AIR AND HEALTH Dispersal of air pollutants

Once in the environment, air pollutants may be dispersed via air, water, soil, living organisms and food. The pathways of dispersion vary greatly, depending upon both the emission source and the pollutant concerned. Rates and patterns of dispersion also depend to a large extent upon environmental conditions. Pollution dispersal in the air is affected by many factors:

• meteorological conditions (especially wind speed, wind direction and atmospheric stability),
• the emission height (e.g. ground level sources such as road traffic or high level sources such as tall chimneys),
• local and regional geographical features,
• the source (e.g. fixed point, such as a chimney, or a diffuse number of sources such as cars and solvents).

During dispersion pollutants undergo a wide array of changes and transfers. Dilution occurs owing to mixing into the air. Separation or accumulation of pollutants occurs on the basis of physical characteristics of the pollutant. Chemical reactions occur, breaking down the original pollutant or converting it into new compounds. Some pollutants can also be removed from the transporting medium through deposition, for example, by settling out under the effects of gravity, by rainwash or by interception (scavenging) by plants and other obstructions.

Many pollutants therefore show extremely complex dispersion patterns, especially in environments such as cities and towns where there are a large number of emission sources and major variations in environmental conditions. This complexity means that it is often very difficult to model or measure pollutant patterns and trends, and thus to predict levels of human exposure.

Temporal variations in pollution levels are important. In many cases long-term trends exist, reflecting underlying changes in the rates of emission (e.g. as a result of technical or economic changes, or due to policy intervention). Superimposed upon these there may be annual variations, reflecting year-to-year differences in climate or source activity. Many pollutants also show marked seasonal, weekly and daily patterns, owing to cycles of activity and short-term climatic and other effects. Major, short-term pollution episodes may also occur as a result of sudden, accidental releases.
Therefore measurements of exposure will vary according to when, where and for how long air monitoring is carried out.