Environmental Engineering Reference
In-Depth Information
or 2) controlled administration of PM metals by bronchial instillation (Ghio and
Devlin
2001
; Ghio
2004
; Lay et al.
1999
; Schaumann et al.
2004
) or inhalation of
CAPs (Huang et al.
2003
; Urch et al.
2004
).
Unlike the epidemiological studies that we reviewed, the exposures to human
volunteers are well defi ned. Major limitations of some, but not all of the studies cor-
relating individual responses with ambient PM were: relatively small numbers of
subjects; self-measurement and self-reporting of PEF and respiratory symptoms
(especially in the studies with young children); personal-level exposures to metals
or other factors (e.g., community respiratory infections) associated with the
response. Innate individual variability in the response being measured over time
complicates interpretation of the results, but refl ects the variability in individual
responses in the general population. In studies in which volunteers were instilled
with PM metals, the investigators controlled both the exposure and the measure-
ment of response, but the number of subjects was necessarily small and the admin-
istration method far removed from breathing ambient air. Inhalation exposure of
human volunteers to CAPs is a relevant route of administration, and only increases
concentrations by factors of 10-40, so can be considered environmentally relevant
to exposures at the upper levels of ambient PM constituents. However, the gaseous
components of air pollution, such as ozone, NOx, and SOx, etc., and any transfor-
mation products, are not represented in exposure to CAPs, so their contribution to
any effects cannot be determined.
The studies that attempted to correlate exposure to metals in ambient PM with
changes in readily-measured indicators of respiratory and vascular function provide
only weak evidence for the involvement of metals. This hardly seems surprising
given the inherent variability in the exposure(s) of interest and variability in response
among subjects, and for the same subject at different times. These study results may
also indicate that metals play little or no role in producing these health effects.
Potential bias from self-reported measurements of PEF and respiratory symptoms in
the Korean and European children studies further compounds the variability. To
some extent, the possibility of fi nding statistically signifi cant associations between
changes in the concentrations of PM-bound metals and changes in the responses
being measured may also have been a matter of luck, with regard to the time period
and locations selected for the studies.
Experiments, in which volunteers were instilled with PM extracts or breathed
CAPs essentially follow the same protocols used in the majority of the
in vivo
ani-
mal studies (except that the human subjects survived the procedures). Moreover, the
fi ndings of increased infl ammatory indicators in the lungs in response to metal-
containing PM extracts are consistent with those observed in the animal studies. The
results are also subject to the same caveats as the in vivo studies, with regard to
amount of material deposited in a limited anatomical location, i.e., by-passing land-
scape of the nose, sinuses and upper respiratory tract, and potential effects related to
the surgical procedure.
It should also be recognized that invasive studies (e.g., bronchoscopy) with
humans are costly to conduct, and the number of subjects is necessarily small.
However, the investigators optimized the information obtained by having each sub-
ject serve as his/her own control. The numbers of human subjects in each treatment
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