Every morning over a period of several weeks in 1971, 16 inmate volunteers reported to the hospital at the Clinton Correctional Facility in Dannemora, NY. Under the supervision of two scientists and a physician, they were divided into four groups. Three of the groups received pills containing different doses of the pesticide Dowco 179, made with a little-known nerve poison called chlorpyrifos. One received placebos. The prisoners' blood and urine were analyzed, and they were monitored for signs of distress.

By March of 1972, the results of this curious experiment were in. Although 12 of the inmates were fed chlorpyrifos, none became violently ill. Volunteer No. 3, in the highest-dose group, "complained of a runny nose, blurred vision, and a feeling of faintness" but was treated for a cold and recovered. All four members of that group also experienced a sharp drop in levels of an enzyme called plasma cholinesterase -- evidence of toxic insult. But the reading were back to normal four weeks after the dosing ended.

At this point, the study's sponsor, the Dow Chemical Co., had two options: step back and ponder the safety of chlorpyrifos, which it began selling in 1965, or forge ahead and try to grab a bigger share of the lucrative pesticide market. Dow chose the latter, while continuing to test. Sales grew steadily. Marketed by a subsidiary under the names Dursban (for structures) and Lorsban (for agriculture), the pesticide was being applied 20 million times a year in homes, schools, and offices by the 1990s.

A U.S. News investigation presents a more complicated picture. Since June 1992, Dow AgroSciences, a predecessor company called DowElanco, and other pesticide manufacturers have sent the EPA some 7,000 reports of adverse reactions to chlorpyrifos. An EPA analysis found that the chemical was suspected in 17,771 incidents reported to U.S. poison-control centers between 1993 and 1996. More than half the cases involved children under 6. In a draft report released last week, the EPA said that those who come in contact with the product in its granular and powdered forms -- whose dust is easily inhaled and absorbed through the skin -- could receive up to 100 times the safe amount. Dow AgroSciences says the document is riddled with errors and omits important data.

This much at least is certain: Chlorpyrifos is nearly everywhere. A survey of 1,000 Americans in 1994 found that 82 percent had its residue in their urine. A survey of 46 California school districts in 1997 found that almost half routinely applied pesticides containing chlorpyrifos. In New York the same year, Dursban Pro was the most popular bug killer: State records show that 3.5 million pounds and 665,000 gallons were applied.

New studies suggest that chlorpyrifos poses a particular threat to the developing nervous system, attacking it in ways that can lower intelligence and cause behavioral problems. It may be more toxic in combination with other chemicals than alone, or more toxic in repeated small doses than in a single large one. "It should not be used inside the home," says Mohamed Abou-Donia, a professor of pharmacology and cancer biology at Duke University.

Dow AgroSciences has spent more than $100 million on 3,600 studies that suggest chlorpyrifos is harmless when properly applied. Why, then, has it elicited 274 lawsuits since 1990? "It's been the biggest kid on the block for a long, long time," says Guy Relford, the company's global legal counsel. "Historically, [it has been] one of the most prevalently used insecticides in the world, and when you're out there in the market place like that, then you're going to be a target." Relford can draw from a impressive pool of scientists. There's Rudy Richardson, Dow professor of toxicology at the University of Michigan, who says: "Caffeine will kill you at the same sort of dose that chlorpyrifos will kill you, and you don't hear too much conversation about banning caffeine from beverages." Richard Kingston, an assistant professor of pharmacology at the University of Minnesota, examined 36,183 known and suspected chlorpyrifos exposures called in to poison-control centers over a 10- year period. His conclusion: Nearly 96 percent "resulted in no significant health effects."

Anecdotal evidence. Others aren't so sanguine. At the EPA, health statistician Jerome Blondell says his figures on individuals exposed to chlorpyrifos --and other organophosphate pesticides like malathion -- might be low because not all exposures are reported to poison centers. And the adverse-reaction reports filed by the manufactures include worrisome anecdotes like these:

* Home interior sprayed for carpenter ants. Dursban used while children were present. Lady re-entered in three hrs.; headache, nausea, dizzy . . ."

* "Lady had two serious reactions to Dursban in 1988 and 1998. Electric feeling through torso like a seizure, sleep disruption, deadened nerves."

* Flea & tick collar was chewed by dog; dog died."

U.S. News has spoken with more than a dozen people who believe that chlorpyrifos has made them or other family members chronically ill. Hours after the inside of her house in Burnsville, Minn., was sprayed for fleas with Dursban in 1991, a still-ailing Diane Lang thought she was going to die. "I hurt so bad all over. My head was just pounding with a headache like I'd never experienced. Shafts of lightning would just go through my vision. I was confused." There are many similar stories.

Tragic -- but not our fault, responds Dow AgroSciences. The chlorpyrifos exposures in all these cases, Relford says, were well within the "safe zone." But what if the zone isn't safe for everyone? New research supports the notion that there is a hypersensitive minority, susceptible to quantities of chlorpyrifos that wouldn't faze the average person. In experiments on mice bred without a crucial pesticide-fighting enzyme, a team from the University of California- Los Angeles and the University of Washington noted a dramatic increase in sensitivity to an oxidized form of chlorpyrifos. It's a concept the EPA must bear in mind as it struggles to carry out its regulatory duties: Is this substance short-circuiting some people's brains even as it snuffs out bugs?

The origins of chlorpyrifos can be traced to the late 1930s, when the German conglomerate IG Farben developed the organophosphate pesticides tabun and sarin. Appropriated by the Nazis as prospective weapons during WW II, the compounds cause symptoms ranging from blurred vision to asphyxia.

All organophosphates inhibit cholinesterase, an enzyme that regulates nerve transmission in the body. The result can be over stimulation of the nervous system; in extreme cases, "you can't run, you can't walk, you can't breathe," says Janette Sherman, a physician in Alexandria, VA. "That's the way it kills the insect."

History

Dow came out with chlorpyrifos in 1965, just as DDT and other pesticides were falling into disrepute. The EPA banned DDT in 1972, opening new markets for organophosphates, but it wasn't until 1988, when the agency blackballed the widely used termite-killer chlordane, that sales of chlorpyrifos took off. The chemical was effective, cheap, and seemed fairly benign. But there was already a substantial -- and troubling -- body of knowledge on organophosphates. In 1930, during Prohibition, 50,000 Americans who drank an alcoholic extract of Jamaica ginger experienced numbness of the arms and legs, followed by either temporary or permanent paralysis. The U.S. Public Health Service blamed the epidemic on triorthocresyl phosphate, used to cut he strong ginger taste. Dow's experiment on the NY inmates --limited though it was by lack of follow-up -- yielded more clues. The runny nose, blurred vision, and faintness suffered by Volunteer No. 3 were dead giveaways, says David Wallinga, a senior scientist with the Natural Resources Defense Council. "That's what people get when they get poisoned."

Limited Withdrawal.

Under pressure from the EPA, DowElanco agreed in 1997 to withdraw the compound from a few specialized markets, such as indoor total-release foggers and pet shampoos. But the EPA didn't touch the chemical's main residential and agricultural uses. "The agreement with Dow is inconsequential in terms of the overall risk to public health," says Jay Feldman, executive director of the National Coalition Against the Misuse of Pesticides. Feldman and others find it even less palatable in light of allegations that the company has hidden or misstated evidence of its product's toxicity. In 1994, DowElanco agreed to discontinue what the NY attorney general characterized as deceptive claims in a Dursban brochure. In 1995, the EPA fined the company $876,000 for belatedly reporting 288 possible adverse reactions to chlorpyrifos. In June, the agency alleged that Dow AgroSciences waited too long to report ailments of a Kansas couple whose home had been treated for termites with Dursban TC.

Even in the fractious world of environmental health, the discord over chlorpyrifos is striking. Duke's Abou-Donia is convinced that chlorpyrifos can ravage the developing nervous system under common exposure scenarios. "A pregnant woman should never be exposed to this chemical," he says. But Dow AgroSciences neurophysiologist Patrick Donnelly argues that Abou-Donia is dosing his laboratory animals with unrealistically high levels, which are "irrelevant for the purposes of . . . regulatory decision making."

The EPA is expected to complete its lengthy reassessment of chlorpyrifos next year and may impose further restrictions on the chemical. "We are certainly not bashful", says Deputy Assistant Administrator Stephen Johnson. "We will take action to mitigate a risk."