Pollutants - air quality

What is it?

Nitrogen dioxide - NO2 - is one of a group of gases called nitrogen oxides ('NOx') formed in the combustion of fossil fuels.

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.

Where are the trouble spots?

Road transport is estimated to be responsible for about 50% of total emissions of nitrogen oxides (NOx), power stations contributing another 25%.

The calculation of how much of this NO is converted to NO2 is an important, but extremely difficult factor in the reduction of pollution levels. Because of the domination of traffic sources, mean nitrogen dioxide levels are highest close to busy roads and in large urban areas.

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.

What are the health effects?

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.