'Breathing asbestos in your community is not magically different from breathing asbestos in an industrial setting,' said a study co-author. 'It would have been a surprise to find otherwise.'

Exposure to asbestos in the workplace, particularly in shipyards, long has been recognized as a risk factor for mesothelioma.

But in the new study, researchers found a consistent and dose-dependent association between mesothelioma and residential proximity to ultramafic rock -- the predominant source of naturally occurring asbestos.

"Our findings indicate that the risks from exposure to naturally occurring asbestos, while low, are real and should be taken seriously," said Marc Schenker, professor and chair of the UC Davis Department of Public Health Sciences and the study's senior author.

"This study provides important supportive evidence that naturally occurring asbestos causes mesothelioma -- and public efforts should now shift to understanding the risk and how we can protect people from this preventable malignancy," Schenker added.

To put the mesothelioma risk in perspective, the disease kills about the same number of Americans each year as passive smoking. About 2,500 people a year die from mesothelioma in the United States, according to National Institute of Occupational Safety and Health statistics.

About 3,000 deaths a year are attributed to exposure to secondhand tobacco smoke, according to US Environmental Protection Agency statistics.

Ultramafic rock is distributed throughout the Sierra Nevada, Coast Ranges and Klamath Mountains in Northern and Central California, and has been a source of increasing concern as new housing developments cut through these areas.

Of most concern are the areas of ultramafic rock associated with tremolite asbestos.

In the largest study yet to examine the problem, Schenker and his colleagues used California Cancer Registry data to identify 2,908 cases of malignant mesothelioma diagnosed between 1988 and 1997 in adults ages 35 and older. In most cases, the registry also provided occupational history.

As a control group, an equal number of age- and gender-matched pancreatic cancer cases was selected (since pancreatic cancer has no known association to asbestos exposure).

For both the mesothelioma and pancreatic cancer cases, the researchers employed sophisticated geographic information system mapping to pinpoint home or street addresses for every diagnosed individual. A map from the California Department of Conservation, Division of Mines and Geology served as the reference for ultramafic rock deposits.

Finally, statistical adjustments were made for sex, occupational asbestos exposure and age at diagnosis.

The researchers found that the risk of developing malignant mesothelioma was directly related to residential proximity to a source of ultramafic rock. Specifically, the odds of having mesothelioma fell by 6.3 percent for every 10 kilometers (about 6.2 miles) farther a person lived from the nearest asbestos source.

The association was strongest in men but also was seen in women. No such association showed up in the pancreatic cancer group.

The study was not designed to determine the "ground zero" risk for those living closest to an asbestos source -- only to test for a relationship between proximity and risk.

"This is very creative, painstaking epidemiology," said Jerrold L. Abraham, professor and director of environmental and occupational pathology at SUNY Upstate Medical University in Syracuse, New York, and a leading authority on mesothelioma.

"The UC Davis researchers have shown a significant association between living near deposits of naturally occurring asbestos and mesothelioma. This is the strongest evidence possible without conducting one-on-one interviews with each diagnosed mesothelioma patient or his or her family," Abraham added.

"We showed that breathing asbestos in your community is not magically different from breathing asbestos in an industrial setting," said Laurel Beckett, professor and vice chair of the UC Davis Department of Public Health Sciences and a study co-author. "It would have been a surprise to find otherwise."

Similarly, she said, it was no real surprise to scientists when passive smoking was found to cause lung cancer.

"Like smoking, exposure to asbestos appears to be very dose-dependent," Beckett said. "Day-in, day-out occupational exposures are more dangerous than intermittent exposures in the community. But the more you can do to reduce your personal exposure, the safer you will be."

While the overall mesothelioma rate was about one case per 100,000 people per year in the California study, the rate varied markedly by gender and age.

For white males, the rate was 2.29 cases per 100,000. For white females, it was 0.49. People over age 60 had ten times the rate of those ages 40 to 59.

Worldwide, epidemiological studies of mesothelioma have found occupational causes for most but not all cases of the disease.

In some undeveloped areas of the world, including parts of Greece and Turkey, mesothelioma cases have been linked to use of naturally occurring asbestos in household materials, such as whitewash.

The UC Davis study suggests naturally occurring asbestos also causes mesothelioma in developed countries, through incidental, non-occupational exposures.

Needed now are field studies to more accurately characterize determinants of exposure to asbestos fibers among residents in areas with naturally occurring asbestos, Schenker said.

In addition, he said, more must be learned about the types and size of fibers in asbestos deposits, the types of human activities that disturb asbestos fibers and the determinants of cancer risk in exposed populations.

"Because mesothelioma takes 20 to 30 years to develop, what we learn today will allow us to protect Californians from this preventable cancer decades into the future," Schenker said.