Increased Risk of Mesothelioma for Sierra Foothills Residents, Others Living near Asbestos

People who live close to a source of naturally occurring asbestos are at increased risk of developing the rare and deadly cancer known as mesothelioma, a new study led by Marc Schenker at the University of California Davis has found. The study is of special concern to residents of the Sierra foothills and other parts of central and northern California where asbestos-laden deposits lie in close proximity to homes, schools, and playgrounds.

The researchers found a dose-related association between the risk of mesothelioma and the distance lived from a source of ultramafic rock, which contains naturally occurring asbestos. The odds of developing the cancer decreased approximately 6.3% for every ten kilometers (6.2 miles) farther from the nearest asbestos source, after controlling for occupational asbestos exposure.

“This is the first study to link living near an area of naturally occurring asbestos with health outcomes in California,” said Schenker, professor of medicine and chair of the UC Davis Department of Public Health Sciences. The study was published in the American Journal of Respiratory and Critical Care Medicine online in June and will appear in print this fall.

Asbestos fibers have been long recognized as causing mesothelioma, a cancer of the membrane that ­surrounds the lungs and lines the wall of the chest cavity. However, the risk was thought to lie mainly among those with occupational exposures in shipyards, factories, and mining operations. More recently, studies have documented an increased risk of the disease in individuals with non-occupational exposures such as living near asbestos mines or, as in Turkey and Greece, using locally produced asbestos-containing stones or whitewash in homes.

With the recent building of new homes and schools in asbestos-bearing regions of California, local and federal environmental officials have suspected that the natural deposits could put residents at risk. Earth-movers and other heavy construction equipment can disturb these formations, producing fiber-laden dust that can be inhaled by people who exercise or play nearby.

Until now, however, no one had quantified the health risk from living near naturally occurring asbestos. In the case-control study, Schenker and his co-investigators compared 2,908 cases of malignant mesothelioma to an age- and gender-matched control group of an equal number of cases of pancreatic cancer, which has no known association to asbestos exposure. Cases and controls were selected from the California Cancer Registry and consisted of individuals diagnosed from 1988 through 1997. Using Registry data, the researchers located each individual’s place of residence at the time of diagnosis. They used Registry information to adjust for sex, age at diagnosis, and occupational asbestos exposure.

Schenker notes that relying solely on Cancer Registry data meant that the researchers did not have full records as to the number of years that an individual lived at an address at time of diagnosis. A complete ­residential history would increase the accuracy of the study results because the lag time between exposure and disease onset can be three decades or more. “Exposures do not have to be of long duration to cause the disease,” said Schenker. “It is one of the unfortunate characteristics of this cancer.”

The researchers found an increased mesothelioma risk related to residential proximity in both males and females. If the asbestos risk had been strictly occupational, there would have been no residential proximity effect, and there would have been a greater effect among men.

Within California, natural asbestos occurs in 42 of 58 counties, but the largest deposits are in the northern and central regions, often near earthquake faults. The primary areas of concern are in Humboldt County, areas of San Benito and Monterey counties, and western El Dorado County. Ultramafic rock contains both the amphibole mineral group that is highly associated with mesothelioma and the serpentine group that poses less risk for this cancer. Schenker’s team used ultramafic rock as a surrogate for asbestos deposits because no such statewide map yet exists.

Children may be at special risk because they spend more time outdoors than adults and have a long life-expectancy, and thus could develop the disease in their 30s or 40s. The U.S. Environmental Protection Agency released results of a study of asbestos exposure in the town of El Dorado Hills in May of this year. The agency found asbestos in nearly every one of the 450 samples collected at area playgrounds and schools.

Schenker’s study provides data that can help affected regions evaluate health risks and develop health protective policies. For example, in July, the El Dorado County Air Quality Management District board adopted new dust control measurements and testing requirements that apply to construction projects involving more than 20 cubic yards of soil and within a quarter-mile of areas expected or found to contain the fibrous mineral. Senator Deborah Ortiz, D-Sacramento, has proposed a bill that would require the California EPA to organize a task force to examine policies on naturally occurring asbestos.

“People are beginning to take this issue more seriously, and that is what I would hope,” said Schenker. “The debate needs to move beyond whether naturally occurring asbestos is an issue at all to what is appropriate to do, what is reasonable to do, and what is good public health policy.”

To further solidify the connection between living near a natural asbestos source and the risk of mesothelioma, said Schenker, a next step would be to look at lung samples from autopsies for the presence of fibers. Donated lung samples from two deceased pet dogs that had lived near such formations in El Dorado county yielded high levels of fibers, said Bruce Case, a pathologist at McGill University in Montreal. “I would expect to find similar results in human lungs,” said Case.

Mesothelioma kills about 2,500 people each year nationwide. The overall rate of the disease in California is about one case per 100,000 persons per year. California has not experienced a recent increase in the rate of this cancer.

However, Schenker says that given the decades it takes to develop the deadly illness, it is vital that researchers learn as much as they can about the risk of living near asbestos-bearing mineral formations. “If all the growth and activity going on in these areas now is associated with an increased risk of mesothelioma,” said Schenker, “it will take 30 to 40 years to find that out. Believing as we do in prevention, that is not the way we want to discover that the risk is significant.”