John H. Ross

Assessing Human Exposures to Pesticides

Pesticide use is inevitably associated with chemical exposures that range from inferred nondetectable levels to easily measurable ones using sensitive, readily available analytical tools. Whether these exposures are of any biological significance is determined by duration, dose, and biological reactivity. The overwhelming majority of human exposures occur in a diverse chemical milieu of a nutritive substances and are of no known significance. Technologies that minimize human chemical exposures and maximize pesticide effectiveness are favored. The risk characterization process is ideally suited to assist decision makers concerning the protection of human health and evaluation of agricultural tools. It is the best means available to balance the review of pesticide impacts on health and agriculture. Regulators must be cautious to acknowledge the relative rather than absolute nature of the risk characterization process. Workplace biological monitoring must become more commonplace as a means to evaluate the chemical exposure potential of various work tasks and greater attention must be given to the biological validation of methods. Earlier needs for data to develop workplace hygiene strategies have been replaced in recent years by demands of the risk assessment process, which utilizes direct estimates of exposure and absorbed dose. Animal models, no matter how attractive, are not presently a substitute for human experience. Opportunities to gather more information on human experience associated with pesticide exposures must be more aggressively identified and pursued. Only a very small time lag should exist between identification of pesticide metabolites in rats and evaluation of metabolic similarities in humans. At the present levels of analytical sensitivity, most of our current uncertainty about the extent of worker exposure and patterns of metabolism between species can be at least clarified with the cooperation of persons who are exposed during normal day-to-day activities in the workplace. Only with better human data will the risk assessment process warrant greater reliance in decision making concerning our chemical exposures and human experience.

Biomonitoring and whole body cotton dosimetry to estimate potential human dermal exposure to semivolatile chemicals.

Current methods of estimating absorbed dosage (AD) of chemicals were evaluated to determine residue transfer from a carpet treated with chlorpyrifos (CP) to humans who performed a structured exercise routine. To determine the dislodgeability of residue, a California Department of Food and Agriculture (CDFA) roller was applied to a flat cotton cloth upon a treated carpet. Levels ranged from 0.06 to 0.99 µg CP/cm2. Cotton whole body dosimeters (WBD) were also used to assess residue transfer. The dosimeters retained 1.5 to 38 mg CP/person. Urine biomonitoring (3 days) for 3,5,6-trichloro-2-pyridinol (TCP) of persons who wore only swimsuits revealed a mean AD of 176 µg CP equivalents/person. The results show that the AD depends on the extent of contact transfer and dermal absorption of the residue. Default exposure assessments based upon environmental levels of chemicals and hypothetical transport pathways predict excessive exposure. The cotton WBD retains chemical residues and may be effectively used to predict dermal dose under experimental conditions.

Structure-activity correlations of amines inhibiting active uptake of paraquat (methyl viologen) into rat lung slices.

Abstract Analysis of amine structure with respect to inhibitory potency utilized a new method for determining equipotent inhibitor concentrations of paraquat uptake by lung slices. Fifteen N -alkyl homologues of paraquat (viologens) were tested and inhibition of lung uptake of paraquat was found to be a function of the inductive effect and steric bulk of groups attached to the nitrogens of 4,4′-bipyridyl. Several classes of amine inhibitors were examined. Polyamines were generally more potent than compounds containing only one quaternizable nitrogen at pH 7.4. α, ω-Diaminoalkanes were the most potent inhibitors of paraquat accumulation by lung slices.

Preformed biomarkers including dialkylphosphates (DAPs) in produce may confound biomonitoring in pesticide exposure and risk assessment.

Low levels of pesticides and their metabolites/degradates occur in produce when pesticides are used in conventional or organic crop protection. Human dietary and nonoccupational urine biomonitoring studies may be confounded by preformed pesticide biomarkers in the diet. The extent of formation of putative urine biomarkers, including malathion specific (MMA, MDA; malathion mono- and diacids), organophosphorus generic (DMP, DMTP, DMDTP; dimethyl-, dimethylthio-, and dimethydithiophosphate), pyrethroid generic (3-PBA; 3-phenoxybenzoic acid), and captan-specific metabolites (THPI; tetrahydrophthalimide), was measured in produce samples containing the parent pesticide. Every produce sample of 19 types of fruits and vegetables contained biomarkers of potential human exposure. A total of 134 of 157 (85%) samples contained more molar equivalent biomarkers than parent pesticide. Malathion and fenpropathrin were sprayed (1 lb/A), and the time-dependent formation of pesticide biomarkers in strawberries was investigated under field conditions typical of commercial production in California. Malathion and fenpropathrin residues were always below established residue tolerances. Malathion, MMA, and MDA dissipated, while DMP, DMTP, and DMDTP increased, during a 20 day study period following the preharvest interval. The mole ratios of biomarkers/(malathion + malaoxon) were always greater than 1 and increased from day 4 to day 23 postapplication. Fenpropathrin and 3-PBA also dissipated in strawberries during each monitoring period. The mole ratios of 3-PBA/fenpropathrin were always less than 1 and decreased from day 4 to day 14. The absorption of pesticide biomarkers in produce and excretion in urine would falsely indicate consumer pesticide exposure if used to reconstruct dose for risk characterization.

Comparative evaluation of absorbed dose estimates derived from passive dosimetry measurements to those derived from biological monitoring: validation of exposure monitoring methodologies.

Passive dosimetry (PD) methods for measuring and estimating exposure to agricultural workers (i.e., persons handling agricultural chemicals and working in treated crops) have been in use since the 1950s. A large number of studies were conducted in the 1950s through 1970s to characterize exposure. Since the 1980s quantitative dermal PD methods are used in conjunction with inhalation PD methods to measure whole-body exposure. These exposure or absorbed dose estimates are then compared to “no effect” exposure levels for hazards identified in toxicology studies, and have become the standard for risk assessment for regulatory agencies. The PD methods used have never been validated. Validation in the context of human exposure monitoring methods means that a method has been shown to measure accurately a delivered dose in humans. The most practical alternative to isolating parts of the body for validating recovery methods is to utilize field exposure studies in which concurrent or consecutive measurements of exposure and absorbed dose have been made with PD and biomonitoring in the same cohorts of individuals. This ensures that a direct comparison can be made between the two estimates of absorbed dose, one derived from PD and the other from biomonitoring. There are several studies available (published and proprietary) employing both of these approaches. Reports involving 14 concurrent or consecutive PD-biomonitoring studies were quantitatively evaluated with 18 different methods of application or reentry scenarios for eight different active ingredients for which measured human kinetics and dermal absorption data existed. This evaluation demonstrated that the total absorbed dose estimated using PD for important handler and reentry scenarios is generally similar to the measurements for those same scenarios made using human urinary biomonitoring methods. The statistical analysis of individual worker PD:biomonitoring ratios showed them to be significantly correlated in these studies. The PD techniques currently employed yield a reproducible, standard methodology that is valid and reliably quantifies exposure.

Implications of estimates of residential organophosphate exposure from dialkylphosphates (DAPs) and their relevance to risk

Recent epidemiological studies have claimed to associate a variety of toxicological effects of organophosphorus insecticides (OPs) and residential OP exposure based on the dialkyl phosphates (DAPs; metabolic and environmental breakdown products of OPs) levels in the urine of pregnant females. A key premise in those epidemiology studies was that the level of urinary DAPs was directly related to the level of parent OP exposure. Specific chemical biomarkers and DAPs representing absorbed dose of OPs are invaluable to reconstruct human exposures in prospective occupational studies and even in non-occupational studies when exposure to a specific OP can be described. However, measurement of those detoxification products in urine without specific knowledge of insecticide exposure is insufficient to establish OP insecticide exposure. DAPs have high oral bioavailability and are ubiquitously present in produce at concentrations several-fold greater than parent OPs. Studies relying on DAPs as an indicator of OP exposure that lack credible information on proximate OP exposure are simply measuring DAP exposure and misattributing OP exposure.

Estimation of the percutaneous absorption of permethrin in humans using the parallelogram method.

The objective of this study was to develop an estimate of the percent dermal absorption of permethrin in humans to provide more accurate estimates of potential systemically absorbed dose associated with dermal exposure scenarios. Piperonyl butoxide (PBO) was used as a reference compound. The human percutaneous absorption estimate was based on the assumption that the ratio of in vivo dermal absorption (expressed as a percentage during a given time period) of permethrin through rat skin to in vitro dermal absorption through rat skin was the same as the ratio of in vivo dermal absorption in humans to in vitro dermal absorption with human skin, known as the parallelogram method. The ratio of dermal absorption by in vitro rat skin to absorption by in vitro human skin ranged from 6.7 to 15.4 (for a 24-h exposure period) with an average of 11. Data suggest in vivo human dermal absorption values for permethrin ranging from 1.4 to 3.3% when estimated based on 24-h in vivo rat values, and 2.5 to 5.7% based on 5-d in vivo rat values. The parallelogram method used to estimate dermal absorption of permethrin and PBO is supported by results from several other compounds for which in vivo and in vitro rat and human dermal absorption data exist. Collectively, these data indicate that estimating human dermal absorption from in vitro human and rat plus in vivo rat data are typically accurate within ±3-fold of the values measured in human subjects.

2,4-D exposure and risk assessment: comparison of external dose and biomonitoring based approaches

Conventional chemical exposure assessment relies upon measurements or estimates of chemical concentrations in environmental media, food, or products, in combination with assumptions regarding contact rates, in order to estimate external doses (ppm in air) or intake rates of chemicals (e.g., mg/kg/day ingested). A risk assessment is conducted by comparing these external or intake dose estimates to appropriate (e.g., route-specific) exposure guidance values (e.g., Reference Dose or Reference Concentration) to assess whether exposures are exceeding levels of concern. Human biomonitoring, in which concentrations of chemicals are measured in blood or urine, is being increasingly used as an alternative or complementary exposure assessment. The Biomonitoring Equivalent, which is the translation of a Reference Dose to an equivalent concentration of a compound in blood or urine, provides a parallel means to interpret biomonitoring data in order to assess whether chemical-specific exposures exceed levels of concern. This manuscript presents a side-by-side comparison of the two approaches for assessing exposures and risks for a case study compound, 2,4-dichlorophenoxyacetic acid (2,4-D). The findings from this case study indicate that the external dose-based assessments result in estimates of exposure and resulting hazard quotients that are consistently several-fold higher than those based on biomonitoring data. These comparisons support a conclusion that exposure assessments conducted as part of the registration process for 2,4-D incorporate sufficiently conservative assumptions.

Assessing exposure to allied ground troops in the Vietnam War: A comparison of AgDRIFT and Exposure Opportunity Index models

The AgDRIFT aerial dispersion model is well validated and closely related to the AGDISP model developed by the USDA Forest Service to determine on- and off-target deposition and penetration of aerially applied pesticide through foliage of trees. The Exposure Opportunity Index (EOI) model was developed to estimate relative exposure of ground troops in Vietnam to aerially applied herbicides. We compared the output of the two models to determine whether their predictions were in substantial agreement, but found a total lack of concordance. While the AgDRIFT model estimated that ground-level deposition through foliage was reduced more than 20 orders of magnitude at less than 1 km from the flight line, the EOI model predicted deposition declines less than one order of magnitude 4 km from the flight line. Interestingly the EOI model predicts a four-fold variability in EOI on the flight line, where exposure should be essentially invariant because the spray apparatus is designed to apply herbicide at a constant rate. We believe that the EOI model cannot be used to provide individual exposure estimates for the purpose of conducting epidemiologic studies. Moreover, evaluation of the position data for both herbicide spray swaths and troop locations, together with the actual patterns of spray deposition predicted by the AgDRIFT model, suggests that precise individual-level exposure assessments for ground troops in Vietnam are impossible. However, we suggest that well-validated tools like AgDRIFT can be used to estimate exposure to groups of individuals.

Assessing exposure to allied ground troops in the Vietnam War: a quantitative evaluation of the Stellman Exposure Opportunity Index model.

The Exposure Opportunity Index (EOI) is a proximity-based model developed to estimate relative exposure of ground troops in Vietnam to aerially applied herbicides. We conducted a detailed quantitative evaluation of the EOI model by using actual herbicide spray missions isolated in time and space. EOI scores were calculated for each of 36 hypothetical receptor location points associated with each spray mission for 30 herbicide missions for two time periods - day of herbicide application and day 2-3 post-application. Our analysis found an enormous range of EOI predictions with 500-1000-fold differences across missions directly under the flight path. This quantitative examination of the EOI suggests that extensive testing of the models code is warranted. Researchers undertaking development of a proximity-based exposure model for epidemiologic studies of either Vietnam veterans or the Vietnamese population should conduct a thorough and realistic analysis of how precise and accurate the model results are likely to be and then assess whether the model results provide a useful basis for their planned epidemiologic studies.