Temporal Trends in the General Population’s Intake Levels of TCDD: Calculated Based on Thirty Years of Biomonitoring Data. L. Aylward and S. Hays, Exponent
There are substantial uncertainties with using biomonitoring data to assess exposures (intake or absorbed dose), especially when the levels are changing with respect to time. It is always easiest to assess exposures when the biomarker being measured is constant over time (i.e., the individuals are at steady-state). However, when the biomarker of exposure is changing over time, the half-life of the compound must be taken into account to accurate reconstruct changes in absorbed dose that would result in the observed changes in the biomarker. Data on lipid levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the general population in the United States, Canada, Germany, and France have exhibited a steady decrease over the past 30 years by nearly a factor of 10 over this time period, with current lipid-adjusted TCDD levels of about 2 parts per trillion (ppt). To determine the absorbed dose that must have occurred among the general population to result in the trends in TCDD body burden, we used a simple one-compartment pharmacokinetic model. Model simulations indicated that absorbed intake levels of TCDD must have decreased by more than 95 percent from levels in 1970 to result in the observed decrease in human lipid levels, with the bulk of this decrease occurring before 1980. Based on this modeling and the pharmacokinetic properties of TCDD in humans, we conclude that mean levels of TCDD in the general population are likely to decrease further over the next 15 years, to between 0.5 and 1 ppt, even if intake levels do not decrease further. Effects of varying assumptions regarding initial intake, age of the population, and intake trends on the modeling will be discussed. These results exhibit why models must some times be used in conjunction with temporal trend data spanning several half-lives of the compound of interest before any meaningful conclusions can be made about absorbed dose when relying on biomonitoring data.
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