and integration. The exposure ontology, ExO, is extended to describe human exposure to consumer products, while an inventory modeling ontology, LciO, is formulated to support automated data mining. The core ontology pieces are connected using a bridging ontology and discussed through a theoretical example to demonstrate how data from LCA can be leveraged to support rapid exposure modeling within a life cycle context.

Advances on a Decision Analytic Approach to Exposure-Based Chemical Prioritization

The volume and variety of manufactured chemicals is increasing, although little is known about the risks associated with the frequency and extent of human exposure to most chemicals. The EPA and the recent signing of the Lautenberg Act have both signaled the need for high-throughput methods to characterize and screen chemicals based on exposure potential, such that more comprehensive toxicity research can be informed. Prior work of Mitchell et al. using multicriteria decision analysis tools to prioritize chemicals for further research is enhanced here, resulting in a high-level chemical prioritization tool for risk-based screening. Reliable exposure information is a key gap in currently available engineering analytics to support predictive environmental and health risk assessments. An elicitation with 32 experts informed relative prioritization of risks from chemical properties and human use factors, and the values for each chemical associated with each metric were approximated with data from EPA's CP_CAT database. Three different versions of the model were evaluated using distinct weight profiles, resulting in three different ranked chemical prioritizations with only a small degree of variation across weight profiles. Future work will aim to include greater input from human factors experts and better define qualitative metrics.

Consensus Modeling of Median Chemical Intake for the U.S. Population Based on Predictions of Exposure Pathways

Prioritizing the potential risk posed to human health by chemicals requires tools that can estimate exposure from limited information. In this study, chemical structure and physicochemical properties were used to predict the probability that a chemical might be associated with any of four exposure pathways leading from sources-consumer (near-field), dietary, far-field industrial, and far-field pesticide-to the general population. The balanced accuracies of these source-based exposure pathway models range from 73 to 81%, with the error rate for identifying positive chemicals ranging from 17 to 36%. We then used exposure pathways to organize predictions from 13 different exposure models as well as other predictors of human intake rates. We created a consensus, meta-model using the Systematic Empirical Evaluation of Models framework in which the predictors of exposure were combined by pathway and weighted according to predictive ability for chemical intake rates inferred from human biomonitoring data for 114 chemicals. The consensus model yields an R² of ∼0.8. We extrapolate to predict relevant pathway(s), median intake rate, and credible interval for 479 926 chemicals, mostly with minimal exposure information. This approach identifies 1880 chemicals for which the median population intake rates may exceed 0.1 mg/kg bodyweight/day, while there is 95% confidence that the median intake rate is below 1 μg/kg BW/day for 474572 compounds.

Computational Exposure Science: An Emerging Discipline to Support 21st-Century Risk Assessment

BACKGROUND

Computational exposure science represents a frontier of environmental science that is emerging and quickly evolving.

OBJECTIVES

In this commentary, we define this burgeoning discipline, describe a framework for implementation, and review some key ongoing research elements that are advancing the science with respect to exposure to chemicals in consumer products.

DISCUSSION

The fundamental elements of computational exposure science include the development of reliable, computationally efficient predictive exposure models; the identification, acquisition, and application of data to support and evaluate these models; and generation of improved methods for extrapolating across chemicals. We describe our efforts in each of these areas and provide examples that demonstrate both progress and potential.

CONCLUSIONS

Computational exposure science, linked with comparable efforts in toxicology, is ushering in a new era of risk assessment that greatly expands our ability to evaluate chemical safety and sustainability and to protect public health.

CITATION

Egeghy PP, Sheldon LS, Isaacs KK, Özkaynak H, Goldsmith M-R, Wambaugh JF, Judson RS, Buckley TJ. 2016. Computational exposure science: an emerging discipline to support 21st-century risk assessment. Environ Health Perspect 124:697-702; http://dx.doi.org/10.1289/ehp.1509748.

Flame retardant exposures in California early childhood education environments

Infants and young children spend as much as 50h per week in child care and preschool. Although approximately 13 million children, or 65% of all U.S. children, spend some time each day in early childhood education (ECE) facilities, little information is available about environmental exposures in these environments. We measured flame retardants in air and dust collected from 40 California ECE facilities between May 2010 and May 2011. Low levels of six polybrominated diphenyl ether (PBDE) congeners and four non-PBDE flame retardants were present in air, including two constituents of Firemaster 550 and two tris phosphate compounds [tris (2-chloroethyl) phosphate (TCEP) and tris (1,3-dichloroisopropyl) phosphate (TDCIPP)]. Tris phosphate, Firemaster 550 and PBDE compounds were detected in 100% of the dust samples. BDE47, BDE99, and BDE209 comprised the majority of the PBDE mass measured in dust. The median concentrations of TCEP (319 ng g(-1)) and TDCIPP (2265 ng g(-1)) were similar to or higher than any PBDE congener. Levels of TCEP and TDCIPP in dust were significantly higher in facilities with napping equipment made out of foam (Mann-Whitney p-values<0.05). Child BDE99 dose estimates exceeded the RfD in one facility for children<3 years old. In 51% of facilities, TDCIPP dose estimates for children<6 years old exceeded age-specific "No Significant Risk Levels (NSRLs)" based on California Proposition 65 guidelines for carcinogens. Given the overriding interest in providing safe and healthy environments for young children, additional research is needed to identify strategies to reduce indoor sources of flame retardant chemicals.

Development of a consumer product ingredient database for chemical exposure screening and prioritization

Consumer products are a primary source of chemical exposures, yet little structured information is available on the chemical ingredients of these products and the concentrations at which ingredients are present. To address this data gap, we created a database of chemicals in consumer products using product Material Safety Data Sheets (MSDSs) publicly provided by a large retailer. The resulting database represents 1797 unique chemicals mapped to 8921 consumer products and a hierarchy of 353 consumer product "use categories" within a total of 15 top-level categories. We examine the utility of this database and discuss ways in which it will support (i) exposure screening and prioritization, (ii) generic or framework formulations for several indoor/consumer product exposure modeling initiatives, (iii) candidate chemical selection for monitoring near field exposure from proximal sources, and (iv) as activity tracers or ubiquitous exposure sources using "chemical space" map analyses. Chemicals present at high concentrations and across multiple consumer products and use categories that hold high exposure potential are identified. Our database is publicly available to serve regulators, retailers, manufacturers, and the public for predictive screening of chemicals in new and existing consumer products on the basis of exposure and risk.

A decision analytic approach to exposure-based chemical prioritization

The manufacture of novel synthetic chemicals has increased in volume and variety, but often the environmental and health risks are not fully understood in terms of toxicity and, in particular, exposure. While efforts to assess risks have generally been effective when sufficient data are available, the hazard and exposure data necessary to assess risks adequately are unavailable for the vast majority of chemicals in commerce. The US Environmental Protection Agency has initiated the ExpoCast Program to develop tools for rapid chemical evaluation based on potential for exposure. In this context, a model is presented in which chemicals are evaluated based on inherent chemical properties and behaviorally-based usage characteristics over the chemical's life cycle. These criteria are assessed and integrated within a decision analytic framework, facilitating rapid assessment and prioritization for future targeted testing and systems modeling. A case study outlines the prioritization process using 51 chemicals. The results show a preliminary relative ranking of chemicals based on exposure potential. The strength of this approach is the ability to integrate relevant statistical and mechanistic data with expert judgment, allowing for an initial tier assessment that can further inform targeted testing and risk management strategies.

Incorporating exposure information into the toxicological prioritization index decision support framework

The Toxicological Prioritization Index (ToxPi) decision support framework was previously developed to facilitate incorporation of diverse data to prioritize chemicals based on potential hazard. This ToxPi index was demonstrated by considering results of bioprofiling related to potential for endocrine disruption. However, exposure information is required along with hazard information to prioritize chemicals for further testing. The goal of this analysis is to demonstrate the utility of the ToxPi framework for incorporating exposure information to rank chemicals and improve understanding of key exposure surrogates. The ToxPi tool was applied to common exposure surrogates (i.e., fate parameters, manufacturing volume, and occurrence measurements) and the relationship between resulting rankings and higher-tiered exposure estimates was investigated. As information more directly relevant to human exposure potential is incorporated, relative rank of chemicals changes. Binned ToxPi results are shown to be consistent with chemical priorities based on crude measures of population-level exposure for a limited set of chemicals. However, these bins are not predictive of higher tiered estimates of exposure such as those developed for pesticide registration. Although rankings based on exposure surrogates are used in a variety of contexts, analysis of the relevance of these tools is challenging. The ToxPi framework can be used to gain insight into the factors driving these rankings and aid identification of key exposure metrics. Additional exposure data is required to build confidence in exposure-based chemical prioritization.

Comparison of four probabilistic models (CARES(®), Calendex™, ConsExpo, and SHEDS) to estimate aggregate residential exposures to pesticides

Two deterministic models (US EPA's Office of Pesticide Programs Residential Standard Operating Procedures (OPP Residential SOPs) and Draft Protocol for Measuring Children's Non-Occupational Exposure to Pesticides by all Relevant Pathways (Draft Protocol)) and four probabilistic models (CARES(®), Calendex™, ConsExpo, and SHEDS) were used to estimate aggregate residential exposures to pesticides. The route-specific exposure estimates for young children (2-5 years) generated by each model were compared to evaluate data inputs, algorithms, and underlying assumptions. Three indoor exposure scenarios were considered: crack and crevice, fogger, and flying insect killer. Dermal exposure estimates from the OPP Residential SOPs and the Draft Protocol were 4.75 and 2.37 mg/kg/day (crack and crevice scenario) and 0.73 and 0.36 mg/kg/day (fogger), respectively. The dermal exposure estimates (99th percentile) for the crack and crevice scenario were 16.52, 12.82, 3.57, and 3.30 mg/kg/day for CARES, Calendex, SHEDS, and ConsExpo, respectively. Dermal exposure estimates for the fogger scenario from CARES and Calendex (1.50 and 1.47 mg/kg/day, respectively) were slightly higher than those from SHEDS and ConsExpo (0.74 and 0.55 mg/kg/day, respectively). The ConsExpo derived non-dietary ingestion estimates (99th percentile) under these two scenarios were higher than those from SHEDS, CARES, and Calendex. All models produced extremely low exposure estimates for the flying insect killer scenario. Using similar data inputs, the model estimates by route for these scenarios were consistent and comparable. Most of the models predicted exposures within a factor of 5 at the 50th and 99th percentiles. The differences identified are explained by activity assumptions, input distributions, and exposure algorithms.

Biogeographical analysis of chemical co-occurrence data to identify priorities for mixtures research

A challenge with multiple chemical risk assessment is the need to consider the joint behavior of chemicals in mixtures. To address this need, pharmacologists and toxicologists have developed methods over the years to evaluate and test chemical interaction. In practice, however, testing of chemical interaction more often comprises ad hoc binary combinations and rarely examines higher order combinations. One explanation for this practice is the belief that there are simply too many possible combinations of chemicals to consider. Indeed, under stochastic conditions the possible number of chemical combinations scales geometrically as the pool of chemicals increases. However, the occurrence of chemicals in the environment is determined by factors, economic in part, which favor some chemicals over others. We investigate methods from the field of biogeography, originally developed to study avian species co-occurrence patterns, and adapt these approaches to examine chemical co-occurrence. These methods were applied to a national survey of pesticide residues in 168 child care centers from across the country. Our findings show that pesticide co-occurrence in the child care center was not random but highly structured, leading to the co-occurrence of specific pesticide combinations. Thus, ecological studies of species co-occurrence parallel the issue of chemical co-occurrence at specific locations. Both are driven by processes that introduce structure in the pattern of co-occurrence. We conclude that the biogeographical tools used to determine when this structure occurs in ecological studies are relevant to evaluations of pesticide mixtures for exposure and risk assessment.

The exposure data landscape for manufactured chemicals

The U.S. Environmental Protection Agency is developing chemical screening and prioritization programs to evaluate environmental chemicals for potential risk to human health in a rapid and efficient manner. As part of these efforts, it is important to catalog available information on chemical toxicity and exposure from widely dispersed sources. The main objective of this analysis is to define important aspects of the exposure space and to catalog the available exposure information for chemicals being considered for analysis as part of the U.S. EPA ToxCast™ screening and prioritization program. Publicly available exposure data have been extracted into ACToR (Aggregated Computational Toxicology Resource), which combines information for hundreds of thousands of chemicals from >600 public sources. We use data from ACToR to assess the exposure data landscape for environmental chemicals. Of the roughly 100,000 chemicals that have at least limited toxicity information available, less than one-fifth also have exposure information - and for most of these the information is of limited utility (e.g., production volume). Readily accessible data on concentrations in exposure-related media are only available for a much smaller fraction. Among these, the largest number of chemicals is measured in water with over 1150 unique compounds, followed by 788 substances measured in soil, and 670 in air. These small numbers clearly reflect a focus of resources on those substances previously identified as possibly posing a hazard to human health. Exposure to a much broader number of chemicals will need to be measured in order to fully realize the envisioned goal of using exposure information to guide toxicity testing.

Pilot scale application of a method for the analysis of perfluorinated compounds in surface soils

A growing number of studies now indicate that perfluorinated compounds (PFCs) are globally distributed in the environment. Their widespread distribution and presence in remote locations has led to questions about the importance of atmospheric and oceanic transport. Describing their distribution in surface soils is also an essential but neglected element in developing a comprehensive understanding of their occurrence in the environment. Soils are the critical link between global atmospheric and hydrologic processes where both local and distant contaminants can accumulate and be released into aquatic and terrestrial communities. Because PFC concentrations in soils will influence ground and surface water, wildlife, and crops, methods to accurately measure PFCs in soil are clearly needed. To help answer this need, we developed a method for the analysis of nine perfluorinated carboxylic acids and four perfluorinated sulfonic acids in soil. Samples from six nations (n=10 per nation) were analyzed by LC-MS/MS to demonstrate the method performance parameters and to make preliminary observations about the occurrence of the PFCs in soils in different parts of the world. The resulting method shows acceptable performance characteristics for the target compounds in most soils while documenting the widespread occurrence of PFCs in surface soils.

Simple intake and pharmacokinetic modeling to characterize exposure of Americans to perfluoroctanoic acid, PFOA

Models for assessing intakes of perfluorooctanoic acid, PFOA, are described and applied. One model is based on exposure media concentrations and contact rates. This model is applied to general population exposures for adults and 2-year old children. The other model is a simple one-compartment, first-order pharmacokinetic (PK) model. Parameters for this model include a rate of elimination of PFOA and a blood volume of distribution. The model was applied to data from the National Health and Nutritional Examination Survey, NHANES, to backcalculate intakes. The central tendency intake estimate for adults and children based on exposure media concentrations and contact rates were 70 and 26 ng/day, respectively. The central tendency adult intake derived from NHANES data was 56 and 37 ng/day for males and females, respectively. Variability and uncertainty discussions regarding the intake modeling focus on lack of data on direct exposure to PFOA used in consumer products, precursor compounds, and food. Discussions regarding PK modeling focus on the range of blood measurements in NHANES, the appropriateness of the simple PK model, and the uncertainties associated with model parameters. Using the PK model, the 10th and 95th percentile long-term average adult intakes of PFOA are 15 and 130 ng/day.

Review of pesticide urinary biomarker measurements from selected US EPA children's observational exposure studies

Children are exposed to a wide variety of pesticides originating from both outdoor and indoor sources. Several studies were conducted or funded by the EPA over the past decade to investigate children’s exposure to organophosphate and pyrethroid pesticides and the factors that impact their exposures. Urinary metabolite concentration measurements from these studies are consolidated here to identify trends, spatial and temporal patterns, and areas where further research is required. Namely, concentrations of the metabolites of chlorpyrifos (3,5,6-trichloro-2-pyridinol or TCPy), diazinon (2-isopropyl-6-methyl-4-pyrimidinol or IMP), and permethrin (3-phenoxybenzoic acid or 3-PBA) are presented. Information on the kinetic parameters describing absorption and elimination in humans is also presented to aid in interpretation. Metabolite concentrations varied more dramatically across studies for 3-PBA and IMP than for TCPy, with TCPy concentrations about an order of magnitude higher than the 3-PBA concentrations. Temporal variability was high for all metabolites with urinary 3-PBA concentrations slightly more consistent over time than the TCPy concentrations. Urinary biomarker levels provided only limited evidence of applications. The observed relationships between urinary metabolite levels and estimates of pesticide intake may be affected by differences in the contribution of each exposure route to total intake, which may vary with exposure intensity and across individuals.

Methodologies for estimating cumulative human exposures to current-use pyrethroid pesticides

We estimated cumulative residential pesticide exposures for a group of nine young children (4-6 years) using three different methodologies developed by the US Environmental Protection Agency and compared the results with estimates derived from measured urinary metabolite concentrations. The Standard Operating Procedures (SOPs) for Residential Exposure Assessment are intended to provide a screening-level assessment to estimate exposure for regulatory purposes. Nonetheless, dermal exposure estimates were typically lower from the SOP (1-1300 nmol/day) than from SHEDS (5-19,000 nmol/day) or any of the four different approaches for estimating dermal exposure using the Draft Protocol for Measuring Children's Non-Occupational Exposure to Pesticides by all Relevant Pathways (Draft Protocol) (5-11,000 nmol/day). Indirect ingestion exposure estimates ranged from 0.02 to 21.5 nmol/day for the SOP, 0.5 to 188 nmol/day for SHEDS, and 0 to 3.38 nmol/day for the Draft Protocol. Estimates of total absorbed dose ranged from 3 to 37 nmol/day for the SOPs, 0.5 to 100 nmol/day for SHEDS, and 1 to 216 nmol/day for the Draft Protocol. The concentrations estimated using the Draft Protocol and SHEDS showed strong, positive relationships with the 3-phenoxybenzoic acid metabolite measured in the children's urine samples (R²=0.90 for the Draft Protocol; R²=0.92 for SHEDS). Analysis of different approaches for estimating dermal exposure suggested that the approach assuming an even distribution of pesticide residue on the child's body was most reasonable. With all three methodologies providing reasonable estimates of exposure and dose, selection should depend on the available data and the objectives of the analysis. Further research would be useful to better understand how best to estimate dermal exposure for children and what exposure factors (e.g., activities, transfer coefficients, measurement techniques) are most relevant in making dermal exposure estimates.

An assessment of the exposure of Americans to perfluorooctane sulfonate: a comparison of estimated intake with values inferred from NHANES data

To better understand human exposure to perfluorinated compounds (PFCs), a model that assesses exposure to perfluorooctane sulfonate (PFOS) and its precursors from both an intake and a body burden perspective and combines the two with a simple pharmacokinetic (PK) model is demonstrated. Exposure pathways were modeled under "typical" and "contaminated" scenarios, for young children and adults. A range of intakes was also estimated from serum concentrations of PFOS reported in the National Health and Nutrition Examination Survey (NHANES) using a first-order 1-compartment PK model. Total PFOS intakes (medians summed over all pathways) were estimated as: 160 and 2200 ng/day for adults and 50 and 640 ng/day for children under typical and contaminated scenarios, respectively. Food ingestion appears to be the primary route of exposure in the general population. For children, the contribution from dust ingestion is nearly as great as from food ingestion. Pathway-specific contributions span several orders of magnitude and exhibit considerable overlap. PK modeling suggests central tendency PFOS intakes for adults range between 1.6 and 24.2 ng/kg-bw/day, and the forward-based intake estimates are within this range. The favorable comparison reported between the forward-modeled and the back-calculated range of intake predictions lends validity to the proposed framework.

Organophosphorus and pyrethroid insecticide urinary metabolite concentrations in young children living in a southeastern United States city

Pesticide metabolites are routinely measured in the urine of children in the United States. Although the sources of these metabolites are believed to include residues in food from agricultural applications and residues from applications in everyday environments (e.g., homes), few studies have been able to demonstrate an association between indoor residential pesticide applications and pesticide metabolite concentrations. To better quantify the effects of potential risk factors related to demographics, household characteristics, occupation, and pesticide use practices on urinary biomarker levels, we performed a study in a city (Jacksonville, Florida) previously determined to have elevated rates of pesticide use. We enrolled a convenience sample of 203 children ranging in age from 4 to 6 years; their caregivers completed a questionnaire and the children provided a urine sample, which was analyzed for a series of organophosphorus and pyrethroid insecticide metabolites. The questionnaire responses substantiated much higher pesticide use for the study participants as compared to other studies. Urinary metabolite concentrations were approximately an order of magnitude higher than concentrations reported for young children in other studies. Few statistically significant differences (at the p<0.05 level) were observed, however, several trends are worth noting. In general, mean urinary pesticide metabolite concentrations were higher for males, Caucasians, and those children living in homes with an indoor pesticide application occurring within the past four weeks. Comparing the urinary pesticide metabolite concentrations in this study to those reported in the NHANES and GerES studies showed that the children living in Jacksonville had substantially higher pyrethroid pesticide exposures than the general populations of the United States and Germany. Further research is needed in communities where routine pesticide use has been documented to obtain information on the most important routes and pathways of exposure and to develop the most effective strategies for reducing pesticide exposures for children.

American Healthy Homes Survey: a national study of residential pesticides measured from floor wipes

The U.S. Department of Housing and Urban Development in collaboration with the United States Environmental Protection Agency conducted a survey measuring lead, allergens, and insecticides in a randomly selected nationally representative sample of residential homes. Multistage sampling with clustering was used to select the 1131 homes of which a subset of 500 randomly selected homes included the collection of hard surface floor wipes. Samples were collected by trained field technicians between June 2005 and March 2006 using isopropanol wetted wipes. Samples were analyzed for a suite of 24 compounds which included insecticides in the organochlorine, organophosphate, pyrethroid and phenylpyrazole classes, and the insecticide synergist piperonyl butoxide. The most commonly detected were permethrin (89%), chlorpyrifos (78%), chlordane (64%), piperonyl butoxide (52%), cypermethrin (46%), and fipronil (40%). Mean and geometric mean (GM) concentrations varied widely among compounds, but were highest for trans-permethrin (mean 2.22 ng/cm2 and GM 0.14 ng/ cm2) and cypermethrin (mean 2.9 ng/cm2 and GM 0.03 ng/ cm2). Results show that most floors in occupied homes in the U.S. have measurable levels of insecticides that may serve as sources of exposure to occupants.

Adult and children's exposure to 2,4-D from multiple sources and pathways

In this study, we investigated the 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide exposures of 135 preschool-aged children and their adult caregivers at 135 homes in North Carolina (NC) and Ohio (OH). Participants were randomly recruited from six NC and six OH counties. Monitoring was performed over a 48-h period at the participants' homes. Environmental samples included soil, outdoor air, indoor air, and carpet dust. Personal samples collected by the adult caregivers concerning themselves and their children consisted of solid food, liquid food, hand wipe, and spot urine samples. All samples were analyzed for 2,4-D (free acid form) by gas chromatography/mass spectrometry. 2,4-D was detected in all types of environmental samples but most often in carpet dust samples, with detection frequencies of 83% and 98% in NC and OH, respectively. The median level of 2,4-D in the carpet dust samples was about three times higher in OH homes compared to NC homes (156 vs. 47.5 ng/g, P<0.0002). For personal samples, 2,4-D was more frequently detected in the hand wipe samples from OH participants (>48%) than from NC participants (<9%). Hand wipe levels at the 95th percentile were about five times higher for OH children (0.1 ng/cm(2)) and adults (0.03 ng/cm(2)) than for the NC children (0.02 ng/cm(2)) and adults (<0.005 ng/cm(2)). 2,4-D was detected in more than 85% of the child and adult urine samples in both states. The median urinary 2,4-D concentration was more than twice as high for OH children compared to NC children (1.2 vs. 0.5 ng/ml, P<0.0001); however, the median concentration was identical at 0.7 ng/ml for both NC and OH adults. The intraclass correlation coefficient of reliability for an individual's urinary 2,4-D measurements, estimated from the unadjusted (0.31-0.62) and specific gravity-adjusted (0.37-0.73) values, were somewhat low for each group in this study. The variability in urinary 2,4-D measurements over the 48-h period for both children and adults in NC and OH suggests that several spot samples were needed to adequately assess these participants' exposures to 2,4-D in residential settings. Results from this study showed that children and their adult caregivers in NC and OH were likely exposed to 2,4-D through several pathways at their homes. In addition, our findings suggest that the OH children might have been exposed to higher levels of 2,4-D through the dermal and nondietary routes of exposure than the NC children and the NC and OH adults.

Relationships between levels of volatile organic compounds in air and blood from the general population

The relationships between levels of volatile organic compounds (VOCs) in blood and air have not been well characterized in the general population where exposure concentrations are generally at parts per billion levels. This study investigates relationships between the levels of nine VOCs, namely, benzene, chloroform, 1,4-dichlorobenzene, ethylbenzene, methyl tert-butyl ether (MTBE), tetrachloroethene, toluene, and m-/p- and o-xylene, in blood and air from a stratified random sample of the general US population. We used data collected from 354 participants, including 89 smokers and 265 nonsmokers, aged 20-59 years, who provided samples of blood and air in the National Health and Nutrition Examination Survey (NHANES) 1999-2000. Demographic and physiological characteristics were obtained from self-reported information; smoking status was determined from levels of serum cotinine. Multiple linear regression models were used to investigate the relationships between VOC levels in air and blood, while adjusting for effects of smoking and demographic factors. Although levels of VOCs in blood were positively correlated with the corresponding air levels, the strength of association (R(2)) varied from 0.02 (ethylbenzene) to 0.68 (1,4-DCB). Also the blood-air relationships of benzene, toluene, ethylbenzene, and the xylenes (BTEX) were influenced by smoking, exposure-smoking interactions, and by gender, age, and BMI, whereas those of the other VOCs were not. Interestingly, the particular exposure-smoking interaction for benzene was different from those for toluene, ethylbenzene, and the xylenes. Whereas smokers retained more benzene in their blood at increasing exposure levels, they retained less toluene, ethylbenzene, and xylenes at increasing exposure levels. Investigators should consider interaction effects of exposure levels and smoking when exploring the blood-air relationships of the BTEX compounds in the general population.

Comparing surface residue transfer efficiencies to hands using polar and nonpolar fluorescent tracers

Transfer of chemicals from contaminated surfaces such as foliage, floors, and furniture is a potentially significant source of both occupational exposure and children's residential exposure. Increased understanding of relevant factors influencing transfers from contaminated surfaces to skin and resulting dermal-loading will reduce uncertainty in exposure assessment. In a previously reported study, a fluorescence imaging system was developed, tested, and used to measure transfer of riboflavin residues from surfaces to hands. Parameters evaluated included surface type, surface loading, contact motion, pressure, duration, and skin condition. Results of the initial study indicated that contact duration and pressure were not significant for the range of values tested, but that there are potentially significant differences in transfer efficiencies of different compounds. In the study reported here, experimental methods were refined and additional transfer data were collected. A second fluorescent tracer, Uvitex OB, with very different physicochemical properties than riboflavin, was also evaluated to better characterize the range of transfers that may be expected for a variety of compounds. Fluorescent tracers were applied individually to surfaces and transfers to skin were measured after repeated hand contacts with the surface. Additional trials were conducted to compare transfer of tracers and co-applied pesticide residues. Results of this study indicate that dermal loadings of both tracers increase through the seventh brief contact. Dermal loading of Uvitex tends to increase at a higher rate than dermal loadings of riboflavin. Measurement of co-applied tracer and pesticide suggest results for these two tracers may provide reasonable bounding estimates of pesticide transfer.

Multimedia measurements and activity patterns in an observational pilot study of nine young children

A pilot observational exposure study was performed to evaluate methods for collecting multimedia measurements (air, dust, food, urine) and activity patterns to assess potential exposures of young children to pesticides in their homes. Nine children (mean age=5 years) and their caregivers participated in this study, performed in the Duval County, Florida, in collaboration with the Centers for Disease Control and Prevention and the Duval County Health Department. For all nine children, the total time reported for sleeping and napping ranged from 9.5 to 14 h per day, indoor quiet time from 0 to 5.5 h per day, indoor active time from 0.75 to 5.5 h per day, outdoor quiet time from 0 to 1.5 h per day, and outdoor active time from 0.5 to 6.5 h per day. Each home had one to three pesticide products present, with aerosols being most common. Pesticide inventories, however, were not useful for predicting pesticide levels in the home. Synthetic pyrethroids were the most frequently identified active ingredients in the products present in each home. Fifteen pesticide active ingredients were measured in the application area wipes (not detected (ND) to 580 ng/cm(2)), 13 in the play area wipes (ND-117 ng/cm(2)), and 14 in the indoor air samples (ND-378 ng/m(3)) and the socks (ND-1000 ng/cm(2)). Cis-permethrin, trans-permethrin, and cypermethrin were measured in all nine homes. Chlorpyrifos was measured in all nine homes even though it was not reported used by the participants. All urine samples contained measurable concentrations of 3-phenoxybenzoic acid (3-PBA). The median 3-PBA urinary concentration for the nine children was 2.2 mug/l. A wide variety of pesticide active ingredients were measured in these nine homes at median concentrations that were often higher than reported previously in similar studies. These data highlight the need for additional observational studies in regions where pesticides are used in order to understand the factors that affect young children's exposures and the education/mitigation strategies that can be used to reduce children's exposures.

PBTK modeling demonstrates contribution of dermal and inhalation exposure components to end-exhaled breath concentrations of naphthalene

BACKGROUND

Dermal and inhalation exposure to jet propulsion fuel 8 (JP-8) have been measured in a few occupational exposure studies. However, a quantitative understanding of the relationship between external exposures and end-exhaled air concentrations has not been described for occupational and environmental exposure scenarios.

OBJECTIVE

Our goal was to construct a physiologically based toxicokinetic (PBTK) model that quantitatively describes the relative contribution of dermal and inhalation exposures to the end-exhaled air concentrations of naphthalene among U.S. Air Force personnel.

METHODS

The PBTK model comprised five compartments representing the stratum corneum, viable epidermis, blood, fat, and other tissues. The parameters were optimized using exclusively human exposure and biological monitoring data.

RESULTS

The optimized values of parameters for naphthalene were a) permeability coefficient for the stratum corneum 6.8 x 10(-5) cm/hr, b) permeability coefficient for the viable epidermis 3.0 x 10(-3) cm/hr, c) fat:blood partition coefficient 25.6, and d) other tissue:blood partition coefficient 5.2. The skin permeability coefficient was comparable to the values estimated from in vitro studies. Based on simulations of workers' exposures to JP-8 during aircraft fuel-cell maintenance operations, the median relative contribution of dermal exposure to the end-exhaled breath concentration of naphthalene was 4% (10th percentile 1% and 90th percentile 11%).

CONCLUSIONS

PBTK modeling allowed contributions of the end-exhaled air concentration of naphthalene to be partitioned between dermal and inhalation routes of exposure. Further study of inter- and intraindividual variations in exposure assessment is required to better characterize the toxicokinetic behavior of JP-8 components after occupational and/or environmental exposures.

Dermal exposure to jet fuel JP-8 significantly contributes to the production of urinary naphthols in fuel-cell maintenance workers

Jet propulsion fuel 8 (JP-8) is the major jet fuel used worldwide and has been recognized as a major source of chemical exposure, both inhalation and dermal, for fuel-cell maintenance workers. We investigated the contributions of dermal and inhalation exposure to JP-8 to the total body dose of U.S. Air Force fuel-cell maintenance workers using naphthalene as a surrogate for JP-8 exposure. Dermal, breathing zone, and exhaled breath measurements of naphthalene were obtained using tape-strip sampling, passive monitoring, and glass bulbs, respectively. Levels of urinary 1- and 2-naphthols were determined in urine samples and used as biomarkers of JP-8 exposure. Multiple linear regression analyses were conducted to investigate the relative contributions of dermal and inhalation exposure to JP-8, and demographic and work-related covariates, to the levels of urinary naphthols. Our results show that both inhalation exposure and smoking significantly contributed to urinary 1-naphthol levels. The contribution of dermal exposure was significantly associated with levels of urinary 2-naphthol but not with urinary 1-naphthol among fuel-cell maintenance workers who wore supplied-air respirators. We conclude that dermal exposure to JP-8 significantly contributes to the systemic dose and affects the levels of urinary naphthalene metabolites. Future work on dermal xenobiotic metabolism and toxicokinetic studies are warranted in order to gain additional knowledge on naphthalene metabolism in the skin and the contribution to systemic exposure.

Measuring potential dermal transfer of a pesticide to children in a child care center

Currently, the major determinants of children's exposure to pesticides are not fully understood, and approaches for measuring and assessing dermal exposure in a residential setting have not been sufficiently evaluated. In one approach, dermal exposure is estimated using empirically derived transfer coefficients. To assess the feasibility of using this approach for assessing children's exposure to pesticides, we conducted a study was conducted in a child care center that had a preexisting contract with a pest control service for regular monthly pesticide applications. Children in the selected child care center were monitored using full-body cotton garments to measure dermal loading. Pesticide residues on classroom surfaces were measured in the areas where the children spent time. Measured surface-wipe loadings ranged from 0.47 to 120 ng/cm2, and total garment loadings ranged from 0.5 to 660 pg/cm2. The garment and surface loading measurements were used to calculate dermal-transfer coefficients for use in assessing children's residential exposure to pesticides. Dermal-transfer coefficients calculated using these data range from approximately 10 to 6,000 cm2/hr. The wide range in these values demonstrates the importance of developing standard surface-measurement protocols if this approach is to be used to assess dermal exposure in a residential environment. The upper-range values resulting from this study were found to be similar to the default value used by the U.S. Environmental Protection Agency to assess children's dermal exposures resulting from contact with indoor surfaces.

Determinants of temporal variability in NHEXAS-Maryland environmental concentrations, exposures, and biomarkers

The longitudinal NHEXAS-Maryland study measured metals, PAHs, and pesticides in several media to capture temporal variability. Questionnaires were concurrently administered to identify factors that influenced changes in contaminant levels over time. We constructed mixed-effects regression models for lead, phenanthrene, and chlorpyrifos (including metabolites) in indoor air, dust, dermal wipes, and biological fluids. Significant predictors represented time-varying activities as well as unchanging housing and demographic factors. There was little overlap among the models, with predictors generally reflecting the diverse characteristics of the target compounds. We estimated between- and within-person variance components to evaluate the reliability of the measurements. While only one measurement of lead in blood or chlopyrifos in dust was needed for a dependable estimate of an individual's average level, three to eight measurements were needed for most other compound/exposure medium combinations because of considerable temporal variability. Measurements in biological fluids and dust were generally more consistent than those in indoor air. The significant covariates in the full models preferentially reduced the between-person variance component. Since the regression models explained only 1-37% of the within-person variance, the questionnaires in this study provided only modest insight into the factors responsible for the temporal variability in the contaminant levels.

Dose-dependent production of urinary naphthols among workers exposed to jet fuel (JP-8)

BACKGROUND

Jet propulsion fuel-8 (JP-8) is one of the largest sources of chemical exposures among Air Force personnel. Urinary naphthols have been suggested as useful biomarkers of exposure to JP-8.

METHODS

Multivariate linear regression models were applied to evaluate the effects of environmental and work-related factors upon production of urinary naphthols among 323 Air Force personnel.

RESULTS

Naphthalene exposure, smoking status, and their interaction, plus self-reported skin irritation explained about two-thirds of the variation in naphthol levels. The exposure-smoking interaction was consistent with induction by smoking of one or more steps in the metabolism of naphthalene and naphthalene-1,2-oxide (NapO). A supralinear dose-response relationship was observed between urinary naphthols and naphthalene exposure.

CONCLUSIONS

Urinary naphthols were associated with specific sources of exposure to JP-8, arising from both inhalation and dermal contact. Smokers and nonsmokers metabolized naphthalene at different rates, consistent with induction of at least two metabolic pathways by smoking.

Benzene and naphthalene in air and breath as indicators of exposure to jet fuel

AIMS

To estimate exposures to benzene and naphthalene among military personnel working with jet fuel (JP-8) and to determine whether naphthalene might serve as a surrogate for JP-8 in studies of health effects.

METHODS

Benzene and naphthalene were measured in air and breath of 326 personnel in the US Air Force, who had been assigned a priori into low, moderate, and high exposure categories for JP-8.

RESULTS

Median air concentrations for persons in the low, moderate, and high exposure categories were 3.1, 7.4, and 252 microg benzene/m3 air, 4.6, 9.0, and 11.4 microg benzene/m3 breath, 1.9, 10.3, and 485 microg naphthalene/m3 air, and 0.73, 0.93, and 1.83 microg naphthalene/m3 breath, respectively. In the moderate and high exposure categories, 5% and 15% of the benzene air concentrations, respectively, were above the 2002 threshold limit value (TLV) of 1.6 mg/m3. Multiple regression analyses of air and breath levels revealed prominent background sources of benzene exposure, including cigarette smoke. However, naphthalene exposure was not unduly influenced by sources other than JP-8. Among heavily exposed workers, dermal contact with JP-8 contributed to air and breath concentrations along with several physical and environmental factors.

CONCLUSIONS

Personnel having regular contact with JP-8 are occasionally exposed to benzene at levels above the current TLV. Among heavily exposed workers, uptake of JP-8 components occurs via both inhalation and dermal contact. Naphthalene in air and breath can serve as useful measures of exposure to JP-8 and uptake of fuel components in the body.

Urinary biomarkers of exposure to jet fuel (JP-8)

Benzene, naphthalene, and 1- and 2-naphthol were measured in urine samples obtained from 322 U.S. Air Force personnel categorized a priori as likely to have low, moderate, or high exposure to jet fuel [jet propulsion fuel-8 (JP-8)]. In postexposure samples, levels of these analytes in the high-exposure group were 3- to 29-fold greater than in the low-exposure group and 2- to 12-fold greater than in the moderate-exposure group. Heavy exposure to JP-8 contributed roughly the same amount of benzene and more than three times the amount of naphthalene compared with cigarette smoking. Strong correlations were observed among postexposure levels of naphthalene-based biomarkers in urine and naphthalene in air and breath. We conclude that urinary naphthalene and the naphthols can serve as biomarkers of exposure to jet fuel. Of these, the naphthols are probably more useful because of their greater abundance and slower elimination kinetics.

Self-collected breath sampling for monitoring low-level benzene exposures among automobile mechanics

Automobile mechanics are exposed to benzene through their contact with gasoline vapor and engine exhaust. This study investigated the benzene uptake associated with these exposures. We first evaluated the reliability of self-collected breath samples among a subset of subjects and found good agreement between these samples and those collected under expert supervision (intraclass correlation coefficient 0.79, n = 69). We then used self-monitoring together with a longitudinal sampling design (with up to three measurements per worker) to measure benzene in air and benzene in end-exhaled breath among 81 workers from 12 automobile repair garages in North Carolina. A statistically significant difference (P < 0.0001, Mann-Whitney rank sum test) was observed between non-smokers and smokers for post-exposure benzene concentration in breath (median values of 18.9 and 39.1 micro g/m(3), respectively). Comparing pre- and post-exposure breath concentrations within these two groups, the difference was significant among non-smokers (P < 0.0001) but not significant among smokers (P > 0.05). Mixed effects regression analysis using backwards elimination yielded five significant predictors of benzene concentration in breath, namely benzene exposure (P < 0.0001), pre-exposure benzene concentration in breath (P = 0.021), smoking status (P < 0.0001), fuel system work (P = 0.0043) and carburetor cleaner use (P < 0.0001). The between-person variance component comprised only 28% of the total variance in benzene levels in breath, indicating that differences among individuals related to physiological and metabolic characteristics had little influence on benzene uptake among these workers.

Environmental and biological monitoring of benzene during self-service automobile refueling

Although automobile refueling represents the major source of benzene exposure among the nonsmoking public, few data are available regarding such exposures and the associated uptake of benzene. We repeatedly measured benzene exposure and uptake (via benzene in exhaled breath) among 39 self-service customers using self-administered monitoring, a technique rarely used to obtain measurements from the general public (130 sets of measurements were obtained). Benzene exposures averaged 2.9 mg/m(3) (SD = 5.8 mg/m(3); median duration = 3 min) with a range of < 0.076-36 mg/m(3), and postexposure breath levels averaged 160 microg/m(3) (SD = 260 microg/m(3)) with a range of < 3.2-1,400 microg/m(3). Log-transformed exposures and breath levels were significantly correlated (r = 0.77, p < 0.0001). We used mixed-effects statistical models to gauge the relative influences of environmental and subject-specific factors on benzene exposure and breath levels and to investigate the importance of various covariates obtained by questionnaire. Model fitting yielded three significant predictors of benzene exposure, namely, fuel octane grade (p = 0.0011), duration of exposure (p = 0.0054), and season of the year (p = 0.032). Likewise, another model yielded three significant predictors of benzene concentration in breath, specifically, benzene exposure (p = 0.0001), preexposure breath concentration (p = 0.0008), and duration of exposure (p = 0.038). Variability in benzene concentrations was remarkable, with 95% of the estimated values falling within a 274-fold range, and was comprised entirely of the within-person component of variance (representing exposures of the same subject at different times of refueling). The corresponding range for benzene concentrations in breath was 41-fold and was comprised primarily of the within-person variance component (74% of the total variance). Our results indicate that environmental rather than interindividual differences are primarily responsible for benzene exposure and uptake during automobile refueling. The study also demonstrates that self-administered monitoring can be efficiently used to measure environmental exposures and biomarkers among the general public.