Impact of dermal absorption factors in occupational exposure assessment: comparison of two models for agricultural reentry workers exposed to azinphosmethyl.

This analysis compares two deterministic reentry exposure models that differ in their treatment of the time dependence of dermal absorption. The first model, called the "traditional model," assumes that dermal absorption is a fixed fraction of the cumulative load on skin at the end of the workshift and that absorption is independent of residence time on the skin. The second model, called the "time-integrated model," incorporates the time dependence of both exposure and absorption by assuming that absorption begins at the outset of exposure and continues through the workshift and beyond, until an effective washing event occurs. These two models were evaluated using previously collected biological monitoring data from apple thinners exposed to azinphosmethyl. Daily doses predicted by the models were compared to doses estimated from the biological sampling results assuming pseudo steady-state excretion. The geometric mean dose estimated from the biological sampling data was 20 microg/kg/day. Corresponding geometric mean doses produced by the traditional model and the time-integrated model were 79 microg/kg/day and 24 microg/kg/day, respectively. When the doses predicted by the traditional model were plotted against those estimated from the biological measurements, the slope of the regression line was significantly greater than 1 (beta = 1.37). However, when this same analysis was conducted for the doses predicted by the time-integrated model, the confidence interval around the slope encompassed 1 (beta = 1.01). Thus, time-integrated treatment of absorption appeared to provide more realistic dose estimates than did the traditional approach.

Biologically based pesticide dose estimates for children in an agricultural community.

Current pesticide health risk assessments in the United States require the characterization of aggregate exposure and cumulative risk in the setting of food tolerances. Biologic monitoring can aggregate exposures from all sources and routes, and can integrate exposures for chemicals with a common mechanism of action. Its value was demonstrated in a recent study of organophosphorus (OP) pesticide exposure among 109 children in an agricultural community in Washington State; 91 of the children had parents working in agriculture. We estimated individual OP pesticide doses from urinary metabolite concentrations with a deterministic steady state model, and compared them to toxicologic reference values. We evaluated doses by assuming that metabolites were attributable entirely to either azinphos-methyl or phosmet, the two OP pesticides used most frequently in the region. Creatinine-adjusted average dose estimates during the 6- to 8-week spraying season ranged from 0 to 36 microg/kg/day. For children whose parents worked in agriculture as either orchard applicators or as fieldworkers, 56% of the doses estimated for the spray season exceeded the U.S. Environmental Protection Agency (EPA) chronic dietary reference dose, and 19% exceeded the World Health Organization acceptable daily intake values for azinphos-methyl. The corresponding values for children whose parents did not work in agriculture were 44 and 22%, respectively. The percentage of children exceeding the relevant reference values for phosmet was substantially lower (< 10%). Single-day dose estimates ranged from 0 to 72 microg/kg/day, and 26% of these exceeded the EPA acute reference dose for azinphos-methyl. We also generated dose estimates by adjustment for total daily urine volume, and these estimates were consistently higher than the creatinine-adjusted estimates. None of the dose estimates exceeded the empirically derived no-observable-adverse-effect levels for these compounds. The study took place in an agricultural region during a period of active spraying, so the dose estimates for this population should not be considered representative of exposures in the general population. The findings indicate that children living in agricultural regions represent an important subpopulation for public health evaluation, and that their exposures fall within a range of regulatory concern. They also demonstrate that biologically based exposure measures can provide data for health risk evaluations in such populations.

Adult proxy responses to a survey of children's dermal soil contact activities.

Contaminated site cleanup decisions may require estimation of dermal exposures to soil. Telephone surveys represent one means of obtaining relevant activity pattern data. The initial Soil Contact Survey (SCS-I), which primarily gathered information on the activities of adults, was conducted in 1996. Data describing adult behaviors have been previously reported. Results from a second Soil Contact Survey (SCS-II), performed in 1998-1999 and focused on children's activity patterns, are reported here. Telephone surveys were used to query a randomly selected sample of U.S. households. A randomly chosen child, under the age of 18 years, was targeted in each responding household having children. Play activities as well as bathing patterns were investigated to quantify total exposure time, defined as activity time plus delay until washing. Of 680 total survey respondents, 500 (73.5%) reported that their child played outdoors on bare dirt or mixed grass and dirt surfaces. Among these "players," the median reported play frequency was 7 days/week in warm weather and 3 days/week in cold weather. Median play duration was 3 h/day in warm weather and 1 h/day in cold weather. Hand washes were reported to occur a median of 4 times per day in both warm and cold weather months. Bath or shower median frequency was seven times per week in both warm and cold weather. Finally, based on clothing choice data gathered in SCS-I, a median of about 37% of total skin surface is estimated to be exposed during young children's warm weather outdoor play.

Strategies for assessing children's organophosphorus pesticide exposures in agricultural communities.

Children can be exposed to pesticides from multiple sources and through multiple pathways. In addition to the standard pathways of diet, drinking water and residential pesticide use, children in agricultural communities can be exposed to pesticides used in agricultural production. A research program on children and pesticides was established at the University of Washington (UW) in 1991 and has focused on two major exposure pathway issues: residential proximity to pesticide-treated farmland and transfer of pesticides from the workplace to the home (paraoccupational or take-home exposure). The UW program selected preschool children of agricultural producers and farm workers in the tree fruit region of Washington state as a population that was likely to have elevated exposures from these pathways. The organophosphorus (OP) pesticides were selected as a common class of chemicals for analysis so that issues of aggregate exposure and cumulative risk could be addressed. This paper provides an overview of key findings of our research group over the past 8 years and describes current studies in this field. Soil and housedust concentrations of OP pesticides were elevated in homes of agricultural families (household members engaged in agricultural production) when compared to non-agricultural reference homes in the same community. Dialkyl phosphate metabolites of OP pesticides measured in children's urine were also elevated for agricultural children when compared to reference children and when compared to children in the Seattle metropolitan area. Proximity to farmland was associated with increased OP pesticide concentrations in housedust and OP pesticide metabolites in urine. Current studies include a community-based intervention to reduce parental transfer of pesticides from the workplace, and a systematic investigation of the role of agricultural spray drift in children's exposure to pesticides.

Adult responses to a survey of soil contact-related behaviors.

Protocols used to assess human exposure to chemicals in soils at contaminated sites often include a dermal pathway. Use of default parameters to assess dermal exposure to soil can easily lead to risk projections that appear to warrant remedial action. However, because those default parameters are typically highly uncertain, risk estimates based upon them inspire little confidence. To better characterize assumptions regarding dermal exposures, a telephone survey instrument was developed to elicit information on behaviors relevant to assessment of dermal contact with soil and dust. Participation in four activities--gardening, other yard work, outdoor team sports, and home construction or repair involving digging--was investigated. Questions were also asked regarding clothing choices and post-activity bathing practices. The survey was administered to two populations of approximately 450 adult respondents each using random digit dialing. The first was a national (U.S.) sample. The second sample was drawn from counties surrounding the Hanford Nuclear Reservation. Seventy-nine percent of the regional respondents and 89% of the national respondents reported participating in at least one of the four targeted activities. Responses of doers regarding clothing choices suggest that median fractions of skin exposed during warm-weather activities typically exceed the 25% often assumed. The Hanford sample differed from the national sample in the fraction residing in single-family homes, the fraction describing their residential surroundings as rural, and in ethnic makeup. The Hanford population displayed greater rates of participation than the national sample in three activities that have an obvious link to residence in a single-family dwelling: home repair involving digging, gardening, and other yard work, but differences were not explained entirely by residence type. The regional population also reported greater frequency of participation in multiple activities. In contrast, clothing choices among doers could not be distinguished between the two groups.

Field measurement of dermal soil loadings in occupational and recreational activities.

Risks associated with dermal exposure to contaminated soil are not well-characterized, but nevertheless must be estimated to define endpoints for remedial strategies. Among the parameters contributing to the uncertainty of these estimates is soil adherence to skin. Pre- and postactivity soil loadings have been obtained from hands, forearms, lower legs, faces, and/or feet of volunteers engaged in various occupational and recreational activities. These data are distinguished from other sources of estimates of soil adherence by the manner of their collection. Soil loads were obtained directly from multiple body parts before and after uncontrived exposure scenarios. Data presented for the first time here supplement prior results and roughly double the available data base. This expanded data base provides a useful perspective on types of behavior likely to lead to soil contact falling within general classes of activity (e.g., background, low, moderate, or high contact). Prior conclusions supported by the additional data include the following: (1) post-activity loadings are typically higher than preactivity levels, demonstrating that exposure is episodic; (2) hand loadings are dependent upon class of activity; (3) hand loadings generally provide conservative estimates of loadings on nonhand body parts within activity classes; and (4) hand loadings do not provide conservative estimates of nonhand loadings across activity classes. Finally quantitative estimates of relative loads on unclothed nonhand body parts are presented.

Uncertainty in estimated half-lives of PCBS in humans: impact on exposure assessment.

Estimates of the half-lives of PCBs in humans derived from successive body burden measurements are reviewed and found to vary widely whether based on congener-specific or aggregate data. Variability due to differences in physiological processes among individuals and in congener properties is to be expected, but does not appear to be a complete explanation. Very short half-lives (i.e. < 1 year) are unlikely for those congeners most frequently found in human blood because the exposures required to sustain observed body burdens are too large to be easily explained. Very long half-lives ( > 10 years) may be artifacts of confounding by ongoing exposures (a common effect at low body burdens) and are also suspect. The loss of significant quantities of PCBs from the blood of occupationally exposed persons with half-lives of 2-6 years is comparatively well documented (i.e. has been observed in studies with relatively large numbers of subjects and high initial body burdens). Therefore, very long half-lives must be limited to subsets of congeners or of populations if they occur at all. The impact of the use of fixed estimates of half-lives drawn from the tails of the observed range on the evaluation of regulatory standards is shown to be substantial.

Effect of soil loading on dermal absorption efficiency from contaminated soils.

The effect of soil loading on the dermal uptake of soil-borne contaminants was examined using an in vitro evaporation/penetration apparatus and abdominal skin from human cadavers. Dermal uptake of two 14C-labeled pesticides, lindane and 2,4-dichlorophenoxyacetic acid (2,4-D), was assessed at nominal soil loadings of 1, 5, and 10 mg/cm2. Sub-150-microns fractions of two soils with differing organic carbon contents were employed. Mean 24-h dermal absorption values ranged from 0.45 to 2.35% for lindane and from 0.18 to 1.64% for 2,4-D, depending upon soil load and type. Mean mass fluxes ranged from 8.8 to 32 pg/cm2/h for lindane and from 1.9 to 6.4 pg/cm2/h for 2,4-D. Results were aggregated as ratios of flux or percent absorption at 1 and 10 mg/cm2 to corresponding values obtained at 5 mg/cm2. Fluxes at 5 and 10 mg/cm2 did not differ significantly, but flux at 1 mg/cm2 was about one-half the value observed at the higher loadings. The most plausible explanation for this decrease in mass flux is incomplete (submonolayer) coverage of the skin. Evidence in the form of electron micrographs is presented in support of this conclusion. Relative percent absorption increased significantly with decreases in soil load from 10 to 5 and from 5 to 1 mg/cm2. This effect was inversely proportional to loading reduction in the former case, but was less than proportional due to the impact of contact area reduction (and, in the case of lindane, volatilization losses) in the latter. Percent dermal absorption data obtained in the laboratory require adjustment for differences in loading and coverage before application to assessment of exposure to contaminants in soils. Description of dermal absorption from soil in a manner comparable to that used to describe absorption from a liquid or vapor (i.e., using a driving force and a mass transfer coefficient) would reduce confusion on this point and is recommended.

Field measurement of dermal soil loading attributable to various activities: implications for exposure assessment.

Estimates of soil adherence to skin are required for assessment of dermal exposures to contaminants in soils. Previously available estimates depend heavily on indirect measurements and/or artificial activities and reflect sampling of hands only. Results are presented here from direct measurement of soil loading on skin surfaces of volunteers before and after normal occupational and recreational activities that might reasonably be expected to lead to soil contact. Skin surfaces assayed included hands, forearms, lower legs, faces and/or feet. Observed hand loadings vary over five orders of magnitude (roughly from 10(-3) to 10(2) mg/cm2) and are dependent upon type of activity. Hand loadings within the current default range of 0.2 to 1.0 mg/cm2 were produced by activities providing opportunity for relatively vigorous soil contact (rugby, farming). Loadings less than 0.2 mg/cm2 were found on hands following activities presenting less opportunity for direct soil contact (soccer, professional grounds maintenance) and on other body parts under many conditions. The default range does not, however, represent a worst case. Children playing in mud on the shore of a lake generated geometric mean loadings well in excess of 1 mg/cm2 on hands, arms, legs, and feet. Post-activity average loadings on hands were typically higher than average loadings on other body parts resulting from the same activity. Hand data from limited activities cannot, however, be used to conservatively predict loadings that might occur on other body surfaces without regard to activity since non-hand loadings attributable to higher contact activities exceeded hand loadings resulting from lower contact activities. Differences between pre- and post-activity loadings also demonstrate that dermal contact with soil is episodic. Typical background (pre-activity) geometric mean loadings appear to be on the order of 10(-2) mg/cm2 or less. Because exposures are activity dependent, quantification of dermal exposure to soil will remain inadequate until data describing relevant human behavior (type of activity, frequency, duration including interval before bathing, clothing worn, etc.) are generated.

Probabilistic prediction of exposures to arsenic contaminated residential soil.

Probabilistic modelling using Monte Carlo simulation has been proposed as a more scientifically valid method of estimating soil contaminant exposures than conservative deterministic methods currently used by regulatory agencies. A retrospective application of probabilistic modelling to an exposure scenario involving arsenic-contaminated residential soil near the former ASARCO smelter near Tacoma, Washington is presented. The population of interest is children, aged 2-6 years, living within one-half mile (0.3 km) of the smelter site. Models that predict urinary arsenic levels based on unintentional soil ingestion and inhalation exposure pathways are used. Distributions of exposure variables are based on site-specific data and previous exposure studies. Simulated urinary arsenic levels are compared with data from two biomonitoring studies performed during the late 1980s. Arsenic distributions produced by simulation and biomonitoring are significantly different, and likely contributors to this difference are discussed. However the probabilistic model provides closer estimations of urinary arsenic levels than conservative deterministic models similar to those used by regulatory agencies, and provides useful information regarding parameter uncertainty. Soil ingestion rate was a driving variable in the probabilistic models. Further quantification of soil ingestion rates is warranted.

Potential impact of deliberately introduced ozone on indoor air quality.

Population subsets whose behavior leads to unusually large exposures to toxicants are of interest to exposure assessors. Despite U.S. Food and Drug Administration (FDA) restriction since 1974, ozone generating devices continue to be marketed in the U.S. for indoor use. Promotional materials cite a host of alleged benefits to indoor environmental quality. Generation rates cited in sales literature are used here to estimate indoor air ozone concentrations that could result from use of such equipment. Predictions exceed relevant air quality standards, in some cases by a substantial margin. Limited available experimental measurements support this finding.