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Pollutants, sources and health effects

Pollutants, sources and health effects for nitrogen oxides, particulates, polynuclear aromatic hydrocarbons and other pollutants.

Nitrogen Oxide

Nitrogen oxides are formed during high temperature combustion processes from the oxidation of nitrogen in the air or fuel. Nitric oxide (NO) and nitrogen dioxide (NO2) are formed and are known collectively as NOx. The species of most interest to human health is nitrogen dioxide. Globally, quantities of nitrogen oxides produced naturally by bacterial, volcanic action and lightening, far outweigh those generated by man's activities. However as they are distributed over the entire surface of the earth the resulting background atmospheric concentrations are very small.

The major source of man made emissions into the atmosphere is from the combustion of fossil fuels i.e. heating, power generation and the internal combustion in motor vehicles. Nitrogen dioxide has several health impacts and includes general irritation to the eyes, irritation of the respiratory system and shortness of breath.


Particulate sources, size and composition vary largely. PM10 (particles less than 10 microns) are a major health concern as these particles are small enough to penetrate deep into the lungs. The major source of PM10's in Europe is from road traffic emissions, particularly diesel. Other sources include construction activities, waste incinerators, quarrying etc.

Particles are either classified as 'primary particulates' and are those which are directly emitted into the atmosphere i.e. road traffic or 'secondary particulates' which are generated in the atmosphere e.g. the oxidation of sulphur dioxide into sulphuric acid. The associated health effects include cardiovascular and respiratory problems.

Polynuclear Aromatic Hydrocarbons

Polynuclear aromatic hydrocarbons (PAH) are a large group of organic compounds which are formed mainly as a result of the incomplete combustion of organic materials. There are many PAH's, however the best known is benzo(a)pyrene (BaP). Long term exposure is suspected to lead to some lung cancers.


Benzene is a volatile organic compound (VOC) and is a major constituent of petrol. The main source in Europe is the combustion of petrol, however refining, distribution and evaporation also accounts for a large percentage. Benzene is a known human carcinogen and long term exposure can lead to cancer.


Ozone (O3) is a secondary pollutants and is produced by a reaction between nitrogen dioxide (NO2), hydrocarbons and sunlight. Nitrogen dioxide participates in the formation of ozone, nitrogen oxide (NO) destroys ozone to form oxygen (O2) and nitrogen dioxide (NO2). Hence where high levels of NO are emitted from vehicles, ozone levels are not as high. Exposure can cause an irritant effect on the lungs, cause airway inflammation and short term respiratory symptoms.

Carbon Monoxide

Carbon Monoxide (CO) is one of the most common and widely distributed air pollutants. It is the product of incomplete combustion of carbon containing materials, and CO is also produced by industry i.e. cement plants, chemical plants etc. In Europe a vast majority of CO comes from road traffic emissions. CO is a toxic gas and reduces the oxygen carrying capacity of blood, at low levels it will reduce concentration and at higher levels it may lead to nausea, cause headaches and dizziness. At very high concentrations it can lead to death.

Sulphur Dioxide

Sulphur Dioxide (SO2) is an acidic gas that combines with water vapour in the atmosphere to produce acid rain. The deposition of acid rain is responsible for the damage of buildings and vegetation. Sulphur Dioxide is derived from the combustion of fossil fuel i.e. power stations . High levels of SO2 tend to occur in areas where coal is still widely used for domestic heating, and in industry. However due to the decline in the use of coal in Europe SO2 levels are no longer considered to pose a significant health risk.SO2 in ambient air is associated with chronic bronchitis and asthma.

Source: World Health Organisation - Air Quality Guidelines for Europe

Updated: 24 February 2021