The data on the number of days when pollution in London is moderate or higher over the last 10 years is outlined below. This is part of the air quality section of the London State of the Environment Report 2010.
How air pollution is assessed
The UK government uses a banding system to categorise the severity of air pollution. It is based on the impact it will have on the health of those who are sensitive to air pollution. This includes the elderly, young children and individuals who suffer from heart and lung conditions such as asthma.
About this indicator
This indicator identifies the number of days when air pollution was moderate or higher in London between 1998 and 2008. It uses data from the London Air Quality Network.
The indicator focuses on a small selection of sites in London. They were chosen because the sites had been consistently monitored for a number of years and had attained a high level of data capture. Each site is representative of other sites in London, for example, Marylebone Road represents the busiest roads in the capital.
The four bands are defined by an air pollution index. Both the bands and the index have been approved by the Committee on Medical Effects of Air Pollution Episodes. These bands also line-up with thresholds identified in the European Commission’s directives on air quality.
The pollutants
The pollution index, and therefore the bands, are based on the concentration of a basket of five pollutants at each site:
- nitrogen dioxide (NO2)
- sulphur dioxide (SO2)
- ozone (O3)
- carbon monoxide (CO)
- particulate matter less than 10 micrometres (µm) (PM10).
The banding identified for any site or region on a given day will be dependent upon the highest index classification at that time. This could mean that it is determined by only one pollutant if its concentration is far higher than the other pollutants.
Air pollution index bands
The index ranges from one when air pollution levels are low, to 10 when they are very high.
| Band |
Index |
Impact on health |
| Low |
1-3 |
Effects are unlikely to be noticed, even by those who are sensitive to air pollution. |
| Moderate |
4-6 |
Sensitive people may notice mild effects, but these are unlikely to need action. |
| High |
7-9 |
Sensitive people may notice significant effects. Action may be required. |
| Very high |
10 |
Effects on sensitive people may worsen. Action may be required. |
Overall trends
There is no clear trend from year to year in the number of days when pollution is moderate or higher. This is because it is dependent on weather conditions.
Marylebone Road had the greatest number of days recorded at any one site when pollution was moderate or high. Research has shown that measurements taken at the kerbside on Marylebone Road are representative of the busiest roads in central London.
For days when pollution is moderate or high at sites which lie adjacent to roads, the primary cause is normally particulate matter.
For background urban sites (urban sites away from major sources of air pollution that are broadly representative of background concentrations), peaks in air pollution will mainly be due to ozone. This secondary pollutant is particularly prevalent during hot, calm summers when conditions favour its formation and it is not easily dispersed.
Days of moderate or higher air pollution
The chart above shows that there was a peak in the number of days when air pollution was moderate or higher in 2003, closely followed by a peak in 2006. Both of these years had hot, dry and calm summers – ideal conditions for allowing pollution to persist in the lower atmosphere as pollutants are prevented from being dispersed.
Days of high or very high air pollution
The graph above shows the number of days when air pollution was high or very high at a number of selected sites between 1998 and 2008.
There was a peak in the number of days in 2003, closely followed by a peak in 2006. During these years weather conditions during the summer months exacerbated the concentration of pollutants in the atmosphere. The patterns are very similar to those shown in the first chart on moderate or higher air pollution.
