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

Noteworthy in the findings of the ILSI Working
Group report is the inadequacy of in vitro
test models to predict the in vivo toxicity of
EMPs. Indeed, many man-made mineral fibers
are positive in cell test systems but do not cause
fibrosis or cancer in chronic animal models.
The in vitro test systems lack predictive ability
because they do not incorporate biopersistence.
For this reason, in vitro tests, other than assays
for durability, are not included in the tiered testing
strategy given below.
Step 1. Preparation and characterization
of test EMPs
This is the initial, required step for any toxicological
evaluation. It should include
• comprehensive chemical and mineralogical
characterization, including crystallinity
and EMP habit.
• size distribution of the EMPs found in the
workplace (total particulate sample), as
well as dimensional characteristics of sizeselected
fraction(s) to be used for hazard
evaluation. A limiting step for detailed toxicological
evaluation is the availability of
sufficient quantities of size-selected EMPs
of known chemistry and mineralogy.
Step 2. Assessment of in vitro durability
Evidence indicates that highly soluble fibrous
particles do not exhibit fibrotic or carcinogenic
potential in animal studies. Rate of dissolution in
simulated body fluids should be measured with
a dynamic flow-through system, as outlined by
Potter et al. [2000]. In brief, EMPs are exposed
by continuous flow to a modified Gamble’s solution,
and fiber diameter is monitored optically
over time. Biopersistence would be an indication
of concern and would indicate the need for
further testing of the pathogenic potential of the
NIOSH CIB 62 • Asbestos
EMP. This step is optional, as one could move
directly to Step 3.
Step 3. Short-term in vivo biopersistence test
Biopersistence of fibers longer than 20 µm has
been found to be an excellent predictor of collagen
deposition in chronic inhalation studies
[Bernstein et al. 2001]. Two alternative methods
are accepted by the European Commission
[1997]: intratracheal instillation and 5-day
inhalation by rats. It is recommended that fiber
burden be measured at time points up to
3 months post-exposure. Biopersistence would
be an indication of concern and would indicate
the need for further testing of the pathogenic
potential of the EMP.
Step 4. Subchronic inhalation study
Parameters that should be measured in such
an inhalation study are noted by EPA [2001].
The test should involve inhalation exposure
for 3 months and should evaluate pulmonary
responses over 6 months post-exposure. Responses
to be measured should include biopersistence,
persistent inflammation, cell proliferation
(bromodeoxyuradine [BrdU] assay),
fibrosis, epithelial cell hyperplasia, lung weight,
and fiber burden. Biopersistence and persistent
inflammation are notable markers of concern.
If the subchronic study is positive, a long-term
inhalation study is necessary to conduct a fullrisk
assessment.
Step 5. Long-term inhalation study
The test would include a 2-year inhalation
study in rats, with lifelong follow-up. Fibrosis,
lung tumors, and mesothelioma should be
measured according to EPA guidelines [EPA
2001] for long-term inhalation studies of fibers.
Obtainment of lung burden, dose-related
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