Air Quality Guidelines Global Update 2005 - World Health ...

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AIR QUALITY GUIDELINES
Uncertainties
Clearly, there are many uncertainties involved in estimating the health effects associated
with outdoor air pollution. Over time, some of these will be reduced as
new research is conducted. Nevertheless, some uncertainty will be inherent in
any estimate. For many of these issues, the bias could be either positive or negative.
We briefly discuss below some of the major uncertainties.
There are several uncertainties and limitations involved in using epidemiological
evidence for quantitative assessment. A key assumption is that the relationship
between air pollution and health effects is causal. Then, the observed
association can reasonably be used to predict how changes in pollution concentrations
will influence the incidence of health effects. Observational epidemiological
studies are able to demonstrate whether an association exists between
health effects and pollution concentrations, but it is more difficult for the studies
to prove that the relationship is causal. It is possible that a statistically significant
relationship is actually due to some unidentified factor that is correlated with
pollution concentrations. Nevertheless, the likelihood of causation is strengthened
when: (a) epidemiological results are replicated by similar findings in different
studies with variable underlying conditions; (b) multiple health outcomes
appear to be affected in a consistent and coherent manner; and (c) the results are
supported by either toxicological or controlled studies on humans. Such is the
case with PM, where numerous studies have found a similar magnitude of effect
for mortality, and have also linked it with a wide spectrum of cardiorespiratory
health outcomes. There is a growing body of laboratory and clinical evidence
regarding the health effects associated with exposure to PM, although biological
mechanisms that underlie the observed epidemiological associations have not
been fully elucidated. This adds some additional uncertainty when using epidemiological
relationships to predict how health may change as a result of changes
in ambient pollution concentrations.
Another large uncertainly concerns dependence on epidemiological studies for
risk estimates. Epidemiological studies have both advantages and disadvantages
in estimating the health impact of air pollution. Epidemiological studies involve
the study of humans in real situations, and thus human health effects are observed
under a wide range of behavioural situations and conditions. The obvious advantage
of such studies is the lack of a need to extrapolate results across species or
dose levels. A wide range of health outcomes can be studied, including mortality,
and a wide range of subgroups can be examined in detail. For example, it is possible
to examine the effects of air pollution on individuals with severe asthma or
on elderly people with chronic bronchitis. In addition, researchers can examine a
wide range of pollutants and pollutant mixes by considering multiple seasons or
locations, and exposures over many days, months and years can be investigated.
Among the disadvantages of this approach are imprecision in measuring exposure
and response, and potential confounding of the air pollutant measure by