What it means: nitrogen dioxide comes mostly from vehicle exhausts, especially diesel, and irritates the lungs. The figure is the average across the UK’s roadside monitoring sites; individual busy roads can be much higher. The legal limit is an annual average of 40 at any site; the World Health Organization guideline is 10. Separately, the UK must cut its total emissions of nitrogen oxides by 73% from 2005 levels by 2030. A change of 1 or more counts.

Source: Defra, air quality statistics: nitrogen dioxide.