Measurements from monitoring stations are only able to report air quality at that particular location. The annual map uses a detailed model to show a prediction of what air quality as like across the whole of Greater London.

The four pollutants chosen are ones which are known to have an effect on health within London and are able to be predicted with this model. Levels of carbon monoxide (CO) are now well below EU Limit Values and have not been mapped, However this pollutant remains important in the formation of O₃ across the northern-hemisphere and indoors, exposure to CO from faulty heating and cooking appliances can be dangerous. Concentrations of sulphur dioxide (SO₂) are too unpredictable to create a map using this method. Each pollutant has different sources and behaviour, so try selecting each pollutant in turn to get a full picture of air quality at your chosen location.

Annual means give a good indication of the average level of a pollutant and long-term exposure. Increasingly evidence is pointing to long-term exposure to air pollution as being more important than exposure to short-term peaks. Where annual mean EU Limit Values exist for a pollutant, the map scale is shown such that areas above the Limit Value are shown as yellow through orange to red. Greatest concentrations of traffic pollutants are often predicted in the middle of road junctions or in the centre of busy roads where the public do not normally have access. These are not generally areas of relevant exposure for the assessment of EU Limit Values and UK air quality objectives.

We have chosen this year because it is the latest year for which an accurate model is available.