BioEd Online

A review committee in Sacramento, California begins on 24 September to assess the science behind methyl iodide — a pesticide that has been approved for agricultural use by the US Environmental Protection Agency (EPA), but which faces a storm of opposition from activists and scientists.

At issue is a set of apparently conflicting assessments of the chemical's health hazard. In 2007, the EPA concluded that health standards could be met by proper use of masks and procedures, such as keeping workers away from newly fumigated fields. A 2009 report from the California Department of Pesticide Regulation (DPR), however, concludes that methyl iodide fumigation "results in significant health risks for workers and the general population", with some scenarios requiring a 3,000-fold reduction in exposure, according to its models and safety limits.

The compound is used to fumigate soils, and is a replacement for methyl bromide — a chemical that has been found to eat away at the ozone layer, and which is being phased out under the Montreal Protocol. Although ozone-friendly, methyl iodide is carcinogenic and neurotoxic, and is thought to affect fetal development by interfering with thyroid hormone production. All chemical soil fumigants are among the nastiest of the 1,200 substances registered for agricultural use, says chemist Susan Kegley of the California-based Pesticide Action Network North America. But methyl iodide is by some measures four times as toxic as methyl bromide, she notes.

The EPA approved methyl iodide in October 2007, prompting protest at the time — including from a group of chemists familiar with the toxic properties of the chemical in the lab. "I have read enough papers with cautions around the use of the chemical that it made me sit up," says Roald Hoffmann, a Nobel-prize-winning chemist at Cornell University in Ithaca, New York.

The states of California, Washington and New York have yet to approve the fumigant. All eyes are particularly on California, as its strawberry farmers are a big potential market for methyl iodide: in 2007, strawberry farmers statewide used 1.2 million kilogrammes of methyl bromide, out of a total allowable usage for the United States of 6.2 million kilogrammes that year. Some 50% of California strawberry growers have already moved away from methyl bromide, often to other fumigants, so the potential market for methyl iodide is potentially larger than these numbers imply. Japan is also considering registering the fumigant for use.

Tokyo-based Arysta LifeScience, which owns the patent for producing methyl-iodide fumigants and had a 2008 revenues of US$1.3 billion, did not respond to press queries from Nature.

Dangerous plumes

The California DPR now awaits the conclusion of the independent panel, headed by chemist John Froines, before making a decision. Froines expects the process to take weeks. "We have about 1,000 to 2,000 pages of material to review," he says.

One reason for the difference between the EPA and the California DPR reports is that the EPA effectively assumed that no one would get hit by a 'plume' of pesticide created by stagnant air pockets or wind, says Kegley. Such plumes of other fumigants, she says, typically send a couple of dozen people in the United States to the emergency room every few years. EPA spokesperson Dale Kemery says his agency's report used data and models that are representative of real field conditions in California. Although methyl iodide has been registered for use for more than a year in some states, Kemery says, "our understanding is that there have been no reports of adverse health effects".

Another difference between the two reports lies in how the safe exposure levels were calculated. The California DPR report divided this by an extra factor of 10 to account for several health unknowns, such as the impact of iodine on human fetal development.

Even if the more optimistic safety estimates are correct, there are still worries that agricultural workers may not have the time, money or know-how to undertake all the required precautions. Alternatives to chemical fumigants are available, including rotating strawberries with crops such as broccoli that contain natural pest deterrents, or using steam to fumigate soils.

The EPA will begin re-reviewing its approved fumigants in 2013, and has said it is open to reconsidering methyl iodide as part of that process — perhaps even speeding up the process if California rejects the pesticide. Hoffmann says he has "faith in the process".