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

level intended to (1) protect against the noncarcinogenic
effects of asbestos; (2) materially
reduce the risk of asbestos-induced cancer
(only a ban can ensure protection against
carcinogenic effects of asbestos); and (3) be
measured by techniques that are valid, reproducible,
and available to industry and official
agencies [NIOSH 1976]. This REL was set at
the limit of quantification (LOQ) for the phase
contrast microscopy (PCM) analytical method
for a 400-L sample, but risk estimates indicated
that exposure at 0.1 f/cm 3 throughout a working
lifetime would be associated with a residual
risk for lung cancer. A risk-free level of exposure
to airborne asbestos fibers has not been
established [NIOSH 1976, 1984].
2.7.1 The NIOSH REL as Revised
in 1990
In 1990, NIOSH [1990b] revised its REL, retaining
the 0.1 f/cm 3 limit but explicitly encompassing
EMPs from the nonasbestiform analogs of
the asbestos minerals:
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NIOSH has attempted to incorporate the
appropriate mineralogic nomenclature in
its recommended standard for asbestos and
recommends the following to be adopted
for regulating exposures to asbestos:
The current NIOSH asbestos recommended
exposure limit is 100,000 fibers greater
than 5 micrometers in length per cubic meter
of air, as determined in a sample collected
over any 100-minute period at a flow rate
of 4 L/min using NIOSH Method 7400, or
equivalent. In those cases when mixed fiber
types occur in the same environment,
would help “to identify and control sporadic exposures
to asbestos and contribute to the overall reduction of
exposure throughout the workshift” [NIOSH 2002].
then Method 7400 can be supplemented
with electron microscopy, using electron diffraction
and microchemical analyses to improve
specificity of the fiber determination.
NIOSH Method 7402 … provides a qualitative
technique for assisting in the asbestos fiber
determinations. Using these NIOSH microscopic
methods, or equivalent, airborne
asbestos fibers are defined, by reference, as
those particles having (1) an aspect ratio of
3 to 1 or greater; and (2) the mineralogic
charz elemental composition) of the asbestos
minerals and their nonasbestiform analogs.
The asbestos minerals are defined as
chrysotile, crocidolite, amosite (cummingtonite-grunerite),
antho phyllite, tremolite,
and actinolite. In addition, airborne cleavage
fragments from the nonasbestiform habits
of the serpentine minerals antigorite and
lizardite, and the amphibole minerals contained
in the series cumming tonite-gruner ite,
tremolite-ferroactinolite, and glauco phanerie
beck ite shall also be counted as fibers provided
they meet the criteria for a fiber when
viewed microscopically.
The NIOSH REL for asbestos has been described
in NIOSH publications and in formal
comments and testimony submitted to the Department
of Labor. The recommendation was
based on the Institute’s understanding in 1990
of potential hazards, the ability of the analytical
methods to distinguish and count fibers,
and the prevailing mineral definitions used to
describe covered minerals.
2.7.1.1 Rationale for Encompassing
Nonasbestiform Analogs of
Asbestos Varieties Within the REL
NIOSH’s rationale for recommending that nonasbestiform
analogs of the asbestos minerals
be encompassed within the policy definition
NIOSH CIB 62 • Asbestos