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

1996; Tran and Buchanan 2000]. Thus, results
from chronic inhalation studies with rodents
exposed to EPs may underestimate risks for
humans, and adjustment for kinetic differences
in particle clearance and retention in rats is required
to predict lung disease risks in humans
[Kuempel et al. 2001].
Studies using in vitro assays have been conducted
with various SVFs and silicate minerals
to determine the dissolution rate in simulated
lung and lysosomal fluids [Hume and Rimstidt
1992; Werner et al. 1995; Hesterberg and Hart
2000; Jurinski and Rimstidt 2001]. In vitro dissolution
studies can provide a rapid and more
controlled alternative to classic long-term toxicity
testing in animals and could provide useful
information when performed as companion
experiments with in vivo studies if conditions
of exposure and test agent can be made similar.
The design of in vitro assays is intended to
mimic the biological conditions that exist in the
lung once the EP comes into contact with lung
tissue or macrophages. Although uncertainties
exist about the specific physiological processes
that occur in the lung, results from in vitro assays
can provide some insight into the chemical
reactions that influence EP dissolution. For
example, it appears that EP (e.g., glass fiber)
dissolution occurs more readily when the EP
is in contact with a fluid that is undersaturated
with respect to the EP’s composition. The condition
of undersaturation must be maintained
at the EP’s surface for dissolution to continue.
If an EP is surrounded by a saturated or supersaturated
solution (compared to the EP composition),
then no further dissolution occurs.
The results from many in vitro experiments
demonstrate different patterns of dissolution
for most of the tested EP types (i.e., glass, asbestos)
under various test conditions. This effect
was most notable in those experiments where
44
different pH conditions were used. Fluid pH
appears to influence the creation of complexes
from the leached elements of the EP, which in
turn alters the rate of solubility. Chrysotile fibers
tend to dissolve readily in acids because of the
preferential leaching of magnesium (Mg) from
the fiber. The leaching of Mg from tremolite and
anthophyllite and of sodium from crocidolite
also occurs more readily in acid conditions.
The rate of EP dissolution has also been observed
to be affected by differing internal and
surface structures. EPs with porous or rough
surfaces have larger surface areas than smooth
EPs with the same gross dimensions. These
larger surface areas interact more readily with
the surrounding medium because of the greater
number of sites where solute molecules can
be absorbed. EMPs with cleavage plane surfaces
will contain varying degrees of defects;
the higher the number of surface defects, the
greater the potential instability of the particle.
Dissolution of these types of EMPs is typically
initiated where surface vacancies or impurities
are present [Searl 1994]. Chrysotile asbestos
is an example of a sheet silicate made up
of numerous fibrils composed of tightly bound
rolled layers of Mg hydroxide. These Mg hydroxide
layers are readily leached by acid solutions
within human tissues [Spurny 1983],
causing disintegration of the fibril’s crystalline
structure. In contrast, the amphibole asbestos
minerals are chain silicates with a crystalline
structure comprising alkali and alkali
earth metals that are tightly bound, making
the fibers less susceptible to dissolution. In
contrast to the crystalline structure of the asbestos
fibers, some high-temperature glass fibers
are more stable than chrysotile fibers because
they are composed of silicate chains,
sheets, and frameworks [Searl 1994]. The absence
of cleavage planes or structural defects
in glass fibers limits the degree to which fluids
NIOSH CIB 62 • Asbestos