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

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AIR QUALITY GUIDELINES
time series studies and applied to the observed scenario of air pollution reduction
(91). This again suggests that the time series approach does not capture the
full range of effects attributed to air pollution exposure, which includes effects of
short- and long-term exposures. Consequently, it will underestimate the benefits
of outdoor air pollution control interventions.
Toxicological studies
By generating supporting evidence on the types of physiological effect exerted
by air pollutants and the biological mechanisms underlying these effects, toxicological
research has played an important role in advancing understanding of
the associations between air pollution and mortality and morbidity reported in
epidemiological studies (97,98).
Much of the evidence contributed by the toxicological sciences on the effects
of air pollution has been generated through inhalation studies, whereby human
volunteers or animals are placed under controlled exposure conditions. When
conducted in humans, such studies are also known as human clinical studies. It
is through these studies that the assessment of subclinical respiratory and cardiovascular
effects, such as changes in lung function, blood pressure and heart rate,
has been possible (97–100).
Controlled exposure conditions may lead to a more adequate assessment of biological
mechanisms and dose–effect relationships than epidemiological studies.
Under controlled conditions, the exposure is well-documented and standardized
while study subjects are exposed to the same concentrations for similar periods.
In addition, it is possible to assess effects from exposure to single pollutants or a
mixture of pollutants. The assessment of single pollutants provides valuable information
on individual mechanisms of action, while the study of mixtures is important
because it may be a better approximation to “real” exposure conditions
(97,101).
Over the last decade, the development of technologies to concentrate particles
derived from outdoor air has led to an improved understanding of effects of particles
on health. The concentrator approach facilitates studies of exposure to fine
(0.1–2.5 μm in diameter) and ultrafine (