Amphiboles are hydrated
mineral silicates five of which occur in asbestiform habits as
asbestos grunerite (
amosite) asbestos, riebeckite (
crocidolite) asbestos,
anthophyllite asbestos,
tremolite asbestos and
actinolite asbestos] and non-asbestiform habits (grunerite, riebeckite,
anthophyllite,
tremolite and
actinolite). The asbestiform varieties are characterized by long, thin fibers while non-asbestiform varieties such as cleavage fragments form short fibers with larger widths. The U.S. regulatory method for counting
asbestos fibers (aspect ratio > or = 3:1, length > or = 5 microm) does not distinguish between
asbestos and cleavage fragments. The method biases toward increased counts of non-asbestiform cleavage fragments compared to long, thin
asbestos fibers. One consequence of this regulatory approach is that workers can be erroneously classified as exposed to concentrations of
asbestos (asbestiform
amphiboles) above the U.S. 0.1 f/mL exposure standard when in fact they are not exposed to
asbestos at all but non-asbestiform
amphibole cleavage fragments. Another consequence is that the known carcinogenic effects of
asbestos may be falsely attributed to non-asbestiform
amphibole cleavage fragments of the same
mineral. The purpose of this review is to assess whether
amphibole cleavage fragments pose the same risk of
lung cancer and
mesothelioma characteristic of
amphibole asbestos fibers. We identified three groups of workers exposed to non-asbestiform
amphiboles: two groups exposed to grunerite (Homestake
gold miners and
taconite miners) and one group exposed to industrial
talc containing non-asbestiform
tremolite and
anthophyllite in St. Lawrence County, NY. In addition to assessing strength of association and exposure-response trends in the non-asbestiform
amphibole cohorts, comparisons were also made with cohorts exposed to the asbestiform counterpart (positive control) and cohorts exposed to the
mineral (e.g.
talc) that does not contain
amphiboles (negative controls). The cohorts exposed to non-asbestiform
amphiboles had no excesses of
lung cancer or
mesothelioma. Similar results were observed in the negative control groups, in stark contrast to the excess risks of
asbestos-related disease found in the
asbestos cohorts. The only possible exception is the twofold increased risk of
lung cancer where exposure was to industrial
talc containing cleavage fragments of
tremolite and
anthophyllite. However, this risk is not considered attributable to the
talc or
amphibole cleavage fragments for several reasons. A similar increased risk of
lung cancer was found in Vermont
talc workers, studied in the same time period. Their exposure was to relatively pure
talc. There was no relationship between
lung cancer mortality and exposure measured as mg/m(3)years and years worked. A case-control study reported that all the
lung cancer cases were smokers (or former smokers) and attributed the excess to smoking. There were two
mesothelioma cases among the NY State
talc workers exposed to cleavage fragments of
tremolite and
anthophyllite, but
talc is not a plausible cause because of too short latency and potential for previous
asbestos exposure. The positive controls of
tremolite asbestos and
anthophyllite asbestos exposed workers showed excess risks of both
lung cancer and
mesothelioma and positive exposure-response trends. St. Lawrence, NY
talc does not produce
mesotheliomas in animals while
amphibole asbestos does. In sum, the weight of evidence fully supports a conclusion that non-asbestiform
amphiboles do not increase the risk of
lung cancer or
mesothelioma.