Explainer

Air Quality: How is air quality measured?

The air around us can have a serious impact on our health. In the UK an estimated 28,000 to 36,000 premature deaths a year are caused by long term exposure to man-made air pollution.

Across the country hundreds of recording stations are constantly checking the quality of the air we breathe. 

They measure the pollutants that are most likely to affect health:

  • Nitrogen dioxide

  • Ground level ozone

  • Particulate matter

  • Sulphur dioxide

All this information is used to create the Daily Air Quality Index, and help forecast pollution levels

But what creates these pollutants?

Nitrogen dioxide NO2

This is a gas that forms the majority of pollution in urban settings. It comes from vehicles, power stations and heating homes and buildings. Diesel vehicles are particularly big polluters and levels of nitrogen dioxide are often highest by busy roads. 

Ozone O3

Not to be confused with the “ozone layer”, ground level ozone is formed when nitrogen dioxide (see above) reacts with sunlight.

It’s usually higher in late spring/summer and lower in winter. Levels also change during the day - it’s often low in the morning and builds during the day to peak late afternoon. Interestingly, levels are usually higher in the countryside. This is because it is suppressed by local traffic emissions in urban areas. 

Particulate Matter: PM10 & PM2.5 

This is particles which can be fine or coarse, man-made or natural. PM10 particles have a diameter smaller than 10 microns (10µm) – that’s 100 times smaller than a millimetre. PM2.5  particles are even smaller.

Man-made sources include diesel and petrol engines, rubber from tyres and brake dust, industrial processes, construction, cooking and farming. Natural sources include wind-blown dust, sea salt, pollen, soil and so on.

Sulphur dioxide SO2

This gas is produced by burning sulphur-containing fuels such as coal and oil. The main sources are vehicles, power stations, steelworks, power plants, refineries and heating. It can travel over long distances and can contribute to ozone.

Pollution episodes

These are when there are elevated levels of pollution in the air. Most pollution episodes in the UK are due to higher levels of particulate matter (namely PM2.5 ) or ozone. But all of the above is measured to create the Daily Air Quality Index (DAQI) to forecast pollution levels. 

The forecast itself is rather like a weather forecast where you can see the air quality index for today, tomorrow and the outlook. It is a good prediction of the pollution episode, the source, the general area affected and the length. However, pollution episodes are even more sensitive than weather forecasts to certain weather conditions. So the forecasts can change significantly in the outlook from what was predicted. It’s also worth noting that it is a general area forecast and won’t capture the possibility of higher pollution levels next to busy roads. 

All this information is used to create the Daily Air Quality Index, and help forecast pollution levels

Air Quality Index Example Credit: ITV Weather - with data from DEFRA and Met Office

 It uses a colour coded scale of 1 to 10.

Low air pollution is between 1 and 3, Moderate between 4 and 6, High is between 7 and 9, and Very High is 10 on the scale 

Air pollution levels Credit: ITV Weather, DEFRA and Met Office
  • 1-3: Enjoy your usual outdoor activities

  • 4-6: At risk individuals consider reducing physical activity outdoors

  • 7-9: Anyone experiencing discomfort should reduce physical activity

  • 10: Everyone should avoid or reduce physical activity

You can find more information on air quality on the CleanAirHub