Pollutants of main concern



In most cities air quality has improved (substantially) over the past decades. The visible and noticeable air pollution (smoke, dust, smog) has disappeared from many cities due to local, national and European initiatives. Occasionally air quality poses an immediate threat: during industrial incidents or pollution episodes. Fortunately this is rare. Nevertheless the current air quality still affects peoples health. In many European cities, air quality is a concern and it is therefore monitored around the clock. In most cities, industrial air pollution is, or tends to be replaced by traffic related air pollution. Air quality is therefore a common problem to almost all major cities.


Problems like global warming, acid rain and ozone depletion are well known but can seem remote from the daily life in our cities. A more direct concern to citizens, health experts and policy makers is the link between air quality and human health. Most of our economic activities are concentrated in urban areas where almost 80% of the European population lives. In urban areas transport routes and residential areas are often very close to each other and therefore transport is a major contributor to urban air pollution. Though residential and industrial areas are often separated air pollution travels over long distances and industries contribute either directly, or through background concentrations to poor air quality as well.

Air pollution causes health effects and environmental problems. Typical air pollutants that cause immediate concern are listed below. Emissions of greenhouse gasses to the air are not the topic of this website though they often occur through the same mechanisms (more info on greenhouse gasses).

Particulate matter (PM10/2.5)

Airborne particulate matter varies widely in its physical and chemical composition, source and particle size. PM10 particles (the fraction of particulates in air of very small size (<10 µm)) and PM2.5 particles (<2.5 µm) are of major current concern, as they are small enough to penetrate deep into the lungs and so potentially pose significant health risks. Larger particles meanwhile, are not readily inhaled, and are removed relatively efficiently from the air by sedimentation. The principal source of airborne PM10 and PM2.5 matter in European cities is road traffic emissions, particularly from diesel vehicles. The limit values are very often exceeded in European cities.

Nitrogen oxides (NOx)

NOX is a term used to describe a mixture of nitric oxide (NO) and nitrogen dioxide (NO2). They are inorganic gases formed by combination of oxygen with nitrogen from the air. NO is produced in much greater quantities than NO2, but oxidises to NO2 in the atmosphere. NO2 causes detrimental effects to the bronchial system. Nitrogen dioxide concentrations frequently approach, and sometimes exceed air quality standards in many European cities. NOx is emitted when fuel is being burned e.g. in transport, industrial processes and power generation.

Ozone (O3)

Ground-level ozone (O3), unlike other pollutants mentioned, is not emitted directly into the atmosphere, but is a secondary pollutant produced by reaction between nitrogen dioxide (NO2), hydrocarbons and sunlight. Ozone levels are not as high in urban areas (where high levels of NO are emitted from vehicles) as in rural areas. Sunlight provides the energy to initiate ozone formation; consequently, high levels of ozone are generally observed during hot, still sunny, summertime weather.

Hydrocarbons (HC) and volatile organic compounds (VOC)

HC belong to a larger group of chemicals known as volatile organic compounds (VOC). HC are compounds of hydrogen and carbon only, while VOC may contain other elements. They are produced by incomplete combustion of hydrocarbon fuels, and also by their evaporation. Because there are many hundreds of different compounds, HC and VOC display a wide range of properties. Some, such as benzene, are carcinogenic; some are toxic and others harmless to health.

Sulphur dioxide (SO2)

Fossil fuels contain traces of sulphur compounds, and SO2 is produced when they are burnt. The majority of the SO2 emitted to the air is from power generation, and the contribution from transport sources is small (shipping being an exception). Exposure to SO2 can damage health by its action on the bronchial system. Sulphuric acid generated from atmospheric reactions of SO2 is the main constituent of acid rain, and ammonium sulphate particles are the most abundant secondary particles found in air.

Carbon monoxide (CO)

CO is an odourless, tasteless and colourless gas produced by the incomplete burning of materials which contain carbon, including most transport fuels. CO is toxic, acting by reaction with haemoglobin and reducing its capacity for oxygen transport in the blood. Even in busy urban centres, CO concentrations rarely exceed health related standards.

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