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Laryngeal
cancer has been considered in two case-control studies,
resulting in risk ratios of 2.4 and 2.3 that relate
to shipyard work and unspecified exposure, respectively
[ref: 40,58] A cohort study of insulation workers showed
a relative risk of 1.9, based on nine cases [ref: 57].
A case series indicated a high frequency of exposure
to asbestos, especially in low-grade smokers [ref: 59].
A risk ratio of 3.2 for laryngeal cancer was reported
among chrysotile miners in an area with generally high
incidence [ref: 60], but no increased risk was seen
in a cohort of workers with exposure to crocidolite
[ref: 61]. Two correlation studies have also indicated
a relationship between laryngeal cancer and exposure
to asbestos [ref: 39,62].
Mesotheliomas related to shipyard work and other exposures,
including household contact with asbestos workers, have
also been subject to epidemiological studies [ref: 36,63-67],
resulting in risk ratios of about 3-15 in comparison
with background rates not clearly referable to asbestos
exposure.
Some studies have specifically considered environmental
exposures with reference to mesotheliomas [ref: 66,67].
Three correlation studies and one case-control study
considering exposure to piped drinking-water [ref: 68-71]
did not show consistently increased risks for any type
of cancer, whereas another study [ref: 72] considering
chrysotile contamination mainly from natural sources
gave some indication of an increase in the incidence
of peritoneal and stomach cancers in persons of each
sex, although no other cancer site was consistent in
this respect.
Exposure to crocidolite has been studied with regard
to risk of lung cancer [ref: 61,73-76], and risk ratios
of about 2-3 have been reported. Three lung cancers
and two mesotheliomas occurred in 20 individuals after
one year of high exposure to crocidolite; at least 17
of the cases had asbestos-induced lung changes on X-ray
films [ref: 77].
One study [ref: 78] of histological types of lung cancers
showed that among persons exposed to crocidolite 45.7%
of cases were squamous-cell carcinomas, as compared
to 35.2% among unexposed persons. In the context of
unspecified and complex exposures, small-cell carcinoma
was found to be relatively more prevalent than other
forms [ref: 50].
Exposure to chrysotile was found in some studies to
result in virtually no increase in risk ratio [ref:
60,79-81], or a slightly elevated relative risk of lung
cancer [ref: 82-86]. Somewhat higher risk ratios, up
to 2.5, 3.5 and 2, respectively, were obtained in one
study of chrysotile miners [ref: 87] and in two independent
studies from one asbestos [chrysotile] textile plant
[ref: 88,89], the latter being the more comprehensive.
With regard to mesotheliomas, one study suggested a
particularly high risk of combined exposure to chrysotile
and amphiboles (risk ratio, 61), thus almost multiplying
the risk ratios (6 and 12, respectively) of exposures
to chrysotile and amphiboles alone [ref: 90]. Another
study showed no mesothelioma among a large worker population
with exposure to chrysotile only [ref: 91].
A slight excess of lung cancer and some mesotheliomas
appeared in some groups with mixed exposures involving
amosite, chrysotile and crocidolite [ref: 92-94]. Exposure
predominantly to amosite, but also to chrysotile, was
reported to be the probable cause of at least four of
five mesotheliomas (one peritoneal) observed in a UK
insulation-board factory [ref: 95]. One cohort with
exposure to cummingtonite-grunerite, which is closely
related to amosite, had no clear excess of lung cancer,
although one case of mesothelioma was observed [ref:
96].
Exposure to tremolite and actinolite has been the subject
of a few studies in investigations of vermiculite mining
and milling [ref: 97,98] and environmental exposure
[ref: 99]. The studies of miners indicated a risk ratio
for lung cancer of up to approximately six fold. Deaths
from mesothelioma were found in the occupational studies,
whereas the study of environmental exposure showed no
increased risk, although pleural plaques were reported.
Publication of one case report of a mesothelioma after
environmental exposure suggests that tremolite was of
etiological importance [ref: 31].
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