Methyl iodide

The debate about using methyl iodide as an agricultural pesticide is going strong in California. The federal Environmental Protection Agency approved methyl iodide for use as a replacement for methal bromide, which harms the ozone layer, back in October, 2008. California, where the pesticide would likely be used mostly for strawberries, required its own risk assessment.  A story at described how the pesticide can be used:

Injected into soil before crops are planted, the fumigant spreads through the soil to kill insects, weed seeds, plant diseases and nematodes. It can be applied by drip irrigation under a special protective tarp or injected into the soil using a tractor that automatically places a tarp over the ground after application.

C&EN’s Britt Erickson reported when the FDA approved the pesticide:

Methyl iodide has several advantages over methyl bromide. It is more reactive and therefore too unstable to make it to the upper atmosphere to damage the ozone layer. And unlike methyl bromide, which is a gas, methyl iodide is a low-boiling liquid, Sims notes. That makes methyl iodide a lot easier to handle, he emphasizes.

Sims also points out that because methyl iodide is so reactive “you can get away with using a little less of it.” This is important because iodine is expensive. In the 1990s, “when I was trying to get companies interested in methyl iodide, one of their major concerns was the cost of iodine,” Sims says.

Methyl iodide is also toxic, a property that perhaps should not be surprising for a pesticide. It’s an alkylating agent that can chemically modify DNA and change gene expression, qualities that mark it as a carcinogen. It also suppresses thyroid hormone synthesis and is neurotoxic.

A report yesterday morning by Quest, a public broadcasting multimedia series about San Francisco Bay Area science and environmental issues, said that the California Department of Pesticide Regulation (DPR) convened a panel of external scientists to peer review the toxicological studies of the chemical. The panel settled on an exposure limit of 0.8 ppb.

In April, DPR announced an exposure limit of 96 ppb, half of the EPA limit, for people handling methyl iodide as a pesticide. For others–say, those living in nearby homes–the limit is 32 ppb, averaged over 24 hours. According to the National Institute for Occupational Safety & Health’s (NIOSH’s) Pocket Guide to Chemical Hazards, the NIOSH recommended exposure limit is 2 ppm and the Occupational Safety & Health Administration permissible exposure limit is 5 ppm.

I’m not sure what to make of this. DPR is being more cautious than EPA, NIOSH, and OSHA (Western Farm Press described the DPR restrictions as “draconian”). Yet the agency still set a limit that is 120 times greater than its own panel recommended. Mostly, though, I wonder exactly how workers will be protected. One of the people interviewed in the Quest story pointed to respirators, but I still recall the 17-year-old pregnant farm worker who died of heat stroke in 2008 after spending nine hours pruning grapevines in nearly 100-degree heat. If farms don’t provide adequate water and shade for workers, why do we think they’ll provide and maintain respirators?

6/18/2010 update: A follow-up story from Quest is at Strawberries and Worker Safety – Part Two.

Author: Jyllian Kemsley

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1 Comment

  1. I noticed concern with farmers providing proper respirators to operators treating soil with methyl iodide. This is a highly regulated field within the agribusiness network,and the operators are constantly montitored, and highly trained. They currently fumigate soil with specialized equipment, and the methyl iodide would simply be another fumigant they would be applying.

    Ordinary farmworkers would not be exposed to methyl iodide, as it disperses soon after finishing its job of fumigating soil.