FRESNO, Calif. — The Central Valley's dairy, cotton, fruit and vegetable farms are the newest front in the fight to clean up one of the nation's dirtiest air basins.

The San Joaquin Valley Air Pollution Control District is requiring large-scale farmers to submit plans by the end of the year showing what they're doing to reduce the microscopic particles of dust, chemicals or other substances that come from their land.

Farmers with more than 100 contiguous acres and dairies with more than 500 cows meet the requirements to participate in the plan; that translates into more than 6,400 farms and dairies in the 270-mile-long valley between San Francisco and Los Angeles.

The farmers can choose from dozens of dust-fighting options. They include measures many already practice, such as watering unpaved roads, switching to organic farming and working at night when winds are lighter.

Environmental activists lauded the new requirements, saying it was about time farmers joined local governments and other industries in controlling dust. But critics said the requirement asks for too little and gives farmers too much room to count measures they already were taking as part of their improvement package.

Despite the concerns, more than two-thirds of farmers with enough land or cows to fall under the new rules had complied and submitted their two-year plans by early December, said Rick McVaigh, the regional air board's permit services manager.

Health advocates said asking farmers to do their part is an important step in addressing the region's pollution problem. Farms raise 51 percent of the tiny specks of dust that help give the valley one of the nation's highest asthma rates.

Farmer John Pucheu said the requirement has raised farmers' awareness of the need to keep dust down. Like many farmers, however, he said the air among the cotton fields where he lives feels a lot cleaner to him than what he sees when he goes into Fresno, the valley's largest city.

"In these urban areas, you have hundreds of thousands of cars," said Pucheu, who farms 3,500 acres in the west Fresno County town of Tranquillity. "Out here, most days the fields are just sitting there, growing."

The latest cleanup plan proposes reducing particulate pollution by 23 percent, or 34 tons a day, by 2010. To date, the region has missed a series of federal deadlines to reduce pollution -- and residents in the area are paying for it with the nation's highest asthma rate.

Medical research has shown that the particles that concern the air regulators and health workers -- called PM10 because they are under 10 micrometers, or one-seventh of a human hair in width -- can lead to chronic respiratory problems.

According to the American Lung Association, the tiniest particles -- those smaller than 2.5 micrometers -- can lodge themselves deep inside lung tissue. They have been linked to heart attacks, strokes and a shorter life expectancy.

The particles can consist of diesel exhaust, soot, ash and organic compounds from dairies such as ammonia, in addition to the dust that can rise from fields during harvest or tilling.

"No one likes to get regulated," said Josette Merced Bello, chief executive officer of the American Lung Association of Central California. "Ag is not the only source, and this is not the only solution. But it's important for everyone to get involved."

Source: Associated Press