Environmental Engineering Reference
In-Depth Information
8.3.2
Exposure Metrics
8.3.2.1
Inhalation
In most early studies that looked at the health effects of inhaled particles, dust
(particle) samples were collected by drawing air through a fi lter or other media
and subsequently analysed off-line to defi ne estimates of exposure, expressed as a
concentration in air. For example, in some of the early studies in the coal industry
the samples were analysed by counting particles on the fi lter under a light micro-
scope (Walton and Vincent, 1998). This resulted in an estimate of exposure in terms
of particle number concentration, expressed as number of particles per cm 3 or per
m 3 of air. Epidemiology studies in the coal industry later showed a good correlation
between pneumoconiosis and mass concentration, expressed typically as mg/m 3 .
Subsequently, the use of Workplace Exposure Limits (WEL), based on mass con-
centrations, have become the norm for measuring or regulating exposure for most
hazardous chemicals or particles (HSE, 2005).
A further refi nement of this approach is that sampling of particles is based on
collection of a biologically relevant aerosol fraction . In this context, biological rel-
evance is characterised by where in the respiratory tract a particle can potentially
deposit, and determined as a function of particle size, measured in terms of aero-
dynamic diameter. The conventions which are used to determine biological rel-
evance are the Inhalable, Thoracic and Respirable fractions defi ned in Section 8.2.1.
The one class of aerosols that does not fall into this category is fi brous aerosols.
Fibrous particles such as asbestos or glass fi bre are interesting in three respects:
they have an extreme shape (aspect ratios); their physical behaviour in the lungs
differs substantially from many more compact particles; and they persist for long
periods in the lungs following deposition. Although some of the toxic mechanisms
associated with asbestos exposure remain unclear, it is known that ill health fol-
lowing exposure is associated with physicochemical properties such as fi bre length
and surface chemistry, and that the signifi cance of these properties is exacerbated
by persistence of the fi bres in the lungs. As a result, exposure is not characterised
in terms of averaged mass and composition, but rather by the number (concentra-
tion) of fi bres in the air with a specifi c shape and composition.
What then is an appropriate metric for measurement of exposure by inhalation?
As in the earlier coal industry example, an ideal approach is to choose a metric
which is correlated with the heath effect of concern and can be relatively easily
measured. At the current time, this is not universally agreed. From the toxicology,
there are strong indications that possible health effects may be better correlated
with specifi c surface area, rather than with mass concentration. A number of recent
studies have shown that the infl ammatory response depends on the specifi c surface
area of particles deposited in the proximal alveolar region of the lungs (Tran et al. ,
2000 ; Faux et al. , 2003). On this basis, particle specifi c surface area is a better metric
than mass for relating the particle dose to the infl ammatory reaction. These issues
are discussed in more detail in Chapter 9.
However, the estimate of specifi c surface area does depend on the measurement
technique used. Depending on this and other factors, measurement can increase
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