Pollutants that are persistent, bioaccumulative, and toxic (PBT), of which a good number are endocrine disruptors, are long-lasting, and can build up in the food chain to levels that are detrimental to human and ecosystem health. Some of these substances can be transported long distances from one region to another. This group of chemicals is often referred to by U.S. EPA and other authorities as PBT pollutants, or simply as PBTs.

PBTs are of great concern because they can lead to severe adverse health outcomes in humans, such as endocrine disruption, even from low levels of release and exposure. Such a concern is actually the focal point of this three-part lecture. This point thus shall be further addressed later. In any case, because of such a concern, several regulatory bodies have established numerical criteria for measuring a substance’s persistence and bioaccumulation.

For example, according to the Canadian Environmental Protection Act (CEPA, 1999), a substance is considered to be persistent when its half-life in air is not less than 2 days, or when it is subject to atmospheric transport from its source to a remote area, or when its half-life in water or soil exceeds 180 days, or when its half-life in sediment exceeds 364 days.

The UK government (U.K. DEFRA, 2002) adopts a similar set of numerical criteria for qualifying a substance as a PBT. The numerical criteria used by U.S. EPA (1999, 2000) are somewhat more health conservative. All in all, many regulatory authorities have a focus more on water, soil, and sediment than on air as the media. This is because atmospheric compartments are unlikely the actual places for long-term chemical bioaccumulation.