Pollutant Information: Nitrogen Oxides 

About Nitrogen Oxides

Category: Air pollutants

Studies have shown that oxides of nitrogen (NOx) can cause lung irritation as well as lowering people's resistance to pneumonia and bronchitis and other respiratory infections. In the presence of sunlight, NOx can react to produce a photochemical smog. If hydrocarbons are also present ozone can be produced, which has a similar health effect to NOx. Although higher concentrations of NOx are found in city areas, resulting ozone concentrations tend to be higher in rural areas, where crop yields can be reduced as a result.

Road transport was the most dominant source of nitrogen oxides (NOx) in the UK in 2015, with industrial combustion and power generation also accounting for a large fraction of the emission total. Around a third of the UK NOx emissions in 2015 arose from road transport, most of which came from diesel vehicles. The estimation of these emissions is complex since the nitrogen can originate from either the fuel or atmospheric nitrogen. The emission is dependent on the conditions of combustion, in particular temperature and excess air ratio, which can vary considerably. Thus combustion conditions, load and even state of maintenance are important. Since 1970, overall NOx emissions have decreased by 69%, although this decrease has not been constant (see plot below). Up to 1984, the NOx emission profile was relatively flat with small peaks in 1973 and 1979, which were largely due to the cold winters in those years. From 1984, emissions rose markedly as a result of the growth in road traffic and an increase in the proportion of cars running on diesel, reaching a peak in 1989. Since then, total emissions have declined by 69% mainly as a result of emission reductions observed from power stations and the road transport sector. Emissions from road transport fell due to the introduction of catalytic converters and stricter regulations (i.e. Euro Standards), and power sector emissions have declined due to changes in fuels burnt (more gas and less coal), and the use of control technologies to minimise emissions. The UK is currently on track to meet the Gothenburg Protocol target in 2020, but emissions will need to be reduced by a further 21% from 2015 to meet this target.

» View and Download Nitrogen Oxides emission summary data

» View Nitrogen Oxides mapping data

Time series graph


Notable events

Start year End year Sector Information Impact
1970 1991 Road Transport Increase in road traffic without any abatement technology fitted. Increase in emissions
1992 2014 Heavy duty vehicles Stricter Euro I - VI emission regulations come in on trucks and buses offsets increasing vehicle km. Impact takes time to bite as only new vehicles need to meet standards. Decrease in emissions
1992 2014 Passenger Cars Stricter Euro I - 5 emission regulations come in for cars offsets increasing vehicle km. Impact takes time to bite as only new vehicles need to meet standards. Decrease in emissions
1970 1999 Public Electricity and Heat Production Increased electricity generated in nuclear plant. Decrease in emissions
1988 2011 Public Electricity and Heat Production The electricity generators programme of progressively fitting low NOx burners to their 500 MWe (megawatt electric) or larger coal fired units to comply with LCPD restrictions. Decrease in emissions
2007 2011 Public Electricity and Heat Production Programme of fitting over-fire-air burners to comply with LCPD restrictions. Decrease in emissions
1990 1999 Passenger Cars Introduction of petrol cars with early 3-way catalysts. Decrease in emissions
1990 2011 Public Electricity and Heat Production Deregulation in the use of gas for electricity production. Increased use of Combined Cycle Gas Turbine (CCGT) stations and other gas fired plant rather than coal for electricity generation. Decrease in emissions
1993 1993 Agriculture Field burning of agricultural waste stopped after 1993. Decrease in emissions
1990 2010 Passenger Cars Switching from petrol to diesel cars Increase in emissions
1970 1985 Iron and Steel Decline in the energy intensive iron and steel industry and other heavy industries. Decrease in emissions