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This is WEEKEND EDITION from NPR News. I'm Liane Hansen.

A controversial pesticide called methyl iodide is pitting scientists against California regulators. Strawberry growers use it to sterilize their soil. A panel of outside scientists hired by the state to evaluate the fumigant says it's too dangerous - not for people who eat strawberries but for farm workers or communities nearby.

Internal documents show that the state's anticipated green light is causing a rift within the California Department of Pesticide Regulation. From member station KQED in San Francisco, Amy Standen reports.

AMY STANDEN: If you've eaten a strawberry lately, odds are it came from California. That's where 90 percent of the berries come from. And if that strawberry wasn't organic, it was likely grown in soil that had been fumigated before the plants went in. Currently, farmers use a fumigant called methyl bromide, but it's being phased out internationally because it damages the ozone layer. The leading alternative, however, has a different set of problems. It's called methyl iodide.

Professor JOHN FROINES (Environmental Health Science, UCLA, Chemist): This is very likely, because of its chemical structure, to be highly toxic.

STANDEN: John Froines is a chemist and professor of environmental health science at UCLA.

Prof. FROINES: It is very worrisome, even frightening, to a chemist and therefore it should be to the public as well.

STANDEN: Frightening because animal studies show that methyl iodide is a carcinogen, a neurotoxin, and can cause miscarriages. So, the task in evaluating how toxic methyl iodide is, is to extrapolate this data from rabbits to humans and to arrive at an exposure level that might be considered safe.

The U.S. EPA approved methyl iodide as a pesticide under the Bush administration in 2007. It was a controversial decision at the time. But California, which would be the country's biggest user of the chemical, has its own review process. In April, the state issued a notice to approve methyl iodide with an exposure level of 96 parts per billion for workers.

Professor ED LOECHLER (Biology. Boston University): I was shocked.

STANDEN: Ed Loechler is a biology professor at Boston University. He served on the independent review panel brought in by state regulators to help staff scientists evaluate methyl iodide.

Prof. LOECHLER: The number in the notice is 120 times higher than the level that both the independent scientific review panel felt was safe, as well as their own internal experts thought would be safe in terms of worker exposure.

STANDEN: According to the assessment produced by the panel and staff scientists, the safe exposure level for workers is .8 parts per billion. Anything over that, they said, would be unsafe. Again, John Froines, he chaired the panel.

Prof. FROINES: I honestly think that this chemical will cause disease and illness, and so does everybody else on the committee.

STANDEN: In setting new exposure levels of methyl iodide, panel members say, the state has overstepped its role, with little explanation for why. Panel member Paul Blanc is a professor of medicine at the University of California San Francisco.

Professor PAUL BLANC (Medicine, University of California San Francisco): They can say anything they want from a policy point of view, but what they cannot do is simply arbitrarily change the science.

STANDEN: An email obtained through California's Public Records Act shows that staff scientists were also perplexed by how exposure levels had jumped from the recommended .8 parts per billion to 96. They stand by the lower numbers.

Managers at the agency say it's not up to scientists to decide whether methyl iodide is approved and at what levels.

Ms. MARY-ANN WARMERDAM (Director, California Department of Pesticide Regulation): We went through a very careful review process of all the scientific information before us.

STANDEN: That's Mary-Ann Warmerdam, director of California's Department of Pesticide Regulation. She says methyl iodide can be used safely. She points to EPA's exposure limit for workers: 193 parts per billion, twice what California is proposing. And she also points to a list of regulations intended to protect people from inhaling or coming into contact with the chemical. Things like face masks, heavy tarps, gloves and buffer zones.

Ms. WARMERDAM: The mitigation factors that we've put forward are extraordinarily health-protective and they stand up against the health protective levels put forward by others, including EPA's.

STANDEN: Meanwhile, the EPA under President Obama appears to be reconsidering the previous administration's approval of methyl iodide. According to state transcripts, EPA officials say they may take another look at the chemical depending on what happens in California.

For NPR News, I'm Amy Standen.