Back to Square One

Methyl bromide destroys ozone, but substitute fumigant warms the planet.

Chemical companies took a long time to find a replacement for ozone-depleting methyl bromide, a fumigant widely used in agriculture and food storage to control insects and rodents. They thought they had found the answer: sulfuryl fluoride. Long used as a fumigant for other applications, it doesn't damage atmospheric ozone and is effective in pest control.

But they need to think again, says a team of researchers from MIT and elsewhere. The substitute chemical may pose no threat to the ozone layer, but it turns out to be a very powerful greenhouse gas. In fact, pound for pound it is 4,800 times more potent than carbon dioxide, and much of it leaks into the air even when it is used in enclosed places like grain silos. One reason it's so damaging is that it is long-lasting: the gas persists in the atmosphere for about 36 years, far longer than had been suspected.

Fortunately, sulfuryl fluoride has not been used as a methyl bromide substitute for long and its other uses have been relatively limited, so its concentration in the air is not yet a significant problem. But now the world is back to square one in the search for a safe fumigation agent.

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Methyl bromide has been widely used for growing crops such as strawberries and in grain storage, commercial seed production, and pest control in buildings. Because these uses are important for some food supplies, a wholesale U.S. ban on methyl bromide, required under the Montreal Protocol, has been delayed until a suitable replacement can be found. "Fumigation is absolutely needed to preserve our buildings and food supply," says Ronald Prinn, ScD '71, director of MIT's Center for Global Change Science and coauthor of a paper on the new findings in the Journal of Geophysical Research.

One "new" replacement has already been proposed: concentrated heat, which can safely kill pests in many of the applications where methyl bromide has been used. It's an ancient method, Prinn says, but it's quite effective, and it has no environmental impact.