NOx nitrogen oxides. The principal source of nitrogen oxides (a combination of nitric oxide (NO) and nitrogen dioxide (NO₂) is road traffic, which is responsible for more than half the emissions the UK.

NO and NO₂ concentrations are greatest in urban areas where traffic is heaviest. Concentrations at rush hour can be up to 20 times greater than rural areas.

Nitric oxide (NO) is an odourless, colourless gas which is produced when fuel burns. Once mixed with air it quickly combines with oxygen, forming nitrogen dioxide (NO₂). Nitrogen dioxide is highly toxic. It causes serious lung damage. It also leads to shortness of breath and chest pains.

Nitrogen oxides also contribute to the formation of ozone, which is a harmful secondary pollutant in the lower atmosphere and also an important greenhouse gas contributing to global warming. Increased levels of ozone increase susceptibility to respiratory disease and irritate the eyes, nose and throat.

Oxides of nitrogen also effect vegetation by reducing growth. In addition they contribute to acid rain.

Cyclists don't emit nitrogen oxides and their lung capacity is enhanced by pedaling..

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