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Another question for Congress is whether to ban a plastic additive from food and drink containers. It's not clear whether this additive, called BPA, is dangerous to people, but lawmakers who support the ban say it's justified by something called the precautionary principle. Trouble is, people have widely different views of what the precautionary principle is and how it should be applied. NPR's Jon Hamilton has the story.

JON HAMILTON: The term precautionary principle gets tossed around a lot in discussions of toxic chemicals. But what does it mean? Senator Dianne Feinstein invoked it when she introduced the BPA bill last month. She said, quote, "If you do not know for certain the chemical is benign, it should not be used," close quote. But that's a standard that no regulatory agency could follow.

Dr. TED SCHETTLER (Science and Environmental Health Network): It's often almost impossible to prove that something will never happen.

HAMILTON: Dr. Ted Schettler is with the Science and Environmental Health Network, an advocacy group that wants to reduce people's exposure to BPA. But he says Feinstein has the precautionary principle wrong. His version goes like this.

Dr. SCHETTLER: When there are credible threats of harm from some proposed activity, precautionary action should be taken even if some cause and effect relationships are not fully understood.

HAMILTON: So if preliminary scientific research indicates there's a risk, the principle says we should consider taking action even if there's no direct evidence of harm.

Dr. SCHETTLER: It does ask you to look carefully at what you know, whether there are alternatives, and then look at the range of precautionary actions that are available to you.

HAMILTON: So where would that interpretation lead BPA? What we know is that BPA acts like a weak form of estrogen. And it's so widely used that low levels can be detected in almost everyone. We don't know whether low levels are a problem for people. But high levels in animals can cause reproductive abnormalities and cancer. Finally, a lot of products, like baby bottles, can be made without BPA. So, Schettler says…

Dr. SCHETTLER: In my view, we've gathered enough evidence to say that we know enough to act.

HAMILTON: But some backers of the precautionary principle don't agree. The European Union, unlike the U.S., explicitly requires some government agencies to follow the precautionary principle, and just last year its Food Safety Authority reaffirmed its view that BPA is safe. Jonathan Wiener is a professor of law, environmental policy and public policy at Duke University. He says you might expect Europe to be more cautious than the U.S.

Professor JONATHAN WIENER (Duke University): On the contrary. What we find is that there's a complex pattern of particular precautions applied to particular risks on each side of the Atlantic.

HAMILTON: Wiener says Europe is more cautious about chemicals. In 2006, it began requiring companies to register the chemicals they use, gather safety information, and switch to safer alternatives when possible. But when mad cow disease started killing people in Europe a few years ago, it was the U.S. that was more cautious. Wiener says the U.S. not only halted beef imports from affected countries, it banned blood donations from anyone who had spent much time in Europe.

Prof. WIENER: A highly precautionary strategy given that the evidence of transmission through blood transfusions was very preliminary and the countervailing risk of not having enough blood in hospital trauma centers was quite real.

HAMILTON: Wiener says the U.S. also has tougher standards for certain types of air pollution and acted sooner to get rid of chemicals that damage the Earth's ozone layer. Linda Birnbaum, who directs the National Institute of Environmental Sciences and the National Toxicology Program, has spent years pondering the precautionary principle. She describes it this way.

Ms. LINDA BIRNBAUM (National Toxicology Program): The precautionary principle says that you act in the presence of concerning information.

HAMILTON: And of course, concerning means different things to different people, which is why applying the principle can be tricky. Birnbaum says she personally finds the information about BPA somewhat concerning, and she is most concerned by studies showing that the chemical tends to leach out of plastic that's been heated.

Ms. BIRNBAUM: So I stopped microwaving in plastic many years ago, not because I was convinced it was going to cause harm, but because it just wasn't a necessity.

HAMILTON: And she didn't have to wait for a new law to act.

Jon Hamilton, NPR News.

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