Last week, Jon Brooks wrote a post about the latest chapter in California’s approval of a controversial strawberry fumigant, called methyl iodide.

Soon after, we received an email from a representative of Arysta Lifescience, the company that manufactures methyl iodide for agricultural use, asserting that we had only presented one side of the story, and offering us an interview with Jeff Tweedy, Arysta’s head of business development. I have been following this story for a while, so I gave Mr. Tweedy a call.

A little background: Methyl iodide is a toxic chemical that causes cancer and miscarriages in animal lab tests. Case studies of humans who have been exposed to methyl iodide suggest that the chemical can also cause brain damage. When California’s Department of Pesticide Regulation approved the use of methyl iodide in December, it did so with a set of conditions. One of these was a maximum exposure level — a ceiling on how much methyl iodide farm workers could be exposed to.

The maximum exposure the DPR set was 96 parts per billion (ppb).

That number, however, is 120 times higher than the exposure level that the DPR’s staff scientists believed was safe. Anything over 0.8 ppb, those scientists had concluded, could put field workers at risk.

(Large PDF of staff scientists’ health risk assessment here. Page 9 contains the scientists’ calculations of human risk levels for cancer and miscarriage, based on lab studies of rats and rabbits.)

So where, I asked Tweedy, did the company’s safe-exposure number, 96 ppb, come from? His answer:

Jeff Tweedy says “risk mitigation” measures were factored into the final determination[audio:]

But what bearing do safety measures, like using tarps and respirators, have on the calculation –- based on animal lab tests — of how much methyl iodide would put people at risk?

Or, to put it another way: Tarps and respirators can cut back on how much of a chemical someone’s exposed to. But they don’t change the amount of the chemical that will prove toxic to human beings. That’s the fundamental difference between the company (and the Department of Pesticide Regulations), and the staff scientists who evaluated the matter. The former says a much higher level of exposure is safer than the latter does.

A DPR spreadsheet, obtained through a public records request, showed that after an eight-hour work day, a worker would be exposed to about 70 ppb of methyl iodide if high-quality tarps and respirators were used. That’s still 88 times the safety level scientists recommended.

So, based on the DPR’s calculations, strawberry field workers who handled methyl iodide – even with the best safety gear — were almost certainly going to be exposed to more methyl iodide than scientists believed was safe.

So I wondered, where did the 96 ppb number come from?

Jeff Tweedy says the mitigation factors are based both on what’s feasible and safe[audio:]

His answer suggests that the 96 ppb was based on best-case mitigation that could be done in the field, rather than what staff scientists had deemed safe. But this itself is speculation, because a clear answer as to where that number came from has yet to be given.

An email obtained through a public records request suggests that scientists who were hired by the state to assess methyl iodide were also unclear about the origins of the 96 ppb figure.

The email shows an exchange between Ed Loechler, a member of the advisory panel (a group of eight scientists who work outside the DPR, but were brought in by the state as peer-reviewers) and two DPR staff scientists.

From Edward L Loechler, to DPR staff scientist
Professor of Biology, Boston University
Member of methyl iodide advisory panel

Dear Lori,

Could you direct us to where there is a discussion of the justification for these changes? Is it spelled out in one of the documents you sent?…If not, then could you summarize their rationale? (I assume that some kind of reasoning was offered.)

Response from staff scientist Lori Lim:

Dear Ed,

The “justification” is on page 5 of the Notice of Proposed Decision document under the heading Regulatory Target Levels. Since we were not involved in the decision, we had to read between the lines to figure out how the target levels were calculated.

There is no way you would “recognize” cancer risk. It is nowhere.

Lori and Ruby

The DPR approved use of methyl iodide in December. Environmental and farm worker groups have sued the state over the decision. On February 22, the State Assembly held a hearing on the matter. KQED’s report on that here.

Methyl Iodide Mystery: Where Did the State Get Its Higher Safe-Level Number From? 1 March,2011Amy Standen


Amy Standen

Amy Standen (@amystanden) is co-host of #TheLeapPodcast (subscribe on iTunes or Stitcher!) and host of KQED and PBSDigital Studios’ science video series, Deep Look.  Her science radio stories appear on KQED and NPR.

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