What is an air pollution episode?

Short term air pollution levels can remain low for weeks at a time, whilst higher levels tend to occur in episodes that can last several days. These episodes are rarely the result of changes in local sources because the amount of traffic on the road remains fairly steady.

Instead, these episodes often happen when the weather conditions cause pollutant concentrations to build up above normal levels. They can also be caused by the route the air has travelled over previous days. For example, if the air passes over Europe it can pick up pollutants on the way.

Most episodes can be termed as either winter smog or summer smog episodes, which have different behaviour.

These episodes can affect health, and raise the profile of air quality when they occur.

• You can see a description and pollutant levels for the latest episode in London, and those back as far as May 2002.
• You can see a map of the latest hourly air quality.