ESTIMATING EXPOSURE TO METHYLMERCURY - EFFECTS OF UNCERTAINTIES

Uncertainties in exposures can lead to biased estimates of slopes and thresholds in the exposure-response relationships that are developed f rom regression analysis. This paper reviews published exposure and epi demiological studies of methylmercury (MeHg) from the perspective of t he accuracy and precision of the estimates used to represent the actua l doses received. Sources of such uncertainties, collectively referred to as ''exposure errors'', include instrumental and analytical errors , sampling and survey uncertainties, and individual variability in the relationships between the exposure metrics and the actual doses to ta rget organs. Because the relationship between maternal intake and the consequent dose to the fetal brain varies among individuals, epidemiol ogical studies of the effects of prenatal exposure must necessarily be accompanied by larger exposure uncertainties than comparable studies of effects on the mothers. The increased exposure errors typically res ult in attenuated slopes of the dose-response functions and under-esti mates of thresholds, so that part of the apparent increased sensitivit y of the fetus that has been developed from epidemiological studies ma y in fact be due to their inherently less certain exposures. Sources a nd magnitudes of exposure error found in the literature are discussed and their statistical ramifications are explored with Monte Carlo simu lations. The paper also finds that, after adjusting for exposure error , the relationship between dietary intake and blood concentration is c onsistent with an average half-life shorter than has typically been us ed and that using population averages yields a consistent but sub-line ar relationship between dietary intake of Hg and hair concentration. I nvestigators are urged to obtain (and present) data on more than one e xposure metric, so that their relative uncertainties may be assessed i ndependently.