Â鶹´«Ã½

Marc B. Schenker, M.D., M.P.H., of the Division of Environmental and Occupational Health, at the University of California, Davis, along with four associates, investigated 2,908 malignant mesothelioma cases reported over a 10-year period (1988 to 1997). Over 50 percent of the men and 58 percent of the women, all of whom were listed in the California Cancer Registry, either had no or low occupational exposure to asbestos.

"People who lived closer to an asbestos source had a greater chance of having mesothelioma, and the chance decreased steadily as the distance increased," said Dr. Schenker.

According to the article, the odds of developing mesothelioma decreased 6.3 percent for every 10 kilometers farther from the asbestos source.

The authors explained that a major strength of the study was the very large number of mesothelioma cases used to assess the potentially weak association between exposure to naturally occurring asbestos and mesothelioma incidence.

"Epidemiological studies have confirmed that occupational exposure to asbestos causes mesothelioma," said Dr. Schenker. "However, almost all population-based studies have found that many mesothelioma cases had no known occupational exposure to asbestos."

Considered rare, mesothelioma usually takes from 30 to 40 years after exposure to develop. The only know causes is exposure to asbestos fibers, which can cause tumors in the two layers of membrane covering the lung (the pleura), or, with greater exposure, in membranes of the abdomen.

According to the authors, California has more naturally occurring asbestos source rocks than any other state in the U.S., but their distribution is patchy, with exposed areas separated from unexposed areas. The group used Geographic Information Systems (GIS) software to pinpoint the sources of asbestos and the location of patient residencies more precisely.

"Geocoding of residential addresses provided a more accurate assessment of potential environmental exposure to naturally occurring asbestos at the individual level than methods using a less precise geographic classification," said Dr. Schenker.

The California Cancer Registry includes occupational information, so the researchers were able to control for, and the study confirmed, the increased risks of mesothelioma among shipyard workers, boilermakers, insulators, plumbers, steam fitters, and other tradespeople involved in construction and shipping.In an editorial on the article in the same issue, Marcel Goldberg, M.D., Ph.D., and Danièle Luce, Ph.D., of the Institut National de la Santé et de la Recherche Médicale (INSERM) in Saint Maurice, France, note that it is of utmost importance, "from a scientific and public health point to view, to know whether exposure to low levels of asbestos is able to induce pleural mesothelioma."

"While exposure in environmental settings is generally much lower than in occupational circumstances, the levels may not be negligible," they write. "In studies in which elevated risk of mesothelioma was demonstrated, people typically lived in close vicinity of naturally occurring asbestos sources, and may have had direct contact with asbestos, when white-washing houses with material containing asbestos or working in polluted fields. It is thus likely that lifelong cumulative exposure may have been as high (if not higher) as in some occupational settings, but it was not—or not adequately—measured, and non-occupational studies have not yet provided adequate answers.

"That is why this work"¦showing a relationship between mesothelioma risk and residential distance from naturally occurring asbestos, and suggesting that there is excess risk of mesothelioma even at long distance from the asbestos source, is important. To our knowledge, this study is the first one that demonstrates such an effect quite convincingly."

The editorialists contend that additional research is now necessary to more accurately assess the levels of cumulative exposure that persons experience in areas where excess risk of pleural mesothelioma was observed.