Environmental Engineering Reference
In-Depth Information
70 kg to convert these concentrations into a daily dose for our “standard human”.
Representative body weights for dogs, rats and mice were 10 kg, 0.3 kg and 0.02 kg,
respectively. The ratio of the study dosage to this mean daily human dose provides
a common metric for all studies. (Please see the footnote to Table
6
for additional
assumptions, e.g., breathing rates and body weights of the test animals, etc.).
3
Results
3.1
Exposure
Understanding the range of ambient concentrations of PM-associated metals is
essential to putting epidemiological and toxicological studies into perspective. We
limited our analysis to data collected in support of large human exposure studies
and national networks and those studies containing ambient, indoor and personal air
data for the ten target metals.
3.1.1
Concentrations of Transition Metals in Ambient Air
Epidemiology studies rely upon measurements of ambient PM and its components
to represent exposure to populations of interest. Airborne elements are generally
measured using actively pumped samplers and analyzing the resulting fi lter loadings
by one of several methods. The most common are X-ray fl uorescence (XRF),
proton-induced X-ray emission (PIXE), atomic absorption spectrometry (AAS),
and inductively coupled plasma mass spectrometry (ICP-MS), which is becoming
more popular, because smaller concentrations of some metals are detectable by
using this technique.
The two main large-scale networks operating in the United States (U.S.), the
IMPROVE and the STN provide useful information on ambient concentrations of
transition metals in urban and rural areas. The IMPROVE network of 110 sites (US
EPA
2010a
) was set up in 1987 to measure visibility and particle and elemental
concentrations in areas where visibility is protected by stringent regulations, such as
National Parks. Approximately 24 elements are being analyzed by XRF in about 60
sites. The STN (US EPA
2010b
) was designed in the 1990s to more completely
analyze the various components of PM
2.5
, and by 2000 it was operating in 174 urban
and suburban sites. STN performs analyses for 48 elements.
Thus, the two networks are expected to represent the range of ambient concentra-
tions experienced by the vast majority of the U.S. population. Both networks report
a detection frequency of 85-100% for Fe, Zn, and Cu. The remaining seven metals
are detected at least 40% of the time in the STN. Ni and As are found more often in
the STN; and, some crustal metals such as Ti and Se are found more often in
IMPROVE. The urban/rural ratios of mean concentrations range between 2 and 4
for 7 of the 10 target metals. Mean ambient concentrations in the PM
2.5
fraction
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