Asbestos Induce
Pleural Mesothelioma
Asbestos is a recognized human carcinogen, causally related to
pleural and peritoneal mesothelioma and to lung cancer (1).
Mesothelioma is of particular interest, as it is a specific outcome
of asbestos exposure and no other causal factor except for exposure
to asbestos (and erionite, a naturally occurring mineral fiber
found in Turkey) (2) has been established or even convincingly
suspected.
The vast majority of asbestos-induced mesotheliomas in the
industrialized world is caused by occupational asbestos exposure,
and occurs among workers engaged in extracting and manufacturing
asbestos, or performing tasks involving contact with asbestos-containing
materials (3). Concern used to be focused on the
occupational environment, but it is now recognized that asbestos
fibers are widely distributed in the general environment. Persons
can be exposed to asbestos in different nonoccupational circumstances:
living with asbestos workers, with regular exposure to
soiled work clothes brought home; environmental exposure in
the neighborhood of industrial sources (asbestos mines and mills,
asbestos processing plants); passive exposure in buildings containing
asbestos; and natural environmental exposure to geological
sources (4).
Studying the effects of nonoccupational asbestos exposure
on mesothelioma risk is important because it could provide information
about the nature of the exposure–response relationship
that cannot be obtained from studies of workers whose exposures
begin in adulthood, are limited to working hours, are typically
much higher in concentration, and who are mainly male. Natural
environmental exposure to geological sources is of special interest,
since populations subjected to natural sources present specific
temporal exposure characteristics: exposure can start during
childhood and be lifelong, and it can occur around the clock,
seven days a week. Evidence on environmental exposure of natural
origin thus provides information about the effect of early
exposure on cancer risk and on latency periods, susceptibility
according to sex, or the potentially different effect of asbestos
fiber types (chrysotile, or amphiboles such as crocidolite, amosite,
or tremolite, which are considered to be more potent carcinogens
for mesothelioma than chrysotile fibers).
The main findings of studies in rural areas of Turkey (5),
Greece (6), some Mediterranean islands (7–9), China (10), and
New Caledonia (11), where occupational exposure to asbestos
is rare, even nonexistent, show that asbestos exposure starting
at birth does not seem to influence the latency period of mesothelioma.
The data also indicate that susceptibility does not differ
according to sex, and confirm that the much higher rates of
mesothelioma among males in the industrialized countries are
most probably due to sex differences in occupational exposure
to asbestos.
Studies of mesothelioma related to environmental exposure
to geological sources of asbestos have yielded important findings.
Am J Respir Crit Care Med Vol 172. pp 939–943, 2005
Internet address: www.atsjournals.org
However, some important issues remain unresolved. Thus, it
would be of utmost importance, from a scientific and public
health point of view, to know whether exposure to low levels
of asbestos is able to induce pleural mesothelioma. There is still
controversy regarding this question (12, 13). While exposure in
environmental settings is generally much lower than in occupational
circumstances, the levels may not be negligible. 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
whitewashing 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 nonoccupational
studies have not yet provided adequate answers
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