‘The dose makes the poison,” said 16th-century Swiss physician Paracelsus, and the modern approach to pesticides proves it. Chemicals for killing bugs and weeds are considered safe at low doses—so safe they have become a fixture of daily life. We eat pesticides on produce, drink them in groundwater, inhale them from flea collars, sprinkle them on lawns, and even spray them on our skin. Yet at high levels they are unquestionably dangerous. So at what dose do they become poisons? Toxicologists and government regulators have long relied on laboratory tests with rats and other animals to answer that question. The shortcomings of such tests are obvious: The results may not apply to people. But the alternative is to test pesticides directly on humans. And that’s just what pesticide manufacturers are doing. Over the past decade manufacturers have submitted to the Environmental Protection Agency results from more than a dozen experiments in which human subjects have deliberately swallowed or inhaled pesticides or absorbed them through the skin. Last year an industry group sued the agency to force it to consider data from those tests in evaluating pesticide risks. The tests involve anywhere from a single slug to four weeks of daily dosing. Volunteers are paid as much as $200 a day, and their blood and urine are screened for pesticide metabolites and physiological markers that may reveal the chemicals’ action in the body.
Public-health advocates maintain that pesticide manufacturers have turned to human tests in an attempt to get around new rules limiting the use of pesticides. “They assume that most of the time [a human test] is going to work out in their favor, and if it doesn’t, they can stop the study,” says Richard Wiles, a senior vice president of the Environmental Working Group in Washington, D.C., which has challenged the ethics of human testing.
“We have an obligation to understand the potential health effects in people,” counters Ray McAllister, a vice president of CropLife America, also in Washington, D.C., which lobbies for the agrochemical industry. “Without these human studies, you don’t know what you should be looking for in terms of metabolites.”
No law prohibits testing pesticides on people, and no policy guides agency action with regard to tests conducted with private funding, either. The EPA normally establishes so-called tolerances—the amount of pesticide residue in produce, grains, and processed food that is considered tolerable—in a three-step process based on animal tests paid for and conducted by the manufacturers themselves. First the agency defines the highest daily dose that does not seem to affect the most sensitive species of animal studied. The agency then lowers that dosage by a factor of 10 to account for possible variations in response between the most sensitive test animal and humans. Then that figure is reduced by another factor of 10—to 1/100 the original dose—to accommodate the potential for differing responses among people. Tolerances can be adjusted if new data warrant a review—if, for example, field workers inadvertently exposed to pesticides become ill.
The Food Quality Protection Act of 1996 toughened standards for pesticide exposure in light of evidence that children are sometimes more vulnerable than adults to environmental contaminants. Among its provisions, the act requires the EPA to reduce by an additional factor of 10 the amount of pesticide residue in food. The legislation also requires the agency to evaluate the cumulative risk posed by entire classes of chemicals that share the same mechanism of action instead of evaluating each compound independently. The EPA has been instructed to draw up new limits for hundreds of pesticide ingredients already in use.
The explicit intent of the legislation is to lower the average person’s exposure to pesticides. But an advisory panel convened by the EPA cautioned that the law may have unintended consequences. “When human data have been available and used, it has generally raised the ‘safe dose,’” the panel reported in 2000. “A higher ‘safe dose’ allows greater use of a pesticide. Thus, the [Food Quality Protection Act] may have inadvertently created an incentive to test pesticides in humans.”