EU limits, WHO guidelines and main sources of ambient (outdoor) air pollutants. Adapted from European Commission Air Quality Standards (updated September 2017), WHO Ambient (outdoor) air quality and health fact sheet (updated Sept 2016), and Lethal and Illegal, Solving London’s Air Pollution Crisis by Institute for Public Policy Research, November 2016
| Pollutants | EU legal limits (averaging period) | WHO guidelines (averaging period) | Main sources |
| Nitrogen dioxide (NO2) | 200 µg/m3 (1 hour) 40 µg/m3 (1 year) | 200 µg/m3 (1 hour) 40 µg/m3 (1 year) | Transport, combustion |
| Ozone (O3) | 120 µg/m3 (8 hours) | 100 µg/m3 (8 hours) | Reaction of hydrocarbons, nitrogen oxides and volatile organic compounds in sunlight |
| Particulate matter (PM10) | 50 µg/m3 (24 hours) 40 µg/m3 (1 year) | 50 µg/m3 (24 hours) 20 µg/m3 (1 year) | Transport (exhaust, tyre, brake wear), combustion, industrial processes and construction |
| Particulate matter (PM2.5) | 25 µg/m3 (1 year) | 10 µg/m3 (24 hours) 25 µg/m3 (1 year) | |
| Sulfur dioxide (SO2) | 350 µg/m3 (1 hour) 125 µg/m3 (24 hours) | 500 µg/m3 (10 min) 20 µg/m3 (24 hours) | Coal combustion and road transport |