Toxicokinetic Modeling as a Tool for Risk Estimation: 2,3,7,8-Tetrachlorodibenzo-P-Dioxin

Identifying Hazards and Risks of Soil Contamination

The effects of contaminated soils on human health have rarely been observed, although models usually predict high exposure from contaminated soil. Such overestimation mostly results from insufficient data on the bioavailability of the chemical, which largely depends on the mobility of the compound in soil and the absorption rate via direct uptake by humans or indirectly via the food chain. It is therefore questionable to regulate soil contamination on the basis of toxicologically derived control levels because the transfer rates from soil to humans are not predictable unless very specific information on the different parameters affecting this transfer are available. Instead, soil control levels should be derived from ecotoxicological considerations or a specific evaluation of the contaminated area should be performed if contamination significantly exceeds the average. Setting soil control levels by considering effects on ecosystems including plants and possibly animals is presently being discussed by the Beratergremium für umweltrelevante Altstoffe and has been proposed previously.

From Epidemiologic Exposure and Risk Assessment to Probabilistic Models: Experience with the Investigation of Health Effects of Soil Contamination in Germany

Exposure and risk assessment can be based on direct epidemiological investigations that study exposed individuals or on models. Examples from situations in Germany for both approaches are presented and discussed here. Epidemiological studies for the health risk assessment of contaminated soil typically have one or more of the following problems: the number of exposed persons is too small: critical confounders cannot be measured and controlled for appropriately; or the expected effects are not large enough to be detected. Therefore, at least for most situations in Germany, epidemiological investigations designed to measure health risks infrequently provide clear answers. In contrast, direct epidemiological investigations are more likely to be able to quantify exposure. If the soil concentrations are high, one is able to identify the location of relevant “hot spots” using biological and environmental monitoring. Further, epidemiology can be used to get information about the distribution of exposure in specific populations, such as young children or amateur gardeners. Exposure assessment by modeling provides answers in situations in which no reasonable answers can be expected using epidemiological means. The model gives quantitative estimates for average and reasonable worst-case situations. The latter can be determined by modeling probability distributions.

Coupled mother-child model for bioaccumulation of POPs in nursing infants

Bioaccumulation of persistent organic pollutants (POPs) leads to high levels in human milk and high doses of POPs for nursing infants. This is currently not considered in chemical risk assessment. A coupled model for bioaccumulation of organic chemicals in breast-feeding mother and nursing infant was developed and tested for a series of organic compounds. The bioaccumulation factors (BAFs) in mother, breast milk and child were predicted to vary with logK(OW) and, for volatile compounds, with K(AW) and concentration in air. The concentrations of POPs in the infant body increase the first half year to about factor 3 above mother and decline thereafter to lower levels. The predicted results are close to empirical data and to an empirical regression. The new mother-child model is compact due to its easy structure and the analytical matrix solution. It could be added to existing exposure and risk assessment systems, such as EUSES.

Elimination half-lives of selected polychlorinated dibenzodioxins and dibenzofurans in breast-fed human infants

Elimination half-life estimates for several polychlorinated dibenzodioxins/furans (PCDD/F) were calculated by modeling the blood and breast milk concentrations in two breast-fed human infants as reported by Abraham et al. (1996, 1998). Our analysis differs from that of other investigators in that we analyzed individual dioxin and furan congeners while the other studies considered TCDD only and we determined the half-lives in infants, rather than simply predicting body burdens in infants and older children. The average half-life values for each consistently measurable congener were determined to be less than about 6 mo and did not vary substantially between the two infants studied. The average elimination half-life values for 2,3,7,8-tetraCDD, 1,2,3,7,8-pentaCDD, 1,2,3,6,7,8-hexaCDD, 1,2,3,4,6,7,8-heptaCDD, and octaCDD were 0.40, 0.32, 0.39, 0.32, and 0.46 yr, respectively, and 0.27 yr for 2,3,4,7,8-pentaCDF. These values are in stark contrast with the 7 to 15+ yr values reported for these congeners in human adults (Michalek et al., 1996). These much shorter half-life values, likely attributable to rapid growth of the adipose tissue volume and enhanced fecal excretion of dioxins for breast-fed infants, explain why the much higher daily dioxin intake during breastfeeding does not translate to proportionately higher tissue concentrations. Thus, the shorter half-life of dioxins during breastfeeding needs to be considered when evaluating the dioxin hazard to children.