One

of the basic steps in the determination of exposure–health associations is exposure assess-ment. With exposure scenarios, when the inception of disease (onset of disease pathol-ogy) has occurred in the past, it becomes nec-essary to estimate the exposure level during the specific window of time in the past in which disease inception took place. Properties of some contaminants facilitate the use of back-ward extrapolation to estimate past exposure. One such property is persistence, wherein the concentration of the contaminant in the host is maintained over a defined period of time. If the substance in question is stable and resis-tant to metabolism, a long exposure half-life will result. With continuing intake, such chemicals will bioaccumulate (Swain 1983). Additionally, bioaccumulated substances will increase when transferred through successive levels of the food chain, a process known as biomagnification. Hence, contaminant con-centrations rise as a result of persistence, bio-magnification, and bioaccumulation. Complicating this scenario, however, are initial ignorance of environmental contamina-tion and later intervention to limit the presence of toxicants in the environment. These phases may introduce variations in environmental exposure levels over time. For instance, concen-trations of toxicants in human food would fol-low an increasing trend until regulated, thus resulting in decreasing concentrations. Second, people may vary in their decision to continue using potentially contaminated products. Thus, backward exposure estimation becomes more difficult because it has to integrate persistence of human body burden (long half-life) and exposure levels in the environment or food that vary over time, as well as the amount and duration of product use over time. One example of this type of scenario is continuing consumption of fish that themselves are char-acterized by varying polychlorinated biphenyl (PCB) levels. However, regarding persistence, various halogenated organic compounds share similar characteristics. PCBs possess half-lives ranging from < 1 year up to 71 years and are capable of ascending the food chain and accumulating in human tissues possessing high lipid concentra