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

physician from an occupational and environmental
history, assessed with respect to intensity
and duration. Such a history enables
a judgment about whether the observed clinical
abnormalities can be reasonably attributed
to past asbestos exposure, recognizing
that severity of lung fibrosis is related to dose
and latency [ATS 2004]. The presence of
otherwise unexplained characteristic pleural
plaques, especially if calcified, can also be
used as evidence of past asbestos exposure
[ATS 2004]. As explained by the ATS [2004],
“the specificity of the diagnosis of asbestosis
increases with the number of consistent
findings on chest film, the number of clinical
features present (e.g., symptoms, signs,
and pulmonary function changes), and the
significance and strength of the history of
exposure.” In a small minority of cases, particularly
when the exposure history is uncertain
or vague or when additional clinical assessment
is required to resolve a challenging
differential diagnosis, past asbestos exposure
is documented through mineralogical analysis
of sputum, bronchoalveolar lavage fluid,
or lung tissue. Light microscopy can be used
to detect and count asbestos bodies (i.e., asbestos
fibers that have become coated with
iron-containing hemosiderin during residence
in the body, more generically referred
to as ferruginous bodies) in clinical samples.
Electron microscopy (EM) can be used
to detect and count uncoated asbestos fibers
in clinical samples. Methods for such clinical
mineralogical analyses often vary, and valid
background levels are difficult to establish.
The absence of asbestos bodies cannot be
used to rule out past exposure with certainty,
particularly chrysotile exposure, because
chrysotile fibers are known to be less persistent
in the lungs than amphibole asbestos
NIOSH CIB 62 • Asbestos
fibers and are less likely to produce asbestos
bodies [De Vuyst et al. 1998; ATS 2004].
2.7 The NIOSH
Recommendation for
Occupational Exposure
to Asbestos
NIOSH has determined that exposure to asbestos
fibers causes cancer and asbestosis in
humans and recommends that exposures be
reduced to the lowest feasible concentration.
NIOSH has designated asbestos to be a “Potential
Occupational Carcinogen” § . Currently, the
designation “Potential Occupational Carcinogen”
is based on the classification system adopted
by OSHA in the 1980s and is the only
designation NIOSH uses for occupational carcinogens.
After initially setting an REL at 2 asbestos
fibers per cubic meter of air (f / cm 3 ) in
1972, NIOSH later reduced its REL to 0.1 f / c m 3 ,
measured as an 8-hour time-weighted average
(TWA) [NIOSH 1976]. The REL was set at a
§ NIOSH’s use of the term “Potential Occupational Carcinogen”
dates to the OSHA classification outlined in
29 CFR 1990.103, and, unlike other agencies, is the
only classification for carcinogens that NIOSH uses.
See Section 6.1 for the definition of “Potential Occupational
Carcinogen.” The National Toxicology Program
[NTP 2005], of which NIOSH is a member, has
determined that asbestos and all commercial forms of
asbestos are known to be human carcinogens based on
sufficient evidence of carcinogenicity in humans. The
International Agency for Research on Cancer (IARC)
concluded that there was sufficient evidence for the carcinogenicity
of asbestos in humans [IARC 1987b].
The averaging time for the REL was later changed to 100
minutes in accordance with NIOSH Analytical Method
#7400 [NIOSH 1994a]. This change in sampling
time was first mentioned in comments and testimony
presented by NIOSH to OSHA [NIOSH 1990a,b] and
was reaffirmed in comments to MSHA in 2002, with
the explanation that the 100-minute averaging time
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