Identifying soil cleanup criteria for dioxins in urban residential soils: how have 20 years of research and risk assessment experience affected the analysis?

Statistical comparison for assessing agreement between two mass spectrometric methods for the analysis of polychlorinated dibenzo-p-dioxins and furans (PCDDs/Fs) in contaminated soils

A gas chromatography coupled to high-resolution mass spectrometry (GC-HR/MS) has been used as the standard method for the quantification of polychlorinated dibenzo-p-dioxins and furans (PCDDs/Fs), which are regulated at screening and action levels in the environment. However, several alternative methods have been attempted due to the disadvantage of its high cost. Although a gas chromatography with triple quadrupole mass spectrometry (GC-QqQ-MS/MS) has been used in a wide variety of sample matrices, showing that they are interchangeable, there has been a lack of comprehensive studies on statistical agreement with GC-HR/MS. In this study, a pairwise comparison of the total concentrations of PCDDs/Fs in 90 soil field samples obtained by two mass spectrometric methods was performed using the Passing-Bablok (P&B) regression and Bland-Altman (B&A) analysis for the method comparison. According to the result of the B&A analysis, the concentration range of PCDDs/Fs was between 98.2 and 1760 pg/g showed good agreement between two methods at the 95 % confidence level (CL). Although there was a large discrepancy between the two methods in the low concentrations (<16.5 pg/g of PCDDs/Fs), this result was similar to the P&B regression analysis. As the verification results by B&A and P&B regression analysis, the interchangeable concentration range between the two methods was confirmed to be adequate for the monitoring of PCDDs/Fs regulating levels in soils.

Elevated congenital heart disease birth prevalence rates found in Central Vietnam and dioxin TCDD residuals from the use of 2, 4, 5-T herbicides (Agent Orange) in the Da Nang region

Congenital heart disease (CHD) birth prevalence rate in Da Nang City and two adjacent provinces in Central Vietnam reported by Giang et al. in 2019 was 20.09/1000 births, much higher than any CHD birth rates previously reported. In this current study, three physicians trained in pediatric cardiology reanalyzed and reclassified the Giang et al 2019 cardiac anomalies data, eliminating singular small PDAs and separating cardiac defects into 27 contemporary CHD subgroups. These CHD subgroups were then statistically compared with Liu et al. 2019 Global CHD birth prevalence study of Asian Low-Middle Income Countries (LMIC) CHD subgroup rate of 9.34/1000 births (95% CI 8.07-10.70). Despite applying newer diagnostic criteria and refining the cardiac anomalies data, the Da Nang region continued to show significantly (p<0.0001) elevated total CHD birth prevalence rates at 14.71/1000 births (95% CI 12.74-16.69) compared to the Asian LMIC CHD birth prevalence rate 9.34/1000 births. This finding raises the question of whether environmental persistence of the contaminant dioxin TCDD from 2,4,5-T herbicides (Agent Orange) used during the Vietnam War (1961-1971) in the Da Nang region might be a factor associated with elevated CHD birth prevalence, as it is not present in other LMIC surrounding Vietnam. We recommend testing of soils and sediments in rural and agricultural areas in Central Vietnam that received high volume applications of contaminated herbicides to assess the relationship of the higher CHD birth prevalence rate and the presence of residual dioxin TCDD. Enhanced fetal cardiac echocardiograpy in the region to screen for CHD would enable early interventions and could improve outcomes for infants and children.

Health risk characterization of maximum legal exposures for persistent organic pollutant (POP) pesticides in residential soil: An analysis

Regulations for pesticides in soil are important for controlling human health risk; humans can be exposed to pesticides by ingesting soil, inhaling soil dust, and through dermal contact. Previous studies focused on analyses of numerical standard values for pesticides and evaluated the same pesticide using different standards among different jurisdictions. To understand the health consequences associated with pesticide soil standard values, lifetime theoretical maximum contribution and risk characterization factors were used in this study to quantify the severity of damage using disability-adjusted life years (DALYs) under the maximum "legal" exposure to persistent organic pollutant (POP) pesticides that are commonly regulated by the Stockholm Convention. Results show that computed soil characterization factors for some pesticides present lognormal distributions, and some of them have DALY values higher than 1000.0 per million population (e.g., the DALY for dichlorodiphenyltrichloroethane [DDT] is 14,065 in the Netherlands, which exceeds the tolerable risk of uncertainty upper bound of 1380.0 DALYs). Health risk characterization factors computed from national jurisdictions illustrate that values can vary over eight orders of magnitude. Further, the computed characterization factors can vary over four orders of magnitude within the same national jurisdiction. These data indicate that there is little agreement regarding pesticide soil regulatory guidance values (RGVs) among worldwide national jurisdictions or even RGV standard values within the same jurisdiction. Among these POP pesticides, lindane has the lowest median (0.16 DALYs) and geometric mean (0.28 DALYs) risk characterization factors, indicating that worldwide national jurisdictions provide relatively conservative soil RGVs for lindane. In addition, we found that some European nations and members of the former Union of Soviet Socialist Republics share the same pesticide RGVs and data clusters for the computed characterization factors.

Worldwide Regulations of Standard Values of Pesticides for Human Health Risk Control: A Review

Abstract: The impact of pesticide residues on human health is a worldwide problem, as human exposure to pesticides can occur through ingestion, inhalation, and dermal contact. Regulatory jurisdictions have promulgated the standard values for pesticides in residential soil, air, drinking water, and agricultural commodity for years. Until now, more than 19,400 pesticide soil regulatory guidance values (RGVs) and 5400 pesticide drinking water maximum concentration levels (MCLs) have been regulated by 54 and 102 nations, respectively. Over 90 nations have provided pesticide agricultural commodity maximum residue limits (MRLs) for at least one of the 12 most commonly consumed agricultural foods. A total of 22 pesticides have been regulated with more than 100 soil RGVs, and 25 pesticides have more than 100 drinking water MCLs. This research indicates that those RGVs and MCLs for an individual pesticide could vary over seven (DDT drinking water MCLs), eight (Lindane soil RGVs), or even nine (Dieldrin soil RGVs) orders of magnitude. Human health risk uncertainty bounds and the implied total exposure mass burden model were applied to analyze the most commonly regulated and used pesticides for human health risk control. For the top 27 commonly regulated pesticides in soil, there are at least 300 RGVs (8% of the total) that are above all of the computed upper bounds for human health risk uncertainty. For the top 29 most-commonly regulated pesticides in drinking water, at least 172 drinking water MCLs (5% of the total) exceed the computed upper bounds for human health risk uncertainty; while for the 14 most widely used pesticides, there are at least 310 computed implied dose limits (28.0% of the total) that are above the acceptable daily intake values. The results show that some worldwide standard values were not derived conservatively enough to avoid human health risk by the pesticides, and that some values were not computed comprehensively by considering all major human exposure pathways.

Interplay of metals and bromine with dioxin-related compounds concentrated in e-waste open burning soil from Agbogbloshie in Accra, Ghana

Open burning of electronic waste (e-waste) releases various metals and organohalogen compounds in the environment. Here we investigated the interplay of metals (Cu, Pb, Zn, Fe, Co, and Sr) and bromine (Br) in the formation of dioxin-related compounds (DRCs), including polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (DL-PCBs), as well as non-regulated DRCs such as polybrominated dibenzo-p-dioxins/furans (PBDD/Fs) and their monobrominated PCDD/Fs in soils sampled from open burning e-waste sites at Agbogbloshie in Accra, Ghana. The predominant DRCs were PBDFs, PCDFs, PCDDs, and DL-PCBs. Statistical analyzes, X-ray absorption spectroscopy, and the PCDF/PCDD ratio suggested possible formation paths of PCDD/Fs and DL-PCBs by catalytic behaviors of copper chlorides (CuCl, CuCl2, and Cu2(OH)3Cl) and thermal breakdown of polyvinyl chloride. Predominant formation of brominated furans may be derived from electron transfer from intermediates of PBDE to copper, Cu(II) → Cu(I). Lead chloride also contributed to generate DRCs and may become highly bioaccessible through the open burning of e-waste. The main zinc species (ZnCl2 and ZnS) suggested a possible relationship to generate DRCs and specific zinc source such as tire burning. Cu, Pb, Zn, and Br contained in various e-wastes, wires/cables, plastics, and tires strongly influenced generation of many DRCs.

Residential surface soil guidance values applied worldwide to the original 2001 Stockholm Convention POP pesticides

Surface soil contamination is a worldwide problem. Many regulatory jurisdictions attempt to control human exposures with regulatory guidance values (RGVs) that specify a soil's maximum allowable concentration. Pesticides are important soil contaminants because of their intentional toxicity and widespread surface soil application. Worldwide, at least 174 regulatory jurisdictions from 54 United Nations member states have published more than 19,400 pesticide RGVs for at least 739 chemically unique pesticides. This manuscript examines the variability of the guidance values that are applied worldwide to the original 2001 Stockholm Convention persistent organic pollutants (POP) pesticides (Aldrin, Chlordane, DDT, Dieldrin, Endrin, Heptachlor, Mirex, and Toxaphene) for which at least 1667 RGVs have been promulgated. Results indicate that the spans of the RGVs applied to each of these pesticides vary from 6.1 orders of magnitude for Toxaphene to 10.0 orders of magnitude for Mirex. The distribution of values across these value spans resembles the distribution of lognormal random variables, but also contain non-random value clusters. Approximately 40% of all the POP RGVs fall within uncertainty bounds computed from the U.S. Environmental Protection Agency (USEPA) RGV cancer risk model. Another 22% of the values fall within uncertainty bounds computed from the USEPA's non-cancer risk model, but the cancer risk calculations yield the binding (lowest) value for all POP pesticides except Endrin. The results presented emphasize the continued need to rationalize the RGVs applied worldwide to important soil contaminants.

Scope of the worldwide effort to regulate pesticide contamination in surface soils

Regulating surface soil contamination is a worldwide problem. Many jurisdictions address this problem with regulatory guidance values (RGVs) that specify the maximum allowable soil concentration of contaminants. Pesticides are a particularly important class of soil contaminants because of their intentional toxicity and widespread application to home, garden, and agricultural soils. Pesticides are also difficult to regulate because they are marketed in thousands of products made from hundreds of potentially toxic chemicals. Worldwide, at least 174 jurisdictions from 54 United Nations member states have promulgated more than 19,400 pesticide RGVs. Values may be found for at least 739 pesticides, identified by unique Chemical Abstract Service numbers (CAS No.). Using CAS numbers helps to avoid confusion that may result from alternative product names, chemical nomenclature conventions, or flawed translations. Assembling the set of pesticide RGVs required translating guidance documents authored in 30 different languages. Results indicate that more than 100 RGVs have been promulgated for each of the 22 most frequently regulated pesticides including over 300 values for DDT. Data are presented on the number of pesticides typically addressed by a regulatory jurisdiction and on the size and variability of the RGV datasets for the 200 most frequently regulated pesticides.

Long-term monitoring and modeling of polychlorinated dibenzo-p-dioxins and dibenzofurans from municipal solid waste incinerators and surrounding area in northern Taiwan

Municipal solid waste incinerators (MSWIs) have long been the major contributors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) to ambient air in Taiwan. After stringent MSWI emission standards were introduced in 2001, the long-term continuous monitoring of flue gas and ambient air quality became necessary to ensure the effectiveness of the related control strategies. Three MSWIs and the surrounding ambient air were investigated in the current study for PCDD/F characteristics during 2006 to 2011. The average concentrations in the flue gas ranged from 0.008 ~ 0.0488 ng I-TEQ/Nm(3), which is much less than the emission standard in Taiwan (0.1 ng I-TEQ/Nm(3)) (I-TEQ is the abbreviation of International Toxic Equivalent). This led to extremely low levels in the ambient air, 0.0255 pg I-TEQ/Nm(3), much less than the levels seen in most urban areas around the world. Additionally, the results obtained using the Industrial Source Complex Short-Term Dispersion Model (ISCST3) indicate that the PCDD/F contributions from the three MSWIs to the ambient air were only in the range from 0.164 ~ 0.723 %. Principal component analysis (PCA) showed that the PCDD/Fs in the air samples had very similar characteristics to those from mobile sources. The results thus show that stringent regulations have been an effective control strategy, especially for urban areas, such as Taipei City.

The University of Michigan Dioxin Exposure Study: estimating residential soil and house dust exposures to young children

The University of Michigan Dioxin Exposure Study provides extensive data on elevated residential soil and house dust concentrations of polychlorinated dioxins and dibenzofurans (PCDD/Fs) and adult body burdens among residents near a chemical manufacturing plant in Midland, Michigan. Recent reports found no significant contribution of residential soil/dust concentrations to serum lipid PCDD/Fs in adults. Although child body burdens were not studied by the University of Michigan, internal dose modeling that incorporates recent findings on demonstrated shorter elimination half life of PCDD/Fs in children (1-2 year half life in children vs. ~7 years in older adults) can be applied to assess this important issue. The model examines children (ages 0-7 years) with background dietary intake and exposure to residential soils at selected concentrations (10, 100 and 1000 pg/g 2,3,7,8-tetrachlorodibenzo-p-dioxin toxic equivalents, TEQ) using the congener patterns observed in Midland. Model predictions assuming 50th percentile TEQ uptake from soil/dust-related dermal and ingestion exposures indicate no measurable changes in serum lipid TEQ concentrations up to 1000 pg/g in soil/dust. Assuming 95th percentile uptake, the model shows no measurable serum lipid TEQ change up to 100 pg/g in soil/dust, but serum lipid TEQ levels rose ~2 pg/g at 1000 pg/g in soil/dust. Since the vast majority of soil/dust data were below 100 pg/g, Michigan children exposed to such soil/dust TEQ concentrations are not reasonably expected to exhibit measurable changes in serum lipid TEQ concentrations when compared to typical background dietary exposures. With adequate data, this approach can be applied to evaluate child dose and risk for other persistent chemicals.

Analysis of worldwide regulatory guidance values for the most commonly regulated elemental surface soil contamination

Guidance values are used worldwide to regulate exposure to surface soil contamination. This analysis examines values applied to element contamination. In the United States, element guidance values have been promulgated by at least 6 federal agencies, 46 states and several regional, city, county, territorial, and autonomous Native American jurisdictions. Guidance values have also been promulgated in at least 71 other United Nations member states. A total of 5949 guidance values for 57 elements have been identified. These values are characterized, and detailed analyses are provided for the eight most frequently regulated elements (Pb, Cd, As, Ni, Cr, Hg, Cu, and Zn). More than 300 guidance values available for each of these are tabulated. These values span from 3.6 orders of magnitude for lead to 6.9 orders of magnitude for arsenic and are distributed in patterns that resemble those of lognormal random variables. However, nonrandom value clusters are also identified in each distribution. On average, the values used in the U.S. are higher than those used elsewhere around the world for noncarcinogenic contaminants, but are lower for carcinogenic contaminants. Approximately 31.4% of all values fall within uncertainty bounds computed from the U.S. Environmental Protection Agency cancer or noncancer risk model, but only 12.9% of carcinogenic arsenic and 7.5% of carcinogenic chromium (Cr-VI) guidance values fall within cancer risk model uncertainty bounds.

Worldwide regulatory guidance values for surface soil exposure to carcinogenic or mutagenic polycyclic aromatic hydrocarbons

Regulatory guidance values are used worldwide to control residential exposure to surface soil contamination. A total of 1,991 values used in 44 United Nations member states plus 4 territories or administrative regions to control exposure to seven polycyclic aromatic hydrocarbons (PAH) generally considered to be carcinogenic or mutagenic (benz(a)anthracene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, dibenzo(a,h)anthracene, chrysene and indeno(1,2,3-c,d)pyrene) are examined. The distributions of these values vary over 6.3-7.2 orders of magnitude and resemble the distributions of lognormal random variables. The U.S. Environmental Protection Agency (USEPA) values fall near the low end of these distributions and exert a modest influence on the values from other U.S. jurisdictions. On average, 17 additional jurisdictions use USEPA values and 21% of all guidance values fall within uncertainty bounds approximated from the USEPA risk model. An average of 62% of U.S. guidance values fall below the median value, but this varies from 43% for chrysene to 70% for dibenz(a,h)anthracene and benzo(a)pyrene. Although the magnitudes of guidance values differ, the ratios of values often reflect the relationships implied by benzo(a)pyrene Toxicity Equivalency Factors (TEFs). This influence is strongest in RGVs from U.S. jurisdictions.

Meta-analysis of mass-balance studies of soil ingestion in children

Ingestion of soil by young children may be an important source of exposure to environmental contaminants. Estimates of soil ingestion have been made by several studies using trace elements in a mass-balance approach, but differ substantially between studies and trace elements. We conduct a meta-analysis of four major mass-balance soil ingestion studies conducted on children between one and seven in the summer/fall in the northern United States. The analysis takes advantage of primary data from all studies, and provides a more complete description of soil ingestion among children. The meta-analysis uses data based on the two most reliable trace elements, Al and Si, that have passed a screening to identify and exclude measures with a high likelihood of bias. Details are described in a companion paper. The best linear unbiased predictor is used in a mixed model to estimate soil ingestion for study subjects. Overall, 11% of subject-periods are identified as outliers and excluded from the analysis. An analysis on 216 children based on Al and Si as tracer elements indicates that the mean (median) soil ingestion is 26 mg/day (33 mg/day), with the 95th percentile estimated as 79 mg/day. This systematic approach provides more reliable estimates than individual study results. There is some evidence that soil ingestion increases with a child's age, but insufficient data to distinguish soil ingestion by gender.

Equation reliability of soil ingestion estimates in mass-balance soil ingestion studies

Exposure to chemicals from ingestion of contaminated soil may be an important pathway with potential health consequences for children. A key parameter used in assessing this exposure is the quantity of soil ingested, with estimates based on four short longitudinal mass-balance soil ingestion studies among children. The estimates use trace elements in the soil with low bioavailability that are minimally present in food. Soil ingestion corresponds to the excess trace element amounts excreted, after subtracting trace element amounts ingested from food and medications, expressed as an equivalent quantity of soil. The short duration of mass-balance studies, different concentrations of trace elements in food and soil, and potential for trace elements to be ingested from other nonsoil, nonfood sources contribute to variability and bias in the estimates. We develop a stochastic model for a soil ingestion estimator based on a trace element that accounts for critical features of the mass-balance equation. Using results from four mass-balance soil ingestion studies, we estimate the accuracy of soil ingestion estimators for different trace elements, and identify subjects where the difference between Al and Si estimates is larger (>3 RMSE) than expected. Such large differences occur in fewer than 12% of subjects in each of the four studies. We recommend the use of such criteria to flag and exclude subjects from soil ingestion analyses.

Nickel tissue residue as a biomarker of sub-toxic exposure and susceptibility in amphibian embryos

Although low level exposure to physicochemical agents is the most common environmental scenario, their effects on living organisms are very controversial. However, there is an increasing need to integrate low level exposures from risk assessment to remediation purposes. This study focus on the possibility to employ Ni tissue residue values as biomarkers of sub-toxic exposure and susceptibility to this metal in a range of almost pristine to sub-toxic concentrations for Rhinella arenarum embryos. For that purpose, three batches of amphibian embryos were pretreated during 10 days with three increasing concentrations of Ni starting in 2, 8 and 20 microg Ni(2+) L(-1) and ending in 16, 64 and 160 microg Ni(2+) L(-1) (in natural fresh waters this value ranges from 2 to 10 microgL(-1); the LC(50)-24h for R. arenarum is 26.2mg Ni(2+) L(-1)). For the experimental conditions, the Ni tissue residue values at 360 h post exposure were 0.5, 2.1 and 3.6 microg Ni g(-1) embryo w/w, respectively, corresponding to BCFs of 31, 33 and 23. The susceptibility to Ni in those experimental embryos was evaluated by means of challenge exposures to three lethal concentrations of this metal (10, 20 and 30 mg Ni(2+) L(-1)), registering survival during the following 10 days of treatment. As a general pattern, the lower, intermediate and higher pretreatments with Ni resulted in enhanced, neutral and adverse effects on embryonic survival, respectively. Thus, sub-toxic exposure to Ni could modify the resistance of the amphibian embryo to this metal and Ni tissue residue values could be considered as biomarkers of both, exposure and susceptibility.

Exposure of Americans to polybrominated diphenyl ethers

Polybrominated diphenyl ethers, PBDEs, are a class of brominated flame retardants that, like other persistent organic pollutants (POPs), have been found in humans, wildlife, and biota worldwide. Unlike other POPs, however, the key routes of human exposure are not thought to be food and fish, but rather are from their use in household consumer products, and to the high levels of PBDEs found in house dust. The exposure of Americans to PBDEs was systematically evaluated in this study. First, exposure media data on PBDE congeners were compiled. Then, an adult intake dose was derived using exposure factors in combination with these data. The exposure pathways evaluated included food and water ingestion, inhalation, and ingestion and dermal contact to house dust. These intakes were converted to a body burden using a simple pharmacokinetic (PK) model. The predicted body burdens were compared with representative profiles of PBDEs in blood and milk. The adult intake dose of total PBDEs was estimated to be 7.7 ng/kg body weight/day, and children's estimated intakes were higher at 49.3 ng/kg/day for ages 1-5, 14.4 ng/kg/day for 6-11, and 9.1 ng/kg/day for 12-19. The much higher dose for the child age 1-5 was due to the doubling of dust ingestion from 50 to 100 mg/day. The predicted adult body burden of total PBDEs was 33.8 ng/kg lipid weight (lwt), compared to representative measurements in blood and milk at 64.0 and 93.7 ng/g lwt, respectively Most of this apparent underprediction in total concentration was due to an underprediction of the key congener, BDE 47. The value for BDE 47 half-life in the body was identified as the variable most likely in error in this exercise. Other congener predictions compared well with measurements, suggesting general validity with the approach. An important finding from this assessment is that the food intake estimate of about 1.3 ng/kg/day (of the 7.7 ng/kg/day total) cannot explain current US body burdens; exposures to PBDEs in house dust accounted for 82% of the overall estimated intakes.

Refinements on the age-dependent half-life model for estimating child body burdens of polychlorodibenzodioxins and dibenzofurans

We modified our prior age-dependent half-life model to characterize the range of child (ages 0-7) body burdens associated with dietary and environmental exposure to polychlorodibenzodioxins and furans (PCDD/Fs). Several exposure scenarios were evaluated. Infants were assumed to be either breast-fed or formula-fed from birth to 6 months of age. They then received intakes of PCDD/Fs through age 7 from foods based on weighted means estimates [JECFA, 2001. Joint FAO/WHO Committee on Food Additives. Fifty-seventh meeting, Rome, June 5-14 , 2001, pp. 24-40], and with or without exposures (ingestion and dermal) to urban residential soils at 1ppb TCDD toxic equivalents (TEQ). A one-compartment (adipose volume) toxicokinetic model for TCDD described by Kreuzer [Kreuzer, P.F., Csanady, Gy.A., et al., 1997. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and congeners in infants. A toxicokinetic model of human lifetime body burden by TCDD with special emphasis on its uptake by nutrition. Arch. Toxicol. 71, 383-400] was expanded to include the key non-TCDD congeners in human breast milk and adipose tissues, and two model parameter refinements were examined: (1) use of updated and more detailed age-correlated body fat mass data [CDC, 2000. Centers for Disease Control. CDC Growth Charts: United States. Advance Data from Vital and Health Statistics of the Centers for Disease Control and Prevention, National Center for Health Statistics, Number 314, December 2000]; (2) use of breast milk PCDD/F concentration data from sampling completed in 2000-2003 [Wittsiepe, J., Fürst, P., et al., 2004. PCDD/F and dioxin-like PCB in human blood and milk from German mothers. Organohalogen Compd. 66, 2865-2872]. The updated body fat mass data nearly halved the predicted peak body burden for breast-feeding and lowered the time-weighted average (TWA) body burdens from ages 0-7 by 30-40% for breast-fed and formula-fed infants. Combined use of the updated breast milk PCDD/F concentration and body fat mass data increased the contribution of breast-feeding but reduced TWA body burdens from diet and soil. We conclude that further refinements are needed, but reliance on these better data sets for body fat mass and breast milk PCDD/F concentration significantly improves the model's ability to accurately predict body burdens during early childhood.

New developments in exposure assessment: the impact on the practice of health risk assessment and epidemiological studies

The field of exposure assessment has matured significantly over the past 10-15 years. Dozens of studies have measured the concentrations of numerous chemicals in many media to which humans are exposed. Others have catalogued the various exposure pathways and identified typical values which can be used in the exposure calculations for the general population such as amount of water or soil ingested per day or the percent of a chemical than can pass through the skin. In addition, studies of the duration of exposure for many tasks (e.g. showering, jogging, working in the office) have been conducted which allow for more general descriptions of the likely range of exposures. All of this information, as well as the development of new and better models (e.g. air dispersion or groundwater models), allow for better estimates of exposure. In addition to identifying better exposure factors, and better mathematical models for predicting the aerial distribution of chemicals, the conduct of simulation studies and dose-reconstruction studies can offer extraordinary opportunities for filling in data gaps regarding historical exposures which are critical to improving the power of epidemiology studies. The use of probabilistic techniques such as Monte Carlo analysis and Bayesian statistics have revolutionized the practice of exposure assessment and has greatly enhanced the quality of the risk characterization. Lastly, the field of epidemiology is about to undergo a sea change with respect to the exposure component because each year better environmental and exposure models, statistical techniques and new biological monitoring techniques are being introduced. This paper reviews these techniques and discusses where additional research is likely to pay a significant dividend. Exposure assessment techniques are now available which can significantly improve the quality of epidemiology and health risk assessment studies and vastly improve their usefulness. As more quantitative exposure components can now be incorporated into these studies, they can be better used to identify safe levels of exposure using customary risk assessment methodologies. Examples are drawn from both environmental and occupational studies illustrating how these techniques have been used to better understand exposure to specific chemicals. Some thoughts are also presented on what lessons have been learned about conducting exposure assessment for health risk assessments and epidemiological studies.

Age- and concentration-dependent elimination half-life of 2,3,7,8-tetrachlorodibenzo-p-dioxin in Seveso children

OBJECTIVE

Pharmacokinetic and statistical analyses are reported to elucidate key variables affecting 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) elimination in children and adolescents.

DESIGN

We used blood concentrations to calculate TCDD elimination half-life. Variables examined by statistical analysis include age, latency from exposure, sex, TCDD concentration and quantity in the body, severity of chloracne response, body mass index, and body fat mass.

PARTICIPANTS

Blood was collected from 1976 to 1993 from residents of Seveso, Italy, who were < 18 years of age at the time of a nearby trichlorophenol reactor explosion in July 1976.

RESULTS

TCDD half-life in persons < 18 years of age averaged 1.6 years while those > or =18 years of age averaged 3.2 years. Half-life is strongly associated with age, showing a cohort average increase of 0.12 year half-life per year of age or time since exposure. A significant concentration-dependency is also identified, showing shorter half-lives for TCDD concentrations > 400 ppt for children < 12 years of age and 700 ppt when including adults. Moderate correlations are also observed between half-life and body mass index, body fat mass, TCDD mass, and chloracne response.

CONCLUSIONS

Children and adolescents have shorter TCDD half-lives and a slower rate of increase in half-life than adults, and this effect is augmented at higher body burdens.

RELEVANCE

Modeling of TCDD blood concentrations or body burden in humans should take into account the markedly shorter elimination half-life observed in children and adolescents and concentration-dependent effects observed in persons > 400-700 ppt.