Feeding Rodents Is No Way To Measure Cancer Risk
The First Rule Of Toxicology Is
June 18, 2001
by Dennis T. Avery
CHURCHVILLE, Va.--One of organic farming's most widely used pesticides--pyrethrum--has been classified as a "likely human carcinogen." An advisory committee to the Environmental Protection Agency made the classification two years ago, after pyrethrum caused higher-than-normal numbers of tumors in two sets of laboratory rodents.
The advisory committee's decision was not made public at the time but recently came to light as the result of a court suit.
The fact that pyrethrum has been classified as a "likely human carcinogen" does not mean the chemical is a danger to consumers.
Pyrethrum is a botanical nerve toxin, but it has low toxicity for people, it's applied at low rates, degrades quickly in sunlight and leaves little residue.
Pyrethrum is made from flowers (a variety of chrysanthemum) that are grown and picked by the billions in East Africa, by women and children, who don't get sick from doing so. In the 19th century, people used the ground-up flowers as a body powder to kill lice, without setting off a cancer epidemic.
The real significance of the pyrethrum classification is this: Organic farmers have been needlessly frightening consumers for decades over synthetic pesticides. The regulatory test for human carcinogens is not whether the pesticide at realistic exposure levels causes cancer in people. Only one approved pesticide has ever flunked that test: lead arsenate, which hasn't been used in 40 years.
The official test for a "likely human carcinogen" is whether giving the rats 100,000 times or 200,000 times the expected human exposure can cause tumors. About half the thousands of chemicals (both natural and man-made) put through the high-dose rodent tests have triggered extra tumors.
The first rule of toxicology is "the dose makes the poison." Many poison are actually beneficial to people in small doses (like iodine and arsenic).
Virtually no substance is as dangerous in low doses as at high ones. That's why government regulators have approved a wide variety of pesticides for use at low rates even though these, like pyrethrum, can cause rat tumors at high doses.
Rotenone, an organic pesticide squeezed from the roots of tropical plants, has produced symptoms like those of Parkinson's disease when massive doses are injected directly into rats.
That certainly doesn't mean eating organic food will cause Parkinson's disease. "We found you can (reduce) the cancer-causing impact of one of the most potent carcinogens from 90 percent down to 3 percent just by cutting rodent caloric intake by 20 percent," said Ronald Hart, then-director of the National Center on Toxicological Research in 1990.
"We feed rats 'all-you-can-eat' buffets every day, yet we know that caloric intake is the single greatest contributing cause of cancer," Hart said. "This means high-dose feeding skews the results."
In short, the high-dose rodent tests are guaranteed to frighten us. That's why they've been so loved by the environmental activists and organic farmers all these years.
In 1990, Science magazine editorialized, "The standard carcinogen tests that use rodents are an obsolescent relic of the ignorance of past decades." We could make the rat tests more useful, by testing at some reasonable safety factor beyond the maximum expected human exposure--say, 1,000 times what we think people will encounter.
Instead, Ron Cummings of the Organic Consumers Association is ready to sacrifice pyrethrum on the altar of the rodent gods. "If pyrethrum is as dangerous as it sounds, then it shouldn't be allowed (for organic farms)," he told me.
The problem is that Cummings and his eco-allies are backing humanity into a tighter and tighter corner on food production and wildlife habitat preservation, without giving us safer food.
An organic farming couple in Michigan is trying to protect their crops without pesticides, using a shop vacuum and portable generator carried in a wheelbarrow. The husband spends huge amounts of time trying to suck the insects off his crops, leaf by leaf. Even that endless stoop labor doesn't help against fungi or plant diseases. You can produce food without pesticides, but you can't produce much.
The other question that pyrethrum's classification as a "likely human carcinogen" brings to mind is: Why didn't the organic industry tell us?
For decades, the organic movement has persecuted the chemical companies for not caring about their consumers and for risking the health of children by marketing "carcinogenic" pesticides. But when one of their own pesticides was caught in the same phony trap, they kept it secret for two years.
We only learned the truth when a pyrethrum manufacturer sued the EPA, demanding that it violate its own guidelines and retract the "likely human carcinogen" classification. So much for the caring of the organic food industry.
This article appeared in Bridge News on June 8, 2001.
Dennis T. Avery is based in Churchville, VA, and is director of the Hudson Institute's Center for Global Food Issues.