Environmentalists and farmworkers have sued to overturn the controversial approval last month of a highly toxic pesticide used mostly in strawberry fields and orchards.

Methyl iodide is a potent fumigant that sterilizes fields to be planted with strawberries, nuts, tomatoes and other crops.

It is expected to be laid down mostly in strawberry fields around Monterey, Santa Cruz and down the Central Coast to Ventura County. In those areas, activists say, the fumigant poses too high a risk to those near fields being treated.

"It's primarily an issue for people living or working next to areas where fumigants are used," said Susan Kegley, a consulting scientist for Pesticide Action Network North America, one of five organizations that filed the lawsuit last week in Alameda County Superior Court.

Methyl iodide was approved by the U.S. Environmental Protection Agency in 2007 in the waning days of the Bush administration. But it was not permitted for use in California until state regulators gave final approval weeks ago.

Conditions placed on its use in California will ensure it is used safely here, regulators say.

"Methyl iodide is the most evaluated pesticide (in) the Department of Pesticide Regulation's history," said department spokeswoman Lea Brooks. "DPR's evaluation determined methyl iodide can be used safely under its toughest-in-the-nation health-protective measures, including stricter buffer zones, more groundwater protections, reduced application rates and stronger protections for workers."

Methyl iodide was sought as an alternative to methyl bromide, a popular fumigant that is being phased out under international treaty because of the damage it does to the Earth's ozone layer.

A committee that reviewed the risks of the chemical in February expressed deep concern that California might approve its use.

"Based on the data available, we know that methyl iodide is a highly toxic chemical and we expect that any anticipated scenario for the agricultural or structural fumigation use of this agent would result in exposures to a large number of the public and thus would have a significant adverse impact on the public health," the scientific review committee wrote.

And before the U.S. EPA gave federal approval for its use in 2007, a number of prominent university chemists warned that in the laboratory they are extremely careful when using methyl iodide.

The chemists said they were "perplexed that U.S. EPA would even consider the introduction of a chemical like methyl iodide into agricultural use."

In California, the pesticide is authorized for use only after obtaining a permit from a county agricultural commissioner.

The new state regulations require larger buffer zones that federal regulators required. Pesticide applicators in California will have to use nearly impermeable tarps to cover fields after methyl iodide is applied, among other restrictions.

Mike Taugher covers the environment. Contact him at 925-943-8257.