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In tests conducted at the University of California in Davis for Amvac, Zeneca and other chemical companies (see above), investigators sought to determine the levels at which methyl isothiocyanate (MITC), the active ingredient in the pesticide metam sodium, can be smelt, and those at which it produces physiological effects.

Eye irritation was measured by running varying concentrations of MITC vapour into goggles worn by volunteers for between one minute and eight hours. Tearing, blinking, subjective discomfort and redness were measured. The study found “no significant increases” in redness or tearing for most test subjects; irritation was deduced from blink rates and subjective discomfort.

The lead investigator, Michael Russell, a neurobiologist at the university's medical school, says the study — which was filed with the state government in September 1996 — produced “no ill effects whatsoever”. Russell, who himself volunteered, says the study was approved without problem by the institutional review board at the medical school. He estimates that volunteers were paid $150 or $300, depending on how long they spent wearing the goggles.

MITC — a less potent cousin of methyl isocyanate, the agent in the Bhopal disaster — is a toxic fumigant that kills virtually all soil organisms. Farmers use it to strip the soil of weeds, insects and microbes before planting. In humans, at high concentrations, it is very irritating to the eyes, skin and respiratory system.

In recent years, dozens of California residents living near farms using metam sodium have been hospitalized with sore throats, burning eyes, breathing difficulties, headaches, nausea and vomiting. In 1991, a train accident dumped at least 13,000 gallons of the pesticide in the Sacramento river, killing tens of thousands of fish. When tests showed that high doses of metam sodium cause birth defects in rats and rabbits, there were calls to pull it from the market.

The companies funded the Davis study because they were dissatisfied with animal data — in particular, a study of tearing in cats — that were being used by California's Department of Pesticide Regulation (DPR) for risk assessments. Russell says that the companies' interest was “to have an [approved exposure] level that is as high as possible but still actually safe”.

Veda Federighi, a DPR spokeswoman, stresses that the department “did not request a human subjects study” from the companies. But she adds, “we would have no problem accepting a [human] study that's done within appropriate guidelines”. Federighi says the California DPR applies a tenfold safety factor to animal data to arrive at acceptable human exposure levels. “Often [companies] feel that that is inappropriate.”

Critics say that metam sodium is such a potent poison that the government should be banning it, not using human data to regulate it. “We ought not to be exposing people to try to keep its use legalized,” says Ralph Lightstone, a staff attorney with the California Rural Legal Assistance Foundation, an advocacy group for farm-workers.

California's DPR placed restrictions on its use in 1994, but the amount used grew in 1995, the most recent year for which data are available. Zeneca Ag Products, based in Wilmington, Delaware, stopped producing metam sodium recently; Amvac continues to make it, along with dichlorvos (see main story).