Nitrogen dioxide (NO2) is one of a group of gases called nitrogen oxides. Road transport is estimated to be responsible for about 50% of total emissions of nitrogen oxides, which means that nitrogen dioxide levels are highest close to busy roads and in large urban areas. Gas boilers in buildings are also a source of nitrogen oxides.

There is good evidence that nitrogen is harmful to health. The most common outcomes are respiratory symptoms such as shortness of breath and cough. Nitrogen dioxide inflames the lining of the lung and reduces immunity to lung infections such as bronchitis. Studies also suggest that the health effects are more pronounced in people with asthma compared to healthly individuals. 

In recent years the average level of nitrogen dioxide within London has not fallen as quickly as predicted. This largely appears to be the result of diesel cars creating more nitrogen dioxide than was anticipated. 

Nitrogen dioxide also reacts with hydrocarbons in the presence of sunlight to create ozone, and contributes to the formation of particles.