September 2011

Volume 20, Number 9

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PBT Chemicals-Persistent, Bioaccumulative, Toxic

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Illustration: Peter Harris

How can pesticides no one has used for decades be found in birds and fish in some of the most remote locations of the globe? These chemicals—along with many carpet treatments, flame retardants, and other additives still commonly found in building products—are persistent, bioaccumulative toxic chemicals (PBTs). While many toxic substances become less potent over time, PBTs can become more harmful the longer they persist due to their activity in ecosystems. Some, like arsenic, are naturally occurring, but many are human-made. Other naturally occurring ones, like mercury and lead, are released into the environment in greater concentrations due to human activities like burning coal.

Persistence means a chemical does not readily break down in the environment. PBTs can be transported long distances through air and ocean currents and the atmosphere. They can remain in soil and silt for decades, being absorbed by plants and microorganisms. The persistence threshold for testing and potential regulation by the U.S. Environmental Protection Agency (EPA) is a half-life longer than two months.

Bioaccumulation is the buildup of a substance in an individual organism. If a PBT remains in silt, bottom feeders take in small quantities, which accumulate in fatty tissue more quickly than they can be metabolized. Predatory fish then eat bottom feeders, storing larger quantities at higher concentrations in their own fat. The higher an animal is on the food chain, the more of the PBT the animal is likely to store and the more harm it may cause. Bioconcentration factor (BCF)—the ratio of a substance’s concentration in an organism to its concentration in surrounding water—is a common measure of bioaccumulation; EPA’s threshold for concern is a BCF of 1,000.

Toxicity includes harm not only to humans but also to individual animals and entire food chains. Toxic effects may include cancers, physical or behavioral reproductive problems, and damage to endocrine and nervous systems. PBTs known as persistent organic pollutants (POPs) include chemicals used in polystyrene insulation, interior fabrics, paint, carpets, and a multitude of materials containing plasticizers. Polyvinyl chloride (PVC), one of the most common building materials, releases many toxic substances during its manufacture and when it is burned; among these are dioxins—potent, carcinogenic POPs with other far-reaching effects on the immune, nervous, and endocrine systems.

Most signatories to the Stockholm Convention on Persistent Organic Pollutants, including the U.S. and Canada, have discontinued use of nine of the most potent PBTs, and the treaty limits use of three others, including the infamous pesticide DDT. Signed in 2001, the Stockholm Convention added nine new chemicals in 2009, a list that included perfluorooctanesulfonate (PFOS), formerly the main ingredient in Scotchgard stain repellent. Hexabromocyclododecane (HBCD), a flame retardant used in polystyrene insulation, is currently under review by the Stockholm Convention.

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August 30, 2011