Quantitative approaches for health risk assessment of environmental pollutants : estimation of differences in sensitivity, relative potencies, and margins of exposure

Abstract

Historically, quantitative health risk assessment of chemical substances is based on deterministic approaches. For a more realistic and informative health risk assessment, however, the variability and uncertainty inherent in measurements should be taken into consideration. The variability and uncertainty may be assessed by applying probabilistic methods when performing dose-response assessment, exposure assessment and risk characterization. The benchmark dose (BMD) method has been suggested as an alternative to the no observed (adverse) effect level (NO(A)EL) approach in dose-response assessment of non-cancer health effects. In contrast to the NO(A)EL that is limited to being one of the experimental dose levels, the BMD is estimated as the dose corresponding to a predetermined change in response, according to a model fitted to the dose-response data.

In the present thesis, quantitative differences in sensitivity between dioxin sensitive Long- Evans (L-E) and dioxin resistant Han/Wistar (H/W) rats following long-term exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was investigated. Sensitivity differences were analyzed by comparing BMDs for the two strains, considering a number of conventional toxicological endpoints, endpoints relevant for the endocrine system, and a group of bone parameters. Differences between the strains were most pronounced for hepatic foci; L-E rats were approximately 20-40 times more sensitive than H/W rats. For body and organ weight parameters, L-E rats were 10-20 times more sensitive than H/W rats. For retinoid parameters and hepatic CYP1A1 induction, estimated differences between the strains were generally about 5-fold. For bone effects, significant strain differences were observed with the L-E rat being the most sensitive strain. This difference was most pronounced (about 49-fold) for cross-sectional area of proximal tibia. It was also concluded that the BMD approach is a more suitable method for evaluation of bone parameters compare to the NOAEL approach. In another application, relative potency values (REPs) were established for a group of dioxinlike (DL) and non-dioxin-like (NDL) polychlorinated biphenyl (PCB) congeners as the ratio between BMDs, median effective doses (ED50s), or NOELs. This analysis was based on increased liver weight, decreased hepatic vitamin A levels, and increased hepatic EROD activity. The findings indicated that the BMD approach results in more reliable REP values compared to the ED50 and NOEL approaches. The BMD approach also provides more information about the precision of the estimated REP values by the calculation of a two-sided 90% confidence interval; a confidence interval may also be established for a ED50 ratio but not for a NO(A)EL ratio. Overall findings in this analysis supported further development and use of endpoint specific systems for assessment of human exposure to mixtures of chemicals with similar as well as different mode-of-actions.

Finally, the potential health impact of a group of PCBs was characterized by estimating the cumulative margins of exposure (MOEs) for the adult Swedish population. A cumulative MOE distribution was quantified by simultaneous integration of a reference dose (RfD) distribution and a distribution for the human dietary exposure. Both a relative potency factor (RPF) based approach and an RPF-free approach were used for estimating the cumulative MOE. Results indicated that the cumulative MOE could be up to four times lower for women compared to men. The cumulative MOE reflected the MOE for PCB 126; other PCB congeners had little contribution. Compared to conventional MOE approaches, the newer approaches considered herein provide an improved tool under which potential health concerns can be assessed by accounting for both variability and various uncertainties, quantitatively, contributing to improving cumulative health risk assessments for the human population.