EL DORADO HILLS, Calif.—El Dorado Hills’ biggest defense against dangerous asbestos dust may be working.

Tests by the park district released this week show that frequently watering a park’s playgrounds, trails and construction sites significantly holds down the dust.

Experts said there were much lower dust levels in this Sierra foothills town east of Sacramento in the fall of 2005, when baseball fields were watered, compared with federal tests a year earlier.

The services district hired Network Environmental Systems Inc. of Folsom to find the greatest levels of dust in 40-acre Community Park. The cancer-causing asbestos fibers occur naturally in foothills rock, but construction and other activity releases them into the air.

If correct, the results are good news for El Dorado County foothills residents who rely on frequent watering as their main protection against asbestos kicked up by construction in the fast-developing region.

“It means that there are relatively cost-effective and relatively straightforward means for minimizing exposure in those areas where asbestos may be present,” said David Bieber, a local geologist and nationally recognized authority on naturally occurring asbestos.

But federal Environmental Protection Agency officials who conducted the first tests said there are too many differences between the methods in the two studies and too many conditions changed in the year between the tests to say conclusively that asbestos exposure was reduced and increased irrigation made a difference.

“It might be that it’s lower. We don’t know,” said Jere Johnson, the EPA official who led the $1 million study of playgrounds. “The important thing is that they are taking steps to reduce exposure.”