If the air in the San Joaquin Valley is clean, then what's that brown haze everywhere as you drive along Interstate 5?

Community groups, public health advocates and environmentalists filed suit against the U.S. Environmental Protection Agency on Friday to overturn an October 2007 rule that allowed valley officials to declare victory in a long battle against the airborne dust technically known as coarse particulate matter (PM-10).

According Earthjustice, the environmental law firm that filed the suit in the U.S. 9th Circuit Court of Appeals, air quality monitors in the valley show that federal standards are not being met. The EPA and the local air district say that the recurring violations are natural ones that do not need to be addressed through further controls.

"At the time of the finding, we said it was either a miracle or they were lying," said Kevin Hall of the Fresno Sierra Club. "As more data came in, we became convinced it was the latter."

Much of the pollution in the valley is due to agricultural activity, such as plowing fields, harvesting crops and truck traffic along unpaved farm roads. Agribusiness, which has been chafing under air pollution rules, is the most politically influential industry in the valley.

The region includes more than a thousand giant dairy farms, many of which house more than 1,500 cows each. Recently, the Bush administration exempted factory farms nationwide from some reporting requirements for ammonia, one of the precursors to fine particle pollution.

--Margot Roosevelt

Photo: Dairy cattle line up to feed near the San Joaquin Valley town of Lamont. State bond money designated for pollution control has financed factory farms that have helped turn the valley into one of the nation's most polluted areas. Credit: David McNew / Getty Images