Student Research: Eva Wong

Use of Children's Activity Patterns in Dermal Soil Exposure Assessment


When performing human health risk assessments, dermal-soil risks frequently need to be calculated. Current regulatory guidance utilizes multiple, non-event specific point estimates of input parameters to produce a single, deterministic estimate of exposure. The Soil Contact Rate (SCR) is proposed as a metric of dermal exposure to soil. The children's SCR utilizes realistic activity pattern data taken from the second Soil Contact Survey (SCS-II), performed in 1998-99. The SCS-II was designed as a nationally representative telephone survey used to gather data regarding the frequency and duration of outdoor play as well as the handwashing and bathing frequency of a randomly chosen child in the household. Respondents reported that their children played outside on grass or dirt surfaces a median of 7 days/wk, 3 hours/day in warm weather and 3 days/wk, 1 hour/day in cold weather months. Respondents reported a median hand wash frequency of 4 times/day and a median bath/shower frequency of 7 times/wk in both warm and cold weather. Washing frequencies were gathered in order to calculate a delay time, the time from the end of an activity leading to soil exposure until soil is removed via washing. Consideration of a delay time can lead to more realistic and increased estimates of total soil exposure time. Based upon survey and empirical data, distributions of activity pattern data, clothing data, and soil adherence were utilized in a two-dimensional Monte Carlo model. Use of two-dimensional Monte Carlo modeling allowed quantification of the contributions of variability and uncertainty to the SCR. Utilizing this methodology, the SCR for player children 5 years or less was calculated to have a median of ~48 g·hr/mo. For comparison, default exposure factor values specified for EPA's reasonably maximally exposed individual scenario can also be used to calculate an SCR. The calculated default equivalent of 406 g·hr/mo corresponds to the 93rd percentile of the median distribution reported here.