Asbestos Fibers and Other Elongate Mineral Particles: State of the ...

and advanced interstitial fibrosis developed in
lung tissue in 10% of all older animals, whereas
intrapleural injection studies produced
mesotheliomas in over 90% of animals [Davis
et al. 1986]. It was noted that very little
chrysotile remained in the lungs of the animals
that survived longest following dust inhalation.
From this it was suggested that chrysotile
is very potent in rodents but, except where exposure
levels are very high and of long duration,
may be less hazardous to man because
chrysotile fibers are removed from lung tissue
more rapidly than are amphibole fibers. In
a further analysis of the data, using regenerated
dusts, Berman et al. [1995] found that although
chrysotile and amphiboles do not differ
in potency for lung tumor induction, mineralogy
is a determinant of the relative potency of
inhaled dusts for mesothelioma induction, and
chrysotile is less potent than amphiboles.
Hodgson and Darnton [2000] reviewed the literature
and estimated that, at exposure levels
seen in occupational cohorts, the exposurespecific
risk of mesothelioma from the three
principal commercial asbestos types is broadly
in the ratio 1:100:500 for chrysotile, amosite,
and crocidolite, respectively, and the risk differential
for lung cancer between chrysotile fibers
and the two varieties of amphibole asbestos
fibers is between 1:10 and 1:50. In 2009, Loomis
et al. [2009] reported the results of a mortality
study of chrysotile textile workers which
found increased risks of both mesothelioma
and lung cancer. The increase in mesothelioma
and pleural cancer in this cohort was consistent
with the average of 0.3% mesothelioma
deaths in the cohorts that previously had been
reported by Stayner et al. [1996] based on
12 studies of workers exposed to chrysotile
in mining and processing plants, but higher
than the estimates reported by Hodgson and
36
Darnton [2000]. Based on the findings of Loomis
et al. [2009], Hodgson and Darnton [2010]
reanalyzed their data. They found that cumulative
risk of mesothelioma for chrysotile-exposed
asbestos workers in processing plants
was approximately an order of magnitude
greater than the risk they had previously reported
for mines and processing plants combined,
and commented that this risk is still at
least an order of magnitude lower than that associated
with exposure to amphibole asbestos
[Hodgson and Darnton 2010].
A recent analysis of potency factors of various
minerals estimated that the relative potency
of chrysotile for producing mesothelioma
ranged between zero and 1/200th that of amphibole
asbestos and that amphibole asbestos
and chrysotile were not equally potent in
producing lung cancer [Berman and Crump
2008b]. However, the Asbestos Committee of
EPA’s Science Advisory Board expressed substantial
concern that the scientific basis for a
similar modeling approach being considered
by EPA for risk assessment was weak and inadequate,
and specifically cited the lack of available
data to estimate TEM-specific levels of exposure
for epidemiological studies used in the
analysis [EPA 2008a]. Subsequently, EPA chose
not to pursue this approach [EPA 2008b].
2.8.2 Asbestiform Amphibole
Minerals
Asbestiform amphibole fibers consist of aggregates
of long, thin, flexible fibrils that separate
along grain boundaries between the fibrils.
Because the fibril diameter of crocidolite
is less than that of anthophyllite asbestos but
the flexibility is greater, there is an indication
that flexibility is a function of fibril diameter.
As with chrysotile, the dimensions of individual
amphibole asbestos fibers depend on
NIOSH CIB 62 • Asbestos