Individual-Level Heterogeneity in Environmental Risk Assessment Models.

Abstract

In this work, I demonstrate the importance of incorporating heterogeneity in risk assessment through the examination of three scenarios of environmental hazards: 1) individual exposure estimates for persistent pollutants; 2) population norovirus risk; and 3) village prevalence of dengue. 1) This study provides congener specific reference half-life values for 29 PCDD, PCDF, and PCB congeners. We use a linear interpolation of these reference values as a basic model of half-life estimation based on age, and add complexity to account for body fat, smoking status and breastfeeding, resulting in a method of individual half-life estimation that incorporates individual heterogeneity. This model can be used to more accurately estimate past exposures and therefore provide a better understanding of individual risk from these chemicals. 2) Models of norovirus transmission often assume a fast rate of recovery. We created a realistic, empirical distribution of norovirus shedding duration with data from the literature. We find that regular shedding individuals shed on average 14 – 16 days, while long shedders shed on average 105 – 136 days. With transmission models we show that the presence of long-shedders increases probability of an outbreak by 33%, the severity of transmission by 20%, and transmission duration by 100%. We demonstrate that ignoring heterogeneity in transmission parameters can lead to underestimates of population norovirus risk. 3) We examined collected serological data from a rural region of costal Ecuador to examine how heterogeneity in road access can affect dengue risk. We found that prevalence of IgG DENV antibodies ranged from 18% to 57%, with most villages between 25% to 35%. We examined potential factors that could lead to heterogeneity, including age distributions and rates of travel to areas of high dengue transmission, but did not find an association with either of these factors. The heterogeneity of infection rates through this region indicate that a village-focused intervention strategy may be more efficient at reducing overall infection than one directed at the household or regional level. By understanding the important underlying variability in these systems, we can create more accurate risk estimates and more informative decisions for the protection of human health.