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

ENVIRONMENTAL EQUITY
While data are limited, racial and socioeconomic characteristics have been
shown to differ in relation to source proximity. Green and colleagues (64) enumerated
all state schools in California that were within 150 metres of a road with
heavy traffic (at least 50 000 vehicles per day) and examined the racial and socioeconomic
characteristics of students attending the schools. A total of 78% of
students at the schools near heavy traffic were non-white, compared to 60% for
schools near very light traffic. Similar differences were seen for indicators of socioeconomic
disadvantage. Finkelstein and colleagues (50) reported higher levels
of TSP and sulfur dioxide in areas with lower socioeconomic conditions in Hamilton,
Ontario, presumably due at least in part to motor vehicle emissions. Affluent
neighbourhoods may also be heavily affected by local sources such as roads.
Hot spots, episodes and cumulative impacts
An urban hot spot is an area within a city where long-term average concentrations
of one or more air pollutants are consistently high compared with other
areas of the same city, thus imposing a potential excess health risk burden. The
hot spot is an especially relevant concept for environmental equity; it implies
disproportionate burdens in certain locations, and the places where people live
and work are often affected by socioeconomic forces. The significance for health
of hot spots may be enhanced where one or more sources contribute a variety
of pollutants to the mix of exposures. Cumulative effects of several pollutants
often are not addressed by regulatory approaches that emphasize the control of
individual pollutants. To the extent that multiple pollutants are at concentrations
close to their guideline levels, the significance for health will be enhanced. Pollutant
indices that take into account concentrations of multiple pollutants are one
approach to addressing this gap.
The hot spot issue is related to, but distinct from, that of the pollution episode.
Whereas a hot spot is characterized by spatially localized elevations in concentrations
that are relatively stable over time, an episode is characterized by temporally
localized elevations in concentrations that are relatively stable over space.
Episodes most often occur as a result of unusual meteorological conditions that
limit the dispersion of locally generated pollution emissions, but an episode will
be exacerbated when there are also unusually large releases of pollution from
one or more nearby sources. Cumulative effects of multiple sources are of special
concern, and may be especially significant for residents and workers closest to
the sources.
Regional air pollution episodes may lead to enhanced concentrations at hot
spots. Day-to-day variations in air pollution concentrations experienced in urban
areas are usually driven by regional-scale weather phenomena, which alter
the degree of dispersion of the locally generated pollution and/or the degree of
delivery of pollution generated at a distance. When stagnant weather conditions
occur (i.e. low wind speed and low mixing height), proportional increases in
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