A Novel Approach to Integrate Human Biomonitoring Data with Model Predicted Dietary Exposures: A Crop Protection Chemical Case Study Using Lambda-Cyhalothrin.

The appropriate use of human biomonitoring data to model population chemical exposures is challenging, especially for rapidly metabolized chemicals, such as agricultural chemicals. The objective of this study is to demonstrate a novel approach integrating model predicted dietary exposures and biomonitoring data to potentially inform regulatory risk assessments. We use lambda-cyhalothrin as a case study, and for the same representative U.S. population in the National Health and Nutrition Examination Survey (NHANES), an integrated exposure and pharmacokinetic model predicted exposures are calibrated to measurements of the urinary metabolite 3-phenoxybenzoic acid (3PBA), using an approximate Bayesian computing (ABC) methodology. We demonstrate that the correlation between modeled urinary 3PBA and the NHANES 3PBA measurements more than doubled as ABC thresholding narrowed the acceptable tolerance range for predicted versus observed urinary measurements. The median predicted urinary concentrations were closer to the median measured value using ABC than using current regulatory Monte Carlo methods.

Developing prototypes of a modernized approach to assess crop protection chemical safety.

In 2019, the US EPA Administrator issued a directive directing the agency away from reliance on vertebrate tests by 2035, whilst maintaining high-quality human health and environmental risk assessments. There is no accepted approach to achieve this. The decade-long duration of the crop protection (CP) chemical R&D process therefore requires both the invention and application of a modernized approach to those CP chemical projects entering corporate research portfolios by the mid-2020s. We conducted problem formulation discussions with regulatory agency scientists which created the problem statement: "Develop, demonstrate, and implement a modern scientifically sound and robust strategy that applies appropriate and flexible exposure and effects characterization without chemical specific vertebrate tests to reliably address risk, uncertainties, and deficiencies in data and its interpretation with equivalent confidence as do the currently accepted test guidelines and meet the regulatory needs of the agencies". The solution must provide the knowledge needed to confidently conclude human health and environmental protective risk assessments. Exploring this led to a conceptual model involving the creation and parallel submission of a new approach without reliance on chemical-specific vertebrate tests. Assessment in parallel to a traditional package will determine whether it supports some, or all, of the necessary risk management actions. Analysis of any deficiencies will provide valuable feedback to focus development of tools or approaches for subsequent iterations. When found to provide sufficient information, it will form the technical foun­dation of stakeholder engagement to explore acceptance of a new approach to CP chemical risk assessment.

The US EPA, and other regulatory agencies, aim to reduce the use of vertebrate animal tests for assessing risks of crop protection chemicals. There is currently no accepted way to do this. We outline a proposal to perform both the assessment using traditional vertebrate testing and a set of new non-animal methods. These data sets must each be combined with a calculated estimate of user exposure to the pesticide based on its intended use. Comparing the outcome of these two assess­ments will show whether the set of non-animal methods needs to be improved further. When the new approach appears to reliably predict the risks, the different stakeholders must be brought together to assess whether the non-animal methods package is acceptable and can replace the tests on vertebrate animals while maintaining the same level of protection of human health and the environment.

Application of a new approach method (NAM) for inhalation risk assessment.

The US Environmental Protection Agency (USEPA) and other regulatory authorities have been working to utilize in vitro studies with human cells and in silico modelling to reduce the use of vertebrate animals for evaluating chemical risk. Using the Source-to-Outcome framework, a novel mathematical procedure was developed to estimate the human equivalent concentration (HEC) for inhalation risk assessment based upon the relevant aerosol characterization, respiratory dosimetry modelling, and endpoints derived from an in vitro assay using human respiratory epithelial tissue. The procedure used the retained doses at the various areas of the inhalation tract estimated from a computational fluid-particle dynamics (CFPD) model coupled with a simple clearance model. The effect of exposure was derived from an in vitro assay. The magnitude of exposure and the particle size distributions (PSDs) of the external aerosol droplets were obtained from Unit Exposure values published by the USEPA and published monitoring studies, respectively. The Source-to-Outcome approach incorporates external and internal exposure metrics with the toxicity pathway. The information was then integrated to conduct a risk assessment for agricultural operators exposed to products containing chlorothalonil (CTN), a broad-spectrum fungicide. The HECs for three different PSDs considered in this work ranged from 0.043 to 0.112 mg-CTN/L for nasal and oral breathing. These were compared with the estimated average daily exposure concentration for six representative application scenarios. The resulting margins of exposure (MOEs) ranged from 230 to 70,000 depending on the application scenario. This New Assessment Method (NAM) that combined human in silico and human in vitro methods, eliminated the typical uncertainties associated with extrapolation from rodent studies, with their associated interspecies toxicokinetics and toxicodynamics differences. The intraspecies toxicodynamics and toxicokinetics, are still relevant and may need to be used in an inhalation risk assessment. The NAM presented in this work is not chemical-specific and may be applied to conduct an inhalation risk assessment for workers as well as bystanders who could be exposed to aerosol particles of any cytotoxic respiratory irritant.

Incorporating human exposure information in a weight of evidence approach to inform design of repeated dose animal studies.

Human health risks from chronic exposures to environmental chemicals are typically estimated from potential human exposure estimates and dose-response data obtained from repeated-dose animal toxicity studies. Various criteria are available for selecting the top (highest) dose used in these animal studies. For example, toxicokinetic (TK) and toxicological data provided by shorter-term or dose range finding studies can be evaluated in a weight of evidence approach to provide insight into the dose range that would provide dose-response data that are relevant to human exposures. However, there are concerns that a top dose resulting from the consideration of TK data may be too low compared to other criteria, such as the limit dose or the maximum tolerated dose. In this paper, we address several concerns related to human exposures by discussing 1) the resources and methods available to predict human exposure levels and the associated uncertainty and variability, and 2) the margin between predicted human exposure levels and the dose levels used in repeated-dose animal studies. A series of case studies, ranging from data-rich to data-poor chemicals, are presented to demonstrate that expected human exposures to environmental chemicals are typically orders of magnitude lower than no-observed-adverse-effect levels/lowest-observed-adverse-effect levels (NOAELs/LOAELs) when available (used as conservative surrogates for top doses). The results of these case studies support that a top dose based, in part, on TK data is typically orders of magnitude higher than expected human exposure levels.

Co-occurrence of Conazole Fungicide Residues in Raw Agricultural Commodities Sampled by the United States Department of Agriculture Pesticide Data Program.

In this study, the residue data for conazole fungicides were collated and analyzed in all crop samples reported by the United States Department of Agriculture Pesticide Data Program over the period of 2009-2019. Considering all individual samples, the overall detection frequencies (DFs) of conazoles are less than 13%. Among the 18 conazoles, imazalil had the highest overall DF of 6%, followed by tebuconazole and myclobutanil, with 4% each. Conazoles were detected most frequently in raisins with 28% DF, followed by cherries (frozen and fresh) and grapes, with 12, 10, and 10%, respectively. The presence of multiple conazoles in single commodity samples is very low, below 2%. The analyses found no more than four conazoles present in any given sample. Out of the 18 conazoles, 8 of them were not detected in more than 99.9% of the commodity samples from 2009 to 2019 and, therefore, can be eliminated from screening-level cumulative risk assessment for dietary contributions from food items. While conazoles are widely used on food commodities, co-occurrence of conazole residues was observed only in a very limited number of food commodities, including raisins, grapes, cherries (frozen), nectarines, and peaches. Considering the remaining individual food commodities, the co-occurrence of conazole residues in single commodity samples is very low or not even present.

Opportunities and challenges related to saturation of toxicokinetic processes: Implications for risk assessment.

Top dose selection for repeated dose animal studies has generally focused on identification of apical endpoints, use of the limit dose, or determination of a maximum tolerated dose (MTD). The intent is to optimize the ability of toxicity tests performed in a small number of animals to detect effects for hazard identification. An alternative approach, the kinetically derived maximum dose (KMD), has been proposed as a mechanism to integrate toxicokinetic (TK) data into the dose selection process. The approach refers to the dose above which the systemic exposures depart from being proportional to external doses. This non-linear external-internal dose relationship arises from saturation or limitation of TK process(es), such as absorption or metabolism. The importance of TK information is widely acknowledged when assessing human health risks arising from exposures to environmental chemicals, as TK determines the amount of chemical at potential sites of toxicological responses. However, there have been differing opinions and interpretations within the scientific and regulatory communities related to the validity and application of the KMD concept. A multi-stakeholder working group, led by the Health and Environmental Sciences Institute (HESI), was formed to provide an opportunity for impacted stakeholders to address commonly raised scientific and technical issues related to this topic and, more specifically, a weight of evidence approach is recommended to inform design and dose selection for repeated dose animal studies. Commonly raised challenges related to the use of TK data for dose selection are discussed, recommendations are provided, and illustrative case examples are provided to address these challenges or refute misconceptions.

New Approach Methodology for Assessing Inhalation Risks of a Contact Respiratory Cytotoxicant: Computational Fluid Dynamics-Based Aerosol Dosimetry Modeling for Cross-Species and In Vitro Comparisons.

Regulatory agencies are considering alternative approaches to assessing inhalation toxicity that utilizes in vitro studies with human cells and in silico modeling in lieu of additional animal studies. In support of this goal, computational fluid-particle dynamics models were developed to estimate site-specific deposition of inhaled aerosols containing the fungicide, chlorothalonil, in the rat and human for comparisons to prior rat inhalation studies and new human in vitro studies. Under bioassay conditions, the deposition was predicted to be greatest at the front of the rat nose followed by the anterior transitional epithelium and larynx corresponding to regions most sensitive to local contact irritation and cytotoxicity. For humans, simulations of aerosol deposition covering potential occupational or residential exposures (1-50 µm diameter) were conducted using nasal and oral breathing. Aerosols in the 1-5 µm range readily penetrated the deep region of the human lung following both oral and nasal breathing. Under actual use conditions (aerosol formulations >10 µm), the majority of deposited doses were in the upper conducting airways. Beyond the nose or mouth, the greatest deposition in the pharynx, larynx, trachea, and bronchi was predicted for aerosols in the 10-20 µm size range. Only small amounts of aerosols >20 µm penetrated past the pharyngeal region. Using the ICRP clearance model, local retained tissue dose metrics including maximal concentrations and areas under the curve were calculated for each airway region following repeated occupational exposures. These results are directly comparable with benchmark doses from in vitro toxicity studies in human cells leading to estimated human equivalent concentrations that reduce the reliance on animals for risk assessments.

Particle Size Characterization of Agricultural Sprays Collected on Personal Air Monitoring Samplers.

Potential inhalation exposure of agricultural workers and bystanders to aerosolized particles emitted by typical agricultural spray nozzles is influenced by the particle size distribution (PSD) of the spray. However, inhalation risk assessments do not currently factor in the human-relevant PSD that may be inhaled during pesticide handling activities. This study was conducted to characterize the PSD of aerosols collected with OSHA Versatile Sampler (OVS) tubes, which are monitoring devices commonly used for inhalation risk assessment in worker exposure studies. An Oxford Lasers N60V particle size analyzer was used for characterizing the spray PSD emitted from various agricultural nozzles. Side-by-side air sampling with OVS tubes and Respicon TM particle samplers was conducted to characterize the size distribution of aerosols collected on the OVS tubes during spraying of a diluted chlorothalonil formulation. Based on this comparison, OVS tubes captured the inhalable fraction (mass median diameter (D50) = 100 µm), with approximately 40% of the total inhalation concentration contributing to systemic exposure (D50 = 10 µm) regardless of nozzle spray quality. In addition, nozzles with fine and medium spray produced higher airborne concentrations compared to nozzles with coarse spray. Thus, the use of modern low-drift nozzles (e.g., air-induction nozzles) that emit larger spray droplets can substantially reduce the airborne concentration levels within inhalable particle size fractions. While the concentrations within these airway fractions (e.g., respirable, thoracic, inhalable) increased from extremely coarse to very fine spray nozzles, the relative proportion of each fraction within the total inhalable concentration remained constant regardless of spray quality or nozzle type. Such information on the PSD of pesticide applications can be used to refine inhalation risk assessments for agricultural workers and bystanders.

A simple problem formulation framework to create the right solution to the right problem.

A systematic approach to formulate consistent, technically robust and scientifically tractable problems will facilitate achieving innovative and effective solutions in risk evaluation. The fundamentals of problem formulation have been adapted from environmental and human health risk assessments. A structured problem formulation enables focus on describing and evaluating the specifics of the problem to be solved, instead of immediately creating solutions. First the problem should be framed to provide clarity and gain agreement on the problem to be addressed, resulting in a specific problem statement. Second the problem is explored in order to transform it into an operational state through questions to answer, hypotheses to test, and represented by a conceptual model. Finally the approach to testing hypotheses is mapped and the analysis plan is developed to address the problem statement. This simple adaptable framework can be applied to any circumstance to resolve a specific problem and describe a path to resolution.

Statistical Techniques to Analyze Pesticide Data Program Food Residue Observations.

The U.S. EPA conducts dietary-risk assessments to ensure that levels of pesticides on food in the U.S. food supply are safe. Often these assessments utilize conservative residue estimates, maximum residue levels (MRLs), and a high-end estimate derived from registrant-generated field-trial data sets. A more realistic estimate of consumers' pesticide exposure from food may be obtained by utilizing residues from food-monitoring programs, such as the Pesticide Data Program (PDP) of the U.S. Department of Agriculture. A substantial portion of food-residue concentrations in PDP monitoring programs are below the limits of detection (left-censored), which makes the comparison of regulatory-field-trial and PDP residue levels difficult. In this paper, we present a novel adaption of established statistical techniques, the Kaplan-Meier estimator (K-M), the robust regression on ordered statistic (ROS), and the maximum-likelihood estimator (MLE), to quantify the pesticide-residue concentrations in the presence of heavily censored data sets. The examined statistical approaches include the most commonly used parametric and nonparametric methods for handling left-censored data that have been used in the fields of medical and environmental sciences. This work presents a case study in which data of thiamethoxam residue on bell pepper generated from registrant field trials were compared with PDP-monitoring residue values. The results from the statistical techniques were evaluated and compared with commonly used simple substitution methods for the determination of summary statistics. It was found that the maximum-likelihood estimator (MLE) is the most appropriate statistical method to analyze this residue data set. Using the MLE technique, the data analyses showed that the median and mean PDP bell pepper residue levels were approximately 19 and 7 times lower, respectively, than the corresponding statistics of the field-trial residues.

Refined aquatic risk assessment for aldicarb in the United States.

Aldicarb is a systemic insecticide applied directly to soil and to control mites, nematodes, and aphids on a variety of crops (e.g., cotton, potatoes, peanuts). It is highly soluble in water (6,000 mg/L) and mobile in soils (K(oc) = 100). As a result, aldicarb has the potential to be transported to aquatic systems close to treated fields. The US Environmental Protection Agency (USEPA) recently conducted an aquatic screening-level ERA for aldicarb as part of the re-registration review process. We conducted a refined risk assessment for aldicarb to characterize better the risks posed by aldicarb to fish and invertebrates inhabiting small freshwater ponds near agricultural areas. For the exposure assessment, tier II PRZM/EXAMS (Predicted Root Zone Model [PRZM] and Exposure Analysis Modelling System [EXAMS]) modelling was conducted to estimate 30-y distributions of peak concentrations of aldicarb and the carbamate metabolites (aldicarb sulfoxide, aldicarb sulfone) in surface waters of a standard pond arising from different uses of aldicarb. The effects assessment was performed using a species sensitivity distribution (SSD) approach. The resulting risk curves as well as available incident reports suggest that risks to freshwater fish and invertebrates from exposure to aldicarb are minor. The available monitoring data did not provide conclusive evidence about risks to aquatic biota.

Characterization of fate and transport of isoxaflutole, a soil-applied corn herbicide, in surface water using a watershed model.

The objective of this study is to conduct a comprehensive assessment of the transport of isoxaflutole and RPA 202248 and their accumulation potential in semistatic water bodies, using a distributed watershed scale model and observed water quality data. A conceptual model was developed to characterize the fate and transport of isoxaflutole residues to and in surface water. The soil and water assessment tool (SWAT), a continuous daily time-step watershed model, was used to simulate the processes identified in the conceptual model. Monitoring data were available for a number of surface water bodies within the major product use area as a result of extensive and intensive residue monitoring. Detailed product use information at the zip-code level was obtained through dealer sales and a grower survey. The hydrologic and chemical transport results from the SWAT model were validated by comparison to available monitoring data from selected water bodies. Upon validation, the model was used to simulate the fate of isoxaflutole-derived residues in the water bodies using long-term historical weather data. The results from this investigation indicate no evidence of long-term accumulation of isoxaflutole and its metabolite RPA 202248 in semistatic water bodies.