The concept of persistence and its modeling discussed thus far is but the tip of the iceberg. The intent of the previous slides is not to belabor students with the details of the conceptual or technical complexities involved. Rather, they are intended for an acceptance of such complexities as well as for an appreciation of the tremendous works that have been done by so many entities in this field.

The discussion in the literature on persistence, bioaccumulation, and toxicity thus far links to adverse health effects in general more than to endocrine disruption. Yet frankly, this lecture has a strong bias towards the latter. In fact, its ultimate goal is to serve as one of the few places that have specifically recognized the health impacts of persistent, bioaccumulative, and toxic (PBT) pollutants present at low concentrations in the environment. For certain types of adverse health effects such as endocrine disruption in particular, even low levels of PBTs in the environment collectively are still of global concern.

There are at least two reasons for such a concern. First, by comparison with other types of adverse health effects, the inducibility of endocrine disruption is not chemical-selective. Second and more importantly, until proven otherwise, the inducibilities of some multiple PBTs are likely additive, if not synergistic. In other words, if two PBTs are antiandrogenic even if they are chemically very different or if they reside in separate places or media, the antiandrogenic effects from subsequent exposures to both of these endocrine disruptors are likely cumulative. It is based on this premise that, for many environmental pollutants, both persistence and bioaccumulation are said to play an important role in endocrine disruption.