Special Report on Completion and Validation Child-Specific Exposure Factor in Taiwan
Harmful substances in soil and dust may enter into human body through direct inhalation, digestion, and dermal contact causing adverse health effects. Owing to the difference in physical and physiological behaviors, infants and toddlers are not only easier to expose to toxic substances in soil, but also less tolerant to toxic substances than adults. Therefore, toxic substances will pose higher risk to children. We have already acquired the soil/dust ingestion data of 0~6 years old children and several parameters pertaining to dermal contact in the past programs. The assessments conducted to explore the influence on risks to the default parameters of SGM’s health risk assessment system indicated that inputting the mean soil ingestion rate and soil-skin adherence factor (all activity groups) derived from this project resulted in a 20-fold decrease in the estimated risks via soil ingestion and dermal absorption exposure, respectively. That is, the soil ingestion rate was an important parameter for risk assessment. Sensitivity analyses revealed that the four most important variables contributing to variability in the predicted soil and dust ingestion values are the soil-skin adherence factor, the skin-soil/surface contact rate, the hand-to-mouth contact frequency, and the fraction of hand area mouthed. In this study, the range of pH value, broad ranges in the contents of organic matter and clay of soil samples were shown among the 45 contaminated soils. The pH value ranged from 4.36 to 8.11. The content of organic matter ranged from 0.02 to 5.56 %. The content of clay ranged from 6.6 to 53.0 %. Based on the in-vitro digestion method, i.e., simple bioaccessibility extraction test (SBET), the average bioaccessibility of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn is 25.9%, 46.3%, 5.5%, 45.0%, 22.0%, 11.2%, 42.3%, and 25.3%, respectively. The results of multiple linear regression model showed that the types and motilities of the metals, pH, organic matter content, clay content, and ferric oxide content of soil had significant impacts on the SBET bioaccessibility of metals. Based on the research of bioavailability and bioaccessibility, EPA's Superfund office (OSWER) lowered the default clean-up value from 100% to 60%, saving billions of dollars in cleanup costs while being protective of human health. Consequently, the bioaccessibility of metals should be considered and used to avoid the overestimates of health risk. In this study, we further found that Cd had the higher bioaccessbility than that of other metals due to its high motilities. Additionally, Cd has been well known as a carcinogen for human. Recent surveys also showed that Cd-contaminated soils account for the high proportion; the background concentration of Cd was also high in soils in Taiwan. Cd can readily access into corps, which threaten human health via Cd-containing rice intake. Based on the available information, we recommend to further assess the bioaccessibility and oral bioavailability (via animal model) of Cd in contaminated soils for future application in human health risk assessments in Taiwan.
Local exposure factors, Bioaccessibility, SHEDS model, Simple bioaccessibility extraction test