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

HUMAN EXPOSURE TO AIR POLLUTION
Spatial variation in ambient air concentrations
Spatial variation determines to what extent ambient concentrations measured at
a single fixed site reflect the outdoor concentrations at other sites in the area. An
important factor for the spatial variation of a primary air pollutant is the geographical
distribution and type of the emission sources (e.g. line source, point
source). After the emission takes place, inert pollutants such as carbon monoxide
simply disperse and a concentration gradient with increasing distance to the
source develops. For chemically reactive pollutants, such as nitric oxide, a steeper
concentration gradient than for inert pollutants develops. In contrast, the formation
of secondary pollutants is a large-scale phenomenon and these pollutants
have quite uniform spatial distributions. Furthermore, diffusion and transport
of pollutants are determined by atmospheric conditions such as wind speed and
solar radiation. The time-scale of the small-scale spatial variation may also be
important. The size of short-term spatial fluctuations is different from spatial
fluctuations in annual means (28).
For PM, the spatial variability depends on the size fraction. For fine particles,
the spatial variability is generally small, whereas ultrafine and coarse particles
have a much stronger spatial variability (29,30). Even for fine particles, however,
the spatial variation can be significant for specific components such as elemental
carbon (e.g. diesel soot) (31). Nitrogen dioxide concentrations exhibit quite
strong spatial variability owing to local sources such as cars. While the spatial
variability of ozone is rather low across larger areas, within city gradients it can
be pronounced owing to the reaction of ozone with traffic and other combustion
emissions of nitrogen oxides (3).
Penetration indoors of outdoor-generated air pollutants
The amount of air pollution penetrating from outdoors to indoors depends on
the penetration coefficient, the ventilation rate and the decay rate (32). Particles
in the ambient air penetrate to the indoor air, although some filtering takes place.
The smallest particles penetrate almost completely from outside to inside. Ozone
is a highly reactive component that reacts quickly with surfaces when penetrating
indoors, which is why ozone levels indoors are generally much lower than
those measured outdoors. Ozone concentrations, however, are generally high
during hot and sunny weather, precisely the conditions under which people open
their windows and doors and spend more time outdoors. Nitrogen dioxide is also
reactive, albeit less than ozone. In the absence of sources in the indoor air, nitrogen
dioxide and sulfur dioxide concentrations indoors will be lower than those
outside.
Household ventilation is a function of both climate and development. Ideally,
such ventilation is appropriate to the surrounding environment, with reduced
ventilation rates in colder climates/seasons and increased rates in more temperate
climates/seasons. Ventilation tends to be better controlled with increased
73