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Causes of Air Pollution

Below is an introduction to the principal pollutants produced by industrial, domestic and traffic sources:.

Download a detailed list of the above pollutants - What are the causes of Air Pollution (PDF 139KB). Historically, the main air pollution problem in both developed and rapidly industrialising countries has typically been high levels of smoke and sulphur dioxide emitted following the combustion of sulphur-containing fossil fuels such as coal, used for domestic and industrial purposes. These days, the major threat to clean air is now posed by traffic emissions. Petrol and diesel-engined motor vehicles emit a wide variety of pollutants, principally carbon monoxide (CO), oxides of nitrogen (NOx), volatile organic compounds (VOCs) and particulate matter (PM10), which have an increasing impact on urban air quality. In addition, pollutants from these sources may not only prove a problem in the immediate vicinity of these sources, but can be transported long distances. Photochemical reactions resulting from the action of sunlight on nitrogen dioxide (NO2) and VOCs, typically emitted from road vehicles, lead to the formation of ozone. Ozone is a secondary pollutant, which often impacts rural areas far from the original emission site as a result of long-range transport. In all except worst-case situations, industrial and domestic pollutant sources, together with their impact on air quality, tend to be steady or improving over time. However, traffic pollution problems are worsening world-wide. Because of their potential impacts on human health, welfare and the natural environment, ambient concentrations for a number of these pollutants are measured continuously at a wide range of rural and urban locations throughout the UK. To find out more about the background to air pollution visit Enviropedia.

Sulphur dioxide (SO2)

Sulphur dioxide is a corrosive, acidic gas which combines with water vapour in the atmosphere to produce acid rain. Both wet and dry deposition have been implicated in the damage and destruction of vegetation and in the degradation of soils, building materials and watercourses. SO2 in ambient air is also associated with asthma and chronic bronchitis.

Nitrogen oxides

Combustion processes emit a mixture of nitrogen oxides (NOX), primarily nitric oxide (NO) which is quickly oxidised in the atmosphere to nitrogen dioxide (NO2). Nitrogen dioxide has a variety of environmental and health impacts. It is a respiratory irritant which may exacerbate asthma and possibly increase susceptibility to infections. In the presence of sunlight, it reacts with hydrocarbons to produce photochemical pollutants such as ozone. NO2 can be further oxidised in air to acidic gases, which contribute towards the generation of acid rain.

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Particular matter (PM10, PM2.5 and PM1)

Airborne PM includes a wide range of particle sizes and different chemical constituents. It consists of both primary components, which are emitted directly into the atmosphere, and secondary components, which are formed within the atmosphere as a result of chemical reactions. Of greatest concern to public health are the particles small enough to be inhaled into the deepest parts of the lung. Air Quality Objectives are in place for the protection of human health for PM102.5 – particles of less than 10 and 2.5 micrometres in diameter, respectively.

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Ozone and volatile organic compounds

Ozone (O3) is not emitted directly into the atmosphere, but is a secondary pollutant generated following the reaction between nitrogen dioxide (NO2), hydrocarbons and sunlight. Whereas nitrogen dioxide acts as a source of ozone, nitric oxide (NO) destroys ozone and acts as a local sink (NOX-titration). For this reason, O3 concentrations are not as high in urban areas (where high levels of NO are emitted from vehicles) as in rural areas. Ambient concentrations are usually highest in rural areas, particularly in hot, still and sunny weather conditions which give rise to summer "smogs".

Monitoring of PMM10 levels in the UK has to date, been largely based upon the use of TEOM analysers. A principal concern with the TEOM instrument is that the filter is held at an elevated temperature (50°C) in order to minimise errors associated with the evaporation and condensation of water vapour. This can also lead to the loss of the more volatile species and has led to the identification of differences between TEOM and gravimetric measurements at co-located sites. In the past, a factor of 1.3 was applied to all TEOM measured concentrations to estimate the gravimetric equivalent. The Volatile Correction Method (VCM, http://www.volatile-correction-model.info/) has recently been developed. The tool uses measurements of volatile particulate matter made by nearby FDMS instruments to correct TEOM measurements for the loss of such volatile material. The corrected measurements have been demonstrated to be equivalent to the gravimetric reference equivalent.

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Toxic Organic Micro-Pollutants (TOMPS)

Toxic organic micropollutants (TOMPs) are produced by the incomplete combustion of fuels. They comprise a complex range of chemicals some of which, although they are emitted in very small quantities, are highly toxic or carcinogenic. Compounds in this category include PAHs (Polycyclic Aromatic Hydrocarbons), PCBs (PolyChlorinated Biphenyls), Dioxins and Furans.

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Benzene(C6H6)

Benzene is an aromatic organic compound which is a minor constituent of petrol (about 2% by volume). The main sources of benzene in the atmosphere in Europe are the distribution and combustion of petrol. Combustion by petrol vehicles is the largest component (70% of total emissions) whilst the refining, distribution and evaporation of petrol from vehicles accounts for approximately a further 10% of total emissions. Benzene is emitted in vehicle exhaust as unburnt fuel and also as a product of the decomposition of other aromatic compounds. Benzene is a known human carcinogen.

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1,3-Butadiene C4H6)

1,3-butadiene, like benzene, is an organic compound emitted into the atmosphere principally from fuel combustion e.g. petrol and diesel vehicles. Unlike benzene, however, it is not a constituent of the fuel but is produced by the combustion of olefins. 1,3-butadiene is also an important chemical in certain industrial processes, particularly the manufacture of synthetic rubber. It is handled in bulk at a small number of industrial locations. Other than in the vicinity of such locations, the dominant source of 1,3-butadiene in the atmosphere is the motor vehicle. 1,3-Butadiene is a known, potent, human carcinogen.

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Carbon monoxide (CO)

Carbon monoxide is a colourless, odourless gas resulting from the incomplete combustion of hydrocarbon fuels. CO interferes with the blood's ability to carry oxygen to the body's tissues and results in adverse health effects.

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Lead and Heavy Metals

Since the introduction of unleaded petrol in the UK there has been a significant reduction in urban lead levels. In recent years industry, in particular secondary non-ferrous metal smelters, have become the most significant contributors to emissions of lead. The highest concentrations of lead and heavy metals are now therefore found around these installations in industrial areas.

Even small amounts of lead can be harmful, especially to infants and young children. In addition, lead taken in by the mother can interfere with the health of the unborn child. Exposure has also been linked to impaired mental function, visual-motor performance and neurological damage in children, and memory and attention span.

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Outdoor air pollution and health

The three main conditions associated with outdoor air pollution are:

  • respiratory disease
  • cardiovascular disease
  • lung cancer

To learn more about the health effects of air pollution go to Public Heath Scotland website.