Controlling the amount of this gas in air has proved particularly difficult for the UK.
For the purposes of air pollution, the UK is divided into 43 zones.
In 2010, when the EU restrictions were meant to come into effect, the levels of nitrogen dioxide were exceeding the limits in 40 of these 43 areas.
Member states were able get an extra five years' grace if they put in place plans to cut levels of NO2. The UK admitted that the limits relating to 16 zones including London, could not be met by the revised deadline of 2015.
For many of these areas, including Greater Manchester, the West Midlands, Merseyside and Glasgow, the government believes the levels can be reached by 2020.
BBC: EU Commission launches legal action over UK air quality
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