The dominant issues determining short-term air quality are nitrogen dioxide, particulate matter and ozone. All three pollutants are strongly related to the use of fossil fuels. In urban environments traffic is the dominant source of nitrogen oxides and particles emissions. Power generation and occasionally an industrial site are also important sources. Heating houses is a source, especially when wood or coal is being used.

In addition to the pollution created by the combustion of fuels (mainly particles, nitrogen dioxide and sulphur dioxide) other pollutants are/were brought into the environment because they are part of consumables: volatile organic carbons (VOCs) in solvents, paints and glues and benzene and lead in fuel. Some are released into the atmosphere as bi-products or leakages in industrial processes. This includes storage and transport leakages of VOCs and the emissions of micro pollutants such as PAHs or dioxins.

Looking at the total emission in tons per year, industries and power generation tend to be the major contributors. However, these emissions mainly occur at considerable heights. The emissions are mixed with large volumes of air and their contribution to ambient concentrations is often relatively small. Traffic emissions on the other hand occur at low levels, in the ambient air layer. Especially in narrow streets, where dispersion and dilution is slow, traffic might cause serious air pollution even when emissions are small. The apparent paradox between emission and concentration contributions of the different sources is shown in the pie-charts from the city of Rotterdam.