Environmental Engineering Reference
In-Depth Information
Figure 2.4
Cooling tower constructed using asbestos fibrocement panels.
Asbestos-containing fibrocement materials were once widely used in the
construction of farm and other utility buildings and mechanical-draft cooling
towers. In the latter case, asbestos-containing cement board was extensively
used in external and internal cooling tower components (
Figure 2.4
)
.
C. Asbestos exposures
Because of the many desirable properties of asbestos and its widespread use
in ACM, it is a ubiquitous contaminant of both indoor and outdoor air. A
number of studies have been conducted to assess levels of asbestos
fibers/structures in indoor and ambient air. Unfortunately, these studies
used a variety of optical methodologies to determine fiber concentrations in
collected samples. Early studies are based on phase-contrast microscopy
used in occupational exposure monitoring. Phase-contrast microscopy can-
not distinguish between asbestos and non-asbestos fibers, and fibers with
small diameters (<0.5
m) cannot be easily seen. Asbestos fibers are best
analyzed using transmission electron microscopy (TEM) and direct sample
preparation techniques.
µ
1.
Units of measurement
Concentrations of airborne asbestos have been expressed in a variety of
ways. These include: (1) fibers with aspect ratios of
≥
3:1 or
≥
5:1 reported as
fibers per cubic centimeter (f/cc or f/ml), (2) structures (
≥
5
µ
m) per liter
(s/l), and (3) fiber mass per unit volume (ng/m
). The term “structure” refers
3
to fibers, clusters, bundles, and matrices.
Only concentrations of asbestos fibers with lengths
m are used for
risk assessment calculations since epidemiological studies have shown that
asbestos-related disease increases significantly with exposure to asbestos
fibers
≥
5
µ
m. Though there is no sharp demarcation of asbestos toxicity
associated with decreasing fiber length, occupational exposure standards are
based on fibers
≥
5
µ
≥
5
µ
m. Such concentrations are best characterized as an index
