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

HEALTH EFFECTS OF AIR POLLUTION: AN OVERVIEW
Studies of effects of air pollution interventions
There have been some epidemiological studies that have attempted to evaluate
the health effects of actions that have resulted in improved air quality. These include
studies that examined the effects of interventions aimed at controlling and
reducing levels of air pollution over urban areas (91,92) or studies of “natural experiments”,
i.e. interventions not designed for air pollution control but that have
resulted in a reduction in pollution levels (93–95). In general terms, these studies
compare periods before and after the intervention and the impact of the observed
reduction in pollution on the mortality and morbidity of the population.
These studies are important because information obtained from them provides
strong support for a causal relationship between air pollution and health
effects. Moreover, when evaluating interventions that produced sharp reductions
in pollution levels over a relatively short period, the possibility of confounding
by other risk factors is reduced. Nevertheless, only a few such studies have been
conducted.
In Utah Valley, a reduction in air pollution levels caused by a year-long strike
at a local steel mill was associated with reductions in total deaths and respiratory
admissions (93,94). During the Olympic Games held in Atlanta in 1996, citywide
changes in transportation patterns reduced vehicle exhaust and related air
pollutants (such as ozone) by about 30%, the number of acute asthma attacks fell
by 40%, and paediatric emergency admissions dropped by 19% (95). Clancy et
al. (91) examined the impact of a ban on coal sales in Dublin and found an 8%
fall in mortality associated with a sustained reduction in average particulate air
pollution. In Hong Kong, China, a restriction introduced over just one weekend,
requiring that all power plants and road vehicles use fuel with a lower sulfur content,
led to an immediate fall in ambient sulfur dioxide levels and a substantial
reduction in seasonal deaths. The average annual trend in deaths from all causes
declined by 2% and that in deaths from respiratory causes by 3.9% (92).
Cohort studies with follow-up during periods of substantial change in air pollution
can also provide an opportunity to assess the effect of recent vs past exposures.
In an extended follow-up analysis of the Six Cities study, when there was
an observed reduction in air pollution levels, Laden et al. (17) detected a reduction
in overall mortality associated with decreased mean levels of PM2.5.
The evidence provided by epidemiological studies designed to assess the
health impact of substantial changes in air pollution levels is of considerable
public health interest. To date, attempts to assess health impact have been based
mostly on risk estimates obtained from other types of epidemiological study and
applied in hypothetical air quality scenarios (96). In most cases, however, these
results have not been confirmed through comparison with studies of real interventions
with clear reductions in air pollution.
The observed impact of the ban on using coal in Dublin was nearly twice as
large as those predicted by models using estimates obtained from traditional
99