Student Research: Jessica Levasseur
, Environmental Health (EH), 2015
Performance Evaluation of a Beta Version of USEPA’s SHEDS-HT Chemical Exposure Model
The United States Environmental Protection Agency (USEPA) is in the process of developing a high throughput (HT) model for exposure-based prioritization in the ExpoCast program to inform toxicity testing and chemical risk assessment. This mechanistic modeling approach is adapted from the prior Stochastic Human Exposure and Dose Simulation (SHEDS) framework, which was implemented in SAS. SHEDS-HT, written in R, reduces user burden and input demands by linking chemicals to particular exposure scenarios based on consumer product categories or food groups. SHEDS-HT rapidly generates probabilistic population distribution estimates of pathway-specific exposures, overall exposures, and intake doses. These predictions are based on particular exposure scenarios, a fugacity-based indoor environmental media model, and information from human activity databases. The predictive ability of SHEDS-HT is best evaluated by comparison to available biomarker measurements or to alternative predictions from prior measurement-intensive exposure studies. A key example of the latter is the USEPA Children’s Total Exposure to Persistent Pesticides and Other Persistent Organic Pollutants (CTEPP) study, which was conducted in Ohio and North Carolina. CTEPP provides both environmental and corresponding urinary biomarker data for a relatively large sample of commonly encountered commercial chemicals. NHANES also provides representative urinary biomarker data for some of the same compounds on a whole US population basis. However, biomarker data can reflect both exposure to a parent compound and direct exposure to its biomarker. Absence of environmental measurements for most biomarkers greatly reduces the number of compounds for which complete accounting for inputs and outputs can be conducted. In this evaluation of SHEDS-HT, focus is placed on 2,4-dichlorophenoxyacetic acid (2,4-D) and the parent compounds of the urinary biomarker 3,5,6-trichloro-2-pyridinol (TCPy), which include chlorpyrifos, chlorpyrifos methyl, and triclopyr.