Introduction

Abstract

It is important, at the outset, for an exposure assessor to consider issues of uncertainty and variability. Uncertainty about the accuracy of exposure and dose estimates and variability in key parameters are critical aspects of every assessment, which must be taken into account when constructing realistic scenarios, selecting appropriate human exposure factors, and interpreting and communicating results. The USEPA recommends that exposure assessors distinguish between uncertainty and variability, where uncertainty represents a lack of knowledge about factors affecting exposure or risk and variability represents true heterogeneity across people, places, and time. Uncertainty is an important issue for assessors because it can lead to inaccurate or biased estimates, whereas variability is important because it can affect the precision of estimates and influence the degree to which they can be generalized. Depending on the circumstances, uncertainty and variability can complement or confound one another (USEPA 1995, 1996).