Exposure to
asbestos from rocks can cause malignant mesothelioma
Newswise — Californians
who live close to naturally occurring asbestos sources and
who are exposed to low levels of the mineral are at
increased risk for developing malignant mesothelioma, a
serious cancer of the membrane covering the lung, according
to a new study published in the second issue of the October
2005 American Thoracic Society’s American Journal of
Respiratory and Critical Care Medicine.
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.
