POPs Culture.

• Author: McGinn, Anne Platt

If there's one form of industrial innovation that we can definitely do without, it's the kind that is continually producing new Persistent Organic Pollutants--toxins so potent and durable that current emissions may still be causing cancer and birth defects 1,000 years from now.

Between 1962 and 1970, U.S. soldiers and their South Vietnamese allies sprayed nearly 12 million gallons of herbicide over vast tracts of Southeast Asian forest and more than half of South Vietnam's arable land. The program was designed to eliminate any cover that might conceal North Vietnamese Army units or Viet Cong guerrillas. The crews on the planes that did the spraying devised a slogan for themselves--a variation on a famous "Smokey the Bear" public service message back in the United States. They said, "only you can prevent forests."

The herbicide came in orange-striped drums, so it was called "agent orange." It was a mixture of two chemicals: 2,4,5-T and 2,4-D, both of them commonly used herbicides at the time. As with complex synthetic chemicals in general, these herbicides contained trace amounts of various unwanted substances that arose as byproducts of production. Among the byproducts were some of the chemicals called dioxins. A 1985 report by the U.S. Environmental Protection Agency called dioxins "the most potent carcinogen ever tested in laboratory animals." More recent laboratory work has linked dioxins with birth defects, spontaneous abortion, and injury to the immune system. When those two herbicides were sold in the United States, they typically contained dioxin concentrations of about 0.05 parts per million. But agent orange had dioxin concentrations up to 1,000 times as high.

At the time, the spraying of agent orange seemed a relatively minor part of the conflict. The dioxins, however, will linger in Vietnam's soil long after the war has vanished from living memory. Yet no one is really sure how much damage has been done. Medical doctors in Vietnam do not, by and large, have the resources to carry out longterm public health studies, but some doctors report that in sprayed areas, certain birth defects have become more common: anencephaly (absence of all or part of the brain), spina bifida (a malformation of the vertebral column), and hydrocephaly (overproduction of cerebrospinal fluid, causing a "swelling" of the skull). Immune deficiency diseases and learning disabilities may also be higher in sprayed areas. And if the human damage is uncertain, the broader ecological impact is a complete mystery.

In part because it is so vague, the agent orange legacy illustrates some of the worst aspects of dealing with dangerous synthetic chemicals like dioxins. For purposes of environmental analysis, dioxins are grouped in a loose class of potent toxins known as POPs, short for "persistent organic pollutants." The full definition of a POP, however, is somewhat more complex than the acronym implies. In addition to being persistent (that is, not liable to break down rapidly), organic (having a carbon-based molecular structure), and polluting (in the sense of being significantly toxic), POPs have two other properties. They are fat soluble and therefore liable to accumulate in living tissue; and they occur in the environment in forms that allow them to travel great distances.

If you put all five of these properties together, you can begin to see the potential for "agent orange scenarios" in many places. We know that POPs are very dangerous, but we can never be sure exactly who will be injured by them. In the 1970s, for example, a group of children developed leukemia (a usually fatal blood disorder involving uncontrolled production of white blood cells) in Woburn, a small town in Massachusetts. The leukemia had apparently been caused by solvents in the tap water. But why did the disease emerge only in certain children and not in many others who also presumably ingested the solvents? It often takes sophisticated statistical analysis to find any connection at all between contamination and injury--that's one of the reasons it's so difficult to assess the public health risks from POPs. But of course statistics can't capture the experience of contamination: such a threat can seem like an evil lottery.

The apparent randomness of the threat is exacerbated by the fact that injury is often delayed or indirect. Extremely toxic chemicals can bide their time, then poison their victims in ways that are very hard to see. Benzene, for example, is a common solvent. It's an ingredient in some paints, degreasing products, gasoline, and various other shop and industrial compounds. If you're heavily exposed to it, you stand a heightened chance of developing cancer--and so may any children that you have after your exposure. That's true even if you're a man, since fetal exposure isn't the only way benzene may poison children: it can reach right into your chromosomes and injure the genes your child will inherit. Benzene may do its damage without ever touching the child directly at all.

POPs are potent ecological poisons as well. And just as in the human body, their ecological effects often exhibit a kind of weird indirection. In the United States in the 1960s, for example, biologists began to find strong field evidence that the pesticide DDT (dichlorodiphenyltrichloroethane) and similar chemicals were dangerous. But the evidence didn't come from the organisms that had absorbed the pesticides directly. It came from birds of prey--eagles and falcons--who were suffering widespread reproductive failure. Too few eggs and egg shells so thin they cracked soon after laying: these were the results of a type of indirect poisoning known as bioaccumulation. The fat solubility of the pesticides allowed them to concentrate in the tissues of their hosts as they moved up the food chain, from insects to rodents to raptors. Even today, the North American Great Lakes basin is showing the effects of certain POPs, like DDT, which have not been used in the region for decades. Eagle populations are still depress ed; tumors continue to appear in fish, birds, and mammals.

But there is one way in which the agent orange scenario deviates from the norm. Most POPs owe their presence in the environment not to the horrible exigencies of war, but to ordinary industrial processes--plastic and pesticide manufacturing, leaky transformers, waste incineration, and so forth. POPs are an inevitable byproduct of business as usual. By design and by accident, we are continually introducing new chemicals into the environment without any clear notion of what they will eventually do--or whether we may one day find ourselves in a desperate scramble to remove them. And among the tens of thousands of chemicals that have been in circulation for decades, relatively few have been studied for their health and environmental effects. Consequently, no one knows exactly how many POPs there are, but it's likely that many thousands of chemicals could qualify for the term.

And beyond their number is the question of their effect: while POPs are toxic by definition, their longterm health and environmental impacts are still largely unknown. Even more complex than evaluating individual POPs is the looming need to understand what kinds of synergistic interactions could be triggered by overlapping exposure--to multiple POPs or to POPs combined with other chemicals. Multiple contamination is the rule, rather than the exception, but virtually nothing is known about it. What we do know is that most of the world's living things are now steeping in a diffuse bath of POPs. And that almost certainly includes you. No matter where you live, you're likely to be contaminated by trace amounts of POPs. They're in your food and water; they may be in the air you breathe; they're probably on your skin...