United Nations Environment Programme

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Tool 1: Air pollution and transport


Primary pollutants -What comes out of a tailpipe: When fuel is burned in a vehicle engine, there will always be some emissions. The most common emissions are:






 Photo: Fleet Forum

Secondary pollutants - Pollutants formed from tailpipe emissions: Some air pollutants are not directly emitted but are formed later through reactions of other emissions in the air. These so-called secondary pollutants include:

  • NOx and VOCs, which form ground-level ozone (O3)
  • Emissions of sulphur oxides (SOx), which form acid rain
  • NOx, SOx and VOCs, which also form very small particulate matter (i.e. PM2.5 - particles with diameters of less than 2.5 microns)

All of these pollutants, except for carbon dioxide (CO2), can seriously affect human health (See Tool 3). Rising concentrations of carbon dioxide (CO2) in the atmosphere have severe effects on the environment (e.g. climate change, see Tool 6), and will indirectly have severe health effects on human populations.

Apart from these 'traditional' air pollutants, there are an increasing number of other toxic and carcinogenic chemicals being detected in the urban atmosphere. Although the absolute concentrations of these chemical pollutants in the atmosphere are low, the toxicity and/or carcinogenic effects can be substantial for human health.

For more info on air pollution and transport >>   www.epa.gov,
                                                                      www.cleanairnet.org,
                                                                      http://themes.eea.europa.eu