A second tool scientists use in deciding if a chemical is carcinogenic are studies that look at broad populations to see if people exposed to the chemical are more likely to get cancer. This is the field of epidemiology, which has made huge contributions to health over the last century and a half. Unfortunately, when epidemiology finds that one thing is associated with another, it is often not because the first thing causes the second. Criminals and tattoos, for example, are highly correlated, but tattoos do not cause crime. And so when epidemiologists showed shipyard workers who handled asbestos had higher rates of cancer, it was a strong clue that asbestos causes cancer, but it was not final proof. That came with later work. “Epidemiology is fantastically difficult,” Bruce Ames says. “You’re talking about studying humans, and there are a million confounders. A study will say this and another will say the opposite.” The steady proliferation of studies saying that one thing is “linked” to another—that there is a correlation, but nothing more— invites abuse. “You can easily hype it up. Our local paper has a scare story every couple of weeks. They like those scare stories. But I don’t believe any of them.” Ames cites the example of a controversy in California’s Contra Costa County. “There are a lot of refineries and there is more lung cancer. Ah, the refineries are causing the lung cancer. But who lives around refineries? Poor people. And who smokes more? Poor people. And when you correct for smoking, there’s no extra risk in the county.”
The limitations of animal testing, epidemiology, and other forms of evidence are why regulatory agencies have classification systems for carcinogens. The terms vary, but they typically label a substance a “potential,” “likely,” or “known” carcinogen. These levels indicate degrees of certainty, based on what the evidence looks like when it is all assembled. Scientists today place much less weight on any one piece of evidence, and high-dose tests on lab animals have particularly fallen out of favor. They’re still treated as valid evidence, but they don’t carry nearly as much weight as laypeople think they do.
Aside from Gut’s ancient aversion to contamination, culture also drives the perception that exposure to traces of synthetic chemicals is dangerous. Corporate marketers love to label products “natural” because they understand that natural is equated with wholesome and nurturing—and safe. “People have this impression that if it’s natural, it can’t be harmful, and that’s a bit naive,” says Bruce Ames. “The other night in Berkeley, I noticed they were selling a bag of charcoal briquettes and it said ‘no artificial additives, all natural.’ And it’s pure carcinogen!”
In Daniel Krewski’s 2004 survey, “natural health products” were deemed by far the safest of the thirty presented—safer even than X-rays and tap water. Prescription drugs were seen to be riskier, while pesticides were judged to be more dangerous than street crime and nuclear power plants. It’s not hard to guess at the thinking behind this, or to see how dominated it is by Gut. “Natural” and “healthy” are very good things so natural health products must be safe. Prescription drugs save lives, so while they may not be as safe as “natural health products”—everyone knows prescription drugs can have adverse effects—they are still good and therefore relatively safe. But “pesticides” are “man-made” and “chemical” and therefore dangerous. The irony here is that few of the “natural health products” that millions of people happily pop in their mouths and swallow have been rigorously tested to see if they work, and the safety regulations they have to satisfy are generally quite weak—unlike the laws and regulations governing prescription drugs and pesticides.
Many companies—and whole industries, in the case of organic foods— actively promote the idea that chemicals are dangerous. “What you don’t know about chlorine could hurt you,” warns the Web site of a Florida company selling water purification systems that remove by-products of chlorine treatment that may raise the risk of cancer by some infinitesimal degree. On occasion, politicians also find it convenient to hype the risk of chemical contamination. “Arsenic is a killer,” declared U.S. congressman Henry Waxman. “If there is one thing we all seem to agree on it is that we do not want arsenic in our drinking water.” Arsenic happens to be common in the natural environment, and tiny levels of it are often found in drinking water—but in the spring of 2001, Waxman suddenly found that fact intolerable after the Bush administration suspended the existing regulation on permissible levels of arsenic in water and ordered more study. This was a dispute over how much arsenic content was safe, not whether there should be any at all, but the congressman and his Democratic colleagues saw obvious advantage in framing it as Bush-wants-to-put-poison-in-the-water.
The media, in pursuit of the dramatic story, are another contributor to prevailing fears about chemicals. Robert Lichter and Stanley Rothman scoured stories about cancer appearing in the American media between 1972 and 1992 and found that tobacco was only the second-most mentioned cause of cancer—and it was a distant second. Man-made chemicals came first. Third was food additives. Number 6 was pollution; 7, radiation; 9, pesticides; and 12 was dietary choices. Natural chemicals came sixteenth. Dead last on the list of twenty-five—mentioned in only nine stories—was the most important factor: aging. Lichter and Rothman also found that of the stories that expressed a view on whether the United States was facing a cancer epidemic, 85 percent said it was.
This has a predictable effect on public opinion. In November 2007, the American Institute of Cancer Research (AICR) released the results of a survey in which Americans were asked about the causes of cancer. The institute noted with regret that only 49 percent of Americans identified a diet low in fruits and vegetables as a cause of cancer; 46 percent said the same of obesity; 37 percent, alcohol; and 36 percent, diets high in red meat. But 71 percent said pesticide residues on food cause cancer. “There’s a disconnect between public fears and scientific fact,” said an AICR spokesperson.
Lichter and Rothman argue that the media’s picture of cancer is the result of paying too little attention to cancer researchers and far too much to environmentalists. As John Higginson noted almost thirty years ago, the idea that synthetic chemicals cause cancer is “convenient” for activists opposed to chemical pollution. If DDT had threatened only birds, Rachel Carson would probably never have created the stir she did with Silent Spring. It’s the connection between pollution and human health that makes the environment a personal concern, and connecting synthetic chemicals to health is easy because the chemicals are everywhere, and Gut tells us they must be dangerous no matter how tiny the amounts may be. Add the explosive word “cancer” and you have a very effective way to generate support for environmental action.
However laudable the ultimate goal, many experts are not pleased with what environmentalists have been telling the public about chemicals and health. “This is irresponsible, hysterical scaremongering,” said Alan Boobis, a toxicologist with the faculty of medicine of Imperial College, London, in 2005. Boobis and other leading British scientists were furious that several environmental organizations, particularly the World Wildlife Fund, were massively publicizing the presence of “hazardous chemicals” in blood, food, and even babies’ umbilical cords. “Most chemicals were found at a fraction of a part per billion. There is no evidence such concentrations pose any threat to people’s health,” Boobis told The Guardian. “The message they are putting across is misleading, and deliberately so,” David Coggon, a specialist in occupational and environmental causes of cancer and other diseases at the University of Southampton, told the BBC. “By and large, I think people shouldn’t be worried. Most chemicals will not do any great harm at these very low levels,” added Richard Sharpe, an expert on endocrine disruptersat the Medical Research Council’s Human Reproductive Unit in Edinburgh. “You have to put this in perspective.”