Just passing through!

According to movies about the Old West, the town sheriff made it a habit to ask strangers in town about their intentions. The response "just passing through" was meant to reassure the sheriff and the local populace the stranger wanted no trouble, and would soon be gone. "Just passing through" could be the exit line for most of the substances that get into our bodies from our daily eating, drinking, breathing and touching. Humans and other living things function by breaking down, or metabolizing, their daily intake, extracting what is useful, and exhaling or excreting the rest in fairly short order. That goes for potential toxins we take in, too. Our body's systems are so good at breaking down or excreting the hundreds of natural and man-made materials we consume every day, some intentionally and some not, that relatively few are able to do any actual harm at the doses we are exposed to. Nature also does a good job of breaking down most of the substances that get into the environment.

But there are a few substances that don't just pass through. They aren't readily broken down in the environment, or in living things. These "strangers" stay on for a while. Man-made chemicals that don't just pass through and are potentially harmful when they accumulate in the body or the environment are put in a special category by government regulators. They are called POPs - Persistent Organic Pollutants. The U.S. Environmental Protection Agency uses different terminology - PBT, which stands for Persistent, Bioaccumulative, and Toxic. Both classifications mean essentially the same thing.

A number of governmental bodies have composed lists of these materials with the intent of restricting or eliminating their use. As part of the Stockholm Convention, the United Nations has composed such a list. It presently contains 12 materials, most of them pesticides no longer sold in the U.S. and either banned or restricted in most other areas of the world. EPA also lists 12 materials, including mercury and lead, on its PBT list.

With increasing emphasis on human and environmental health issues, governments try to protect their populaces from POP or PBT materials, and tend to build upon lists already in existence. Unfortunately, lists that are built upon incorrect lists are also incorrect. Also, improvement in analytical techniques that allow the detection of many substances at increasingly lower levels tends to encourage the addition of materials to such manufactured lists, whether or not they fit the definition of a POP or pose a valid concern. According to some people, the mere detection of their presence is sufficient to warrant listing. But, unless the properties of substances are thoroughly evaluated with respect to agreed-upon criteria, some materials may be improperly listed. As a result, 1) some materials are unnecessarily "labeled" as POPs or PBTs., and 2) the list of POP's and PBT's becomes less meaningful and useful as less harmful materials are added to the list and potentially prevented from providing the benefits they were intended to deliver as useful products.

Some advocate expanding the definition of a POP to include phrases such as "presence in remote areas," and "ubiquitous occurrence." The argument goes like this: if certain chemicals are more or less always detectable in the environment because they are in very wide use, isn't that the same as being persistent and in itself a risk?

The answer is: NO. Not if they are present only at trace levels and well below regulatory safety levels, and not if they break down and disappear as fast as they arrive. That is, not if they are just passing through. By definition, chemicals that persist build up in the environment, and accumulate in living things, perhaps to toxic levels. It is the resistance of certain toxic substances to breakdown, and the tendency to remain and increase in concentration in the environment or in living organisms over time, that is the essence of a POP or a PBT. If a chemical is always present but not persistent in the environment and if it does not tend to accumulate in living things, even consistent presence of materials below safety levels would not pose a concern. Just like the cinematic sheriff, the body can readily handle the small amounts of materials that are just passing through.

Last Updated: October 16, 2003