This week the California legislature will be taking up the question of methyl iodide. Methyl iodide is a highly toxic chemical fumigant that California agribusiness is chomping at the bit to start using. A prime crop will be the valuable strawberry fields of the state - the nation's big producer of ever larger and seemingly less tasty berries. But this is not a matter of taste. It's a matter of health and safety.

Methyl iodide is meant to replace an existing fumigant that is chemically related to it: methyl bromide. Methyl bromide is being phased out through international agreement because, in the atmosphere, it depletes the ozone. Methyl iodide doesn't cause that problem, but back on the ground, it doesn't make our DNA very happy. It turns out that methyl iodide is an ideally reactive chemical if your goal is to modify genetic material, as well as many other biologically-sensitive molecules.

The obvious question is, why would such a reagent even be considered for widespread application? Moreover, it's not just for use in remote agricultural settings, but in particular on strawberries, whose fields often abut residential areas and even elementary schools. The facile answer is that all pesticides are toxic, so it can be argued that it is only a matter of degree. But in fact, there are extensive regulatory requirements before extremely dangerous agricultural chemicals are allowed to be marketed and used, both nationally and at the state level.

In California, a state agency called the Department of Pesticide Regulation must evaluate methyl iodide as a newly proposed pesticide (http://www.kqed.org/quest/radio/strawberries-and-worker-safety). This routinely involves an internal review by staff scientists who study toxicity data and application methods in order to estimate how risky the business at hand is likely to be, for workers in the field and for those who live or work nearby. In the special case of methyl iodide, the Department of Pesticide Regulation went one step further. It appointed a panel of independent scientists to externally review the work of the agency.

I was a member of that scientific review panel. We worked diligently to study reams of data, going over with our colleagues within the pesticide agency every step of their exposure and health risk calculations. In the end, we solidly agreed that the poisonous potential of methyl iodide was striking (http://www.cdpr.ca.gov/docs/risk/mei/peer_review_report.pdf ). The calculated risk factors we arrived at (were any use at all to be considered) included margins of safety recognizing key uncertainties. In particular, we were all deeply worried about methyl iodide's long term effects on the developing nervous system of any infant or child that might come into contact even with very small amounts of this highly toxic material.

The scientific review panel was thanked for its work by the California Department of Pesticide Regulation, which then went on to rewrite the findings of its own scientists, watering down the risk estimates and recommending that methyl iodide go into use. So far, this foolhardy policy has not come into force, but even if it can be stopped in California, this provides cold comfort. Methyl iodide has already been approved by the Federal Environmental Protection Agency (EPA) for use in 47 other states (http://www.epa.gov/opprd001/factsheets/iodomethane.pdf).

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