EPA’s Stochastic Human Exposure and Dose Simulation (SHEDS) Model
Everyday activities – actions as simple as biting into an apple, or walking across a carpeted floor – may expose people to a host of chemicals through a variety of pathways. The air we breathe, the food and water we consume, and the surfaces we touch all can be sources of natural and synthetic chemicals, which can enter our bodies through our skin, digestive systems, and lungs. To make informed decisions that protect human health, EPA needs information regarding all the ways that multiple chemicals can enter the human body and how the human body responds.
EPA scientists have developed an innovative set of tools to estimate total exposures and risks people face from chemicals encountered in everyday activities. EPA’s Stochastic Human Exposure and Dose Simulation (SHEDS) model can estimate the range of total chemical exposures in a population from different exposure pathways (inhalation, skin contact, dietary and non-dietary ingestion) over different time periods, given a set of demographic characteristics. The estimates are calculated using available data, such as dietary consumption surveys; human activity data drawn from EPA's Consolidated Human Activity Database (CHAD); and observed chemical levels in food, water, air, and on surfaces like floors and counters. Data on chemical concentrations and exposure factors used in SHEDS are based on measurements collected in EPA field studies and published literature values.
EPA scientists have developed a number of different SHEDS models and modules that address specific kinds of chemical exposure. SHEDS-ATOX looks at exposures to toxic chemicals in the air, and SHEDS-PM is used to predict exposures to particulate matter. Another SHEDS model is SHEDS-Multimedia, which includes the SHEDS-Residential and SHEDS-Dietary modules.
Results and Impact
SHEDS is EPA’s principal research model for simulating human exposures to a variety of multimedia, multipathway environmental chemicals, including air pollutants, pesticides, metals, and persistent bioaccumulative toxins. The model enhances estimates of exposure in many different contexts, and has been used to inform EPA human health risk assessments and risk management decisions. To date, SHEDS models have been used successfully by EPA to:
- Improve pesticide-related risk assessments (e.g., n-methyl carbamates, pyrethroids)
- Evaluate risks to children posed by wood play-sets treated with chromated copper arsenate
- Improve risk assessments for chemicals in food
- Prioritize data needs, and
- Track air quality control measures in communities
Additionally, the SHEDS-Multimedia model has registered users in more than nine countries and 12 U.S. states including representatives from academia, industry, and consulting firms, and also individual citizens.