But that’s not how toxicologists see it. “All substances are poisons; there is none that is not a poison,” wrote Paracelsus in the sixteenth century. “The right dose differentiates a poison from a remedy.” That is the first principle of toxicology. Drink enough water and the body’s sodium and potassium levels can be thrown out of balance, inducing seizures, coma, and even death. Consume even very lethal substances in a sufficiently tiny portion and no harm will come of it—like the trillions of radioactive uranium atoms that are present in our bodies as a result of eating plants and drinking water that absorb naturally occurring uranium from the soil. What matters isn’t whether a substance is in us or not. It’s how much is in us. “It’s important for people to know that the amounts to which they’re exposed is the first thing they should think about,” says Lois Swirsky Gold, senior scientist at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and director of the Carcinogenic Potency Project at the University of California at Berkeley.
That perspective changes everything. When Paul Slovic surveyed toxicologists in the United States, the United Kingdom, and Canada, he found that large majorities said they do not try to avoid chemicals in their daily lives, they are not bothered by the presence of trace contaminants, and they do not agree that any exposure to a carcinogen means that the person exposed is likely to get cancer. The quantities of synthetic chemicals that turn up in blood analysis are almost always incredibly tiny. They are measured in parts per billion, sometimes even parts per trillion. To most toxicologists, they are simply too tiny to worry about.
But that doesn’t make intuitive sense. Humans instinctively recoil from contamination without the slightest regard for the amounts involved. Paul Slovic calls this “intuitive toxicology.” It can be traced it back to our ancient ancestors. Every time they found drinking water, they had to decide if the water was safe. Every time they picked a berry off a bush or cut up a carcass, they had to judge whether they could eat what was in their hands. Every time someone was taken with fever, they had to think about how to help without becoming sick themselves, and when death came, they had to safely dispose of the corpse and the unlucky person’s possessions. We’ve been dealing with dangerous substances for a very long time.
Consider one of the most threatening contaminants our ancestors faced—human waste. Disease loves it. The entire life cycle of cholera, for example, hinges on feces: Someone who drinks water infected with the bacterium will expel huge quantities of watery diarrhea that can spread the disease to any water sources it touches. And so throughout history, avoiding all contact with feces, or anything that had contact with feces, has been absolutely essential for survival. There could be no exceptions. Any contact with any quantity was dangerous and must be avoided: Those who followed this rule tended to survive better than those who didn’t, and so it became hardwired instinct.
We can understand the toxicological principle that “the poison is in the dose” rationally. But Gut doesn’t get, it doesn’t make intuitive sense, and that can lead to some very odd conclusions. In The Varieties of Scientific Experience , the late astronomer Carl Sagan tells how, when it appeared that the Earth would pass through the long tail of Halley’s comet in 1910, “there were national panics in Japan, in Russia, in much of the southern and midwestern United States.” An astronomer had found that comet tails include, among other ingredients, cyanide. Cyanide is a deadly poison. So people concluded that if the Earth were to pass through a comet tail, everyone would be poisoned. “Astronomers tried to reassure people,” Sagan recounted. “They said it wasn’t clear that the Earth would pass through the tail, and even if the Earth did pass through the tail, the density of [cyanide] molecules was so low that it would be perfectly all right. But nobody believed the astronomers. . . . A hundred thousand people in their pajamas emerged onto the roofs of Constantinople. The pope issued a statement condemning the hoarding of cylinders of oxygen in Rome. And there were people all over the world who committed suicide.”
Our ancestors could analyze the world with only their eyes, nose, tongue, and fingers, and intuitive toxicology makes sense for humans limited to such tools. But science revealed what was in a comet’s tail. It also discovered contamination of earth, water, and air in quantities smaller than the senses can detect. In fact, today, we have technology that can dissect the components of drinking water to the level of one part per billion—equivalent to a grain of sugar in an Olympic-size swimming pool—while even finer tests can drill down to the level of parts per trillion. Gut hasn’t a clue what numbers like that mean. They’re even a stretch for a fully numerate Head—which is why, to even begin to understand them, we have to use images like a grain of sugar in a swimming pool.
The synthetic chemicals in our bodies that disturb people are typically found only in these almost indescribably minute quantities. They are traces, mere whispers, like the radioactive uranium we consume all our lives in blissful ignorance of its benign presence. It’s true that many of those chemicals can cause cancer and other horrible effects, but the science on which those conclusions are based almost never involves these sorts of traces. Quite the contrary.
The first step in testing for a carcinogenic effect is to stuff rats and mice with so much of the suspect substance that they die. This tells researchers that the given quantity is above what is called the maximum tolerated dose (MTD). So it’s reduced a little and injected into some more animals. If they live, the researchers then know the MTD. In the next step, fifty mice are injected with the MTD of the chemical. Another batch of fifty is injected with one-tenth or one-half the MTD. Finally, fifty very lucky mice are put in a third group that isn’t injected with anything. This routine is followed day after day for the entire natural life of the animals—usually about two years. Then scientists cut all the mice open and look for cancerous tumors or other damage. In parallel with this project, the whole procedure is done with other groups of mice and at least one other species, usually rats.
These tests find lots of cancer. Almost a third of rodents develop tumors, even if they aren’t injected with anything. So to identify the chemical as a carcinogen, the animals injected with it must have cancer at even higher rates. And they very often do. “Half of everything tested is a carcinogen in high-dose tests,” says Lois Swirsky Gold. But the relevance of these findings to trace contamination is doubtful because the quantities involved are so spectacularly different. “With pesticide residues, the amounts [found in the body] are a hundred thousand or a million or more times below the doses they gave to rodents in the cancer tests,” says Swirsky Gold. There’s also the question of whether bodies of rats and mice react the same way to the presence of a substance as the body of a human. Lab tests showed, for example, that gasoline causes cancer in male rats, but when scientists did further research to figure out exactly how gasoline was causing cancer, they discovered that the key mechanism involved the binding of a chemical in the gasoline to a protein found in the kidneys of male rats—a protein that doesn’t exist in humans. Unfortunately, rigorous analysis to determine precisely how a chemical causes cancer in lab animals hasn’t been done for most chemicals deemed carcinogens, so while there’s a very long list of chemicals that has been shown to cause cancer in mice and rats, it’s not clear how many of the chemicals on the list actually cause cancer in humans.