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The majority of nitrogen oxides emitted from a vehicle exhaust are in the form of nitric oxide ('NO'), which is not considered harmful to health. However, this gas can react with other gases present both in the exhaust and the atmosphere, to form nitrogen dioxide. Nitrogen dioxide is harmful to health and is also an important component in the formation of ozone.

Road transport is estimated to be responsible for about 50 per cent of total emissions of nitrogen oxides, with power stations contributing another 25 per cent. Nitrogen dioxide levels are highest close to busy roads and in large urban areas.

At very high levels, nitrogen dioxide gas irritates and inflames the airways of the lungs. This irritation causes a worsening of symptoms of those with lung or respiratory diseases.

A shift from coal to gas-turbine power stations and the increased use of catalytic converters during the 1990s should have lead to a decrease in nitrogen dioxide levels over recent years. It is possible that increases in traffic volume have cancelled out these improvements. The situation is further complicated by complex reactions with other pollutants. A long-term trend is not clearly identifiable from monitoring data.