What is an air pollutant

Air pollutants

A large number of air pollutants are known, of which only the most important are discussed here:

Nitrogen oxides

In various combustion processes, nitrogen oxides are often produced from nitrogen and oxygen in the combustion air supplied, and sometimes also from the nitrogen or oxygen content of the fuel supplied. Above all, they are irritating to the respiratory tract, and exposure to sufficient exposure can lead to serious illnesses - probably including cancer. In addition, the entry of nitrogen oxides into the soil via rainwater leads to increasing acidification of soils and bodies of water, which primarily causes forest damage, but also production losses in agriculture and damage to fauna in stagnant water.

The formation of nitrogen oxides occurs mainly when high combustion temperatures occur. A particularly strong source of nitrogen oxides are the internal combustion engines used in road vehicles (cars, buses, trucks, etc.), especially diesel engines, where effective exhaust gas cleaning (“denitrification”) is technically more difficult to accomplish than with gasoline engines (Otto engines). Even with gasoline engines, denitrification with an exhaust gas catalytic converter is not complete, especially in the warm-up phase.

The denitrification in large combustion plants is easier to accomplish than z. B. many small engines.

There are also other sources of nitrogen oxides, for example various power plants such as coal-fired power plants and gas-fired power plants. Here, too, attempts are being made today to achieve the most extensive denitrification possible with appropriate exhaust gas cleaning systems. This can be quite effective in stationary systems that work most of the time with a relatively constant load, and the specific costs of such exhaust gas cleaning are much lower than, for example, in cars.

The article on nitrogen oxides has more details.


Ozone is a pungent smelling gas that irritates the respiratory tract and is poisonous. There is hardly any direct z. B. emitted by internal combustion engines, but arises from other air pollutants such as nitrogen oxides and unburned hydrocarbons, especially in the presence of strong sunlight. The reduction of air pollution with ozone, which is often an essential component of smog, must therefore be achieved by reducing precursor substances, whereby it is not sufficient to just reduce the local emissions of these substances.

The article on ozone has more details.

Sulfur dioxide

The fight against sulfur dioxide emissions has been quite successful over the past few decades. This means that the burden on the forests has also decreased significantly.

Sulfur dioxide is produced when fuels containing sulfur such as coal, crude oil or unpurified biogas are burned. Up until the 1980s, coal-fired power plants in particular caused very high emissions of sulfur dioxide. The use of tall chimneys did not result in extremely high immission values, e.g. B. in cities, but increasingly damage occurred in forests, including through the acidification of soils. The observed forest decline was the reason for politicians to increasingly stricter SO2- Establishing emission limit values ​​for power plants, and the technical reduction of such emissions using continuously developed desulphurisation systems was quite effective. That is why sulfur dioxide emissions are much lower today than they were then. The reduction of the SO2-Emissions from small firing systems (especially for coal and wood), on the other hand, are much more difficult, so that local SO2-Immissions can still be significant.

Petroleum naturally contains significant amounts of sulfur (with large differences between different petroleum sources). Nevertheless, petroleum-based fuels such as diesel fuel and fuels such as heating oil can be very low in sulfur if they are produced accordingly in the petroleum refinery. Unfortunately, this often only means that the sulfur content is not separated, but shifted to other products such as heavy oil, which are then used, for example, as fuel for large ships (bunker oil).

See the article on sulfur dioxide for more details.

particulate matter

Research into the health effects of fine dust is very difficult and time-consuming.

Fine dust is understood to mean finely distributed, very small particles in the air that can consist of a wide variety of substances. In contrast to coarser dust, such tiny particles can hardly be prevented by the airways from getting deep into the lungs. A transition into the blood and from there into all possible organs is also possible. The health consequences of fine dust pollution have so far only been scientifically incompletely clarified. The corresponding research is very time-consuming, among other things. because a large number of different substances have to be taken into account, which can possibly have very different effects. In addition, such damage may only occur after prolonged exposure, which makes it difficult to assign cause and effect.

Road traffic generates different types of particulate matter - and not just through exhaust gases.

In cities, a large part of the fine dust pollution comes from road traffic - on the one hand from exhaust gases from internal combustion engines, v. a. from diesel engines without a soot particle filter or with a filter of limited effectiveness and, on the other hand, from abrasion from tires and brakes as well as from the dispersal of dust that is already on the road. It is now clear that particulate matter in the form of small soot particles is very dangerous; It is less clear to what extent the whirling up of natural dust particles (which even occurs when driving an electric car) is also harmful. The fine dust emissions from diesel engines can be reduced quite effectively by modern soot particle filters, while measures against the whirling up of particles are hardly possible.

Heating systems that run on solid fuels also often contribute significantly to particulate matter pollution.

In winter, a significant part of the air pollution with fine dust can come from heating systems that are operated with solid fuels such as wood or coal. Incorrect operation of such ovens (e.g. with insufficient air supply, insufficiently dried wood, co-incineration of waste, etc.) even produces extremely high amounts of pollutants, but the fine dust emissions of such ovens is by no means negligible, even with optimal operation.

When used indoors, cigarettes and other tobacco products cause considerable concentrations of fine dust, which is mixed with various toxic substances.

See the particulate matter article for more details.

Carbon monoxide

Carbon monoxide (CO) is an odorless, poisonous gas, which is mainly produced during combustion processes with a lack of oxygen. For example, it is formed to a greater or lesser extent in most internal combustion engines (particularly in two-stroke engines), but can be broken down relatively effectively by catalytic converters (except during the warm-up phase).

In the case of carbon monoxide, it is primarily about the danger to life at high concentrations, less about chronic stress.

While a high concentration of carbon monoxide in the air is life-threatening, a long-term low exposure does not necessarily have to be harmful to health, as it only slightly reduces the oxygen supply. For this reason, reducing carbon monoxide emissions does not have a special priority in most places, while avoiding high CO concentrations indoors, for example as a result of the operation of internal combustion engines or barbecues in closed rooms or due to defective heating systems, is very important for safety reasons.

The article on carbon monoxide has more details.

Volatile organic compounds

This group of substances includes hydrocarbons and substances derived from them, for example chlorinated or otherwise halogenated hydrocarbons. In particular, to the extent that such substances are volatile (that is, they evaporate easily), they can be present in the air in significant quantities. There is no general definition of which substances are classified as “volatile organic compounds” (VOC); different countries use different definitions for this. For example, the non-toxic but climate-damaging methane is partly included and partly not.

HC emissions from vehicles cause a significant proportion of the volatile hydrocarbon pollution.

By no means all hydrocarbons are poisonous or harmful to the environment, but so are many of these substances; some are even carcinogenic. The main pollution is caused by unburned hydrocarbons in the exhaust gases of gasoline engines (gasoline engines), which are often referred to as HC emissions. A catalytic converter can normally break down hydrocarbons effectively, but not in the warm-up phase and not when a so-called full load enrichment is carried out. The consequence of the latter is that a vehicle accelerating at full throttle can significantly pollute the occupants of vehicles behind it with toxic hydrocarbons (and, by the way, also carbon monoxide).

Many solvents are volatile organic substances and, when used, are released into the atmosphere through evaporation and thus contribute to the corresponding air pollution. Evaporative emissions also occur when using fuels.

An important example of harmful hydrocarbons is benzene, a carcinogenic substance that is released into the environment primarily through the exhaust gases of gasoline engines (Otto engines). It is naturally contained in petroleum and is desirable in gasoline because of the possible increase in anti-knock properties, but it is not always completely burned or broken down in the catalytic converter. The article on benzene has more details.

There are not only technical but also various natural sources of volatile organic substances, and some of these are not directly harmful, but only after they have reacted with other substances. For example, natural organic substances evaporate from trees and freshly mown lawns, which, in combination with nitrogen oxides, contribute to the formation of ozone - similar to the HC emissions from vehicles.


Ammonia (NH3) is a pungent smelling gas, which can strongly irritate the respiratory tract. This is the main reason why chronic exposure to ammonia is harmful to health. The entry of ammonia into water can also be problematic due to its effects on fish.

There are various natural sources of ammonia, but they usually do not cause significant air pollution. However, in some places, factory farming can release large amounts of ammonia, both from stables and, for example, by spreading manure on fields. Large amounts of ammonia are produced by industry, but these hardly ever escape into the environment and are converted into various products (e.g. nitrogen fertilizers).

Combustion processes hardly produce any ammonia. Conversely, ammonia can even be used to break down nitrogen oxides more effectively in certain catalytic converters (through selective catalytic reduction). As far as this technology is used in diesel vehicles, the necessary ammonia is not carried in a tank, but is generated from a more easily transportable urea solution (Adblue).

Heavy metals

In principle, heavy metals do not tend to stay in the air for long. However, they can occur in the form of fine dusts that can stay in the air for a long time and can easily be whirled up again after being deposited on the ground. This can also get you into the airways. Heavy metals deposited there can often remain in the body for a long time and develop their toxic effects.

A special case is the relatively volatile mercury. In particular, its absorption into the body reduces brain development in young people. It is particularly found in the exhaust gases from coal-fired power plants, the concentration being heavily dependent on the quality of the coal used and the exhaust gas purification. Since it was recognized, especially in the USA, that the reduced mental abilities of young people on average as a result of exposure to mercury result in economic damage running into billions, and that a substantial part of the mercury is released into the air through coal-fired power plants, emission limits have become increasingly strict there decided for power plants. Europe (including Germany) is lagging behind when it comes to air pollution control in that the power plant operators concerned have so far been very effective in defending themselves against the use of the effective exhaust gas cleaning technology (mainly developed in Germany, by the way).

Radioactive substances

Various radioactive substances can also occur as air pollutants - mostly in very low concentrations, but with the risk of inhalation and deposits in the lungs. Radioactive emissions occur not only in nuclear power plants and other nuclear facilities (such as reprocessing plants), but on a larger scale (as long as there is no nuclear disaster) primarily from coal-fired power plants. Coal contains various radioactive trace elements such as radium, uranium, thorium, polonium and potassium 40. The majority of the radioactive substances remain in solid residues such as fly ash and granulate with modern exhaust gas cleaning, which should be disposed of with care; In view of the huge quantities of exhaust gas emitted, however, the quantities of pollutants remaining in the exhaust gas are by no means negligible. Many of these substances are problematic as heavy metals simply because of their chemical toxicity.

Climate-affecting air pollutants

Carbon dioxide (CO2) is non-toxic in moderate concentrations, also for plants and animals, but leads to an intensification of the greenhouse effect and thereby causes climate hazards. The same applies to methane (CH4) as well as for certain fluorinated and / or chlorinated hydrocarbons.

So these are not classic pollutants with toxic effects, but “only” substances that are harmful to the climate. However, in the long term, the climatic dangers for mankind and nature are likely to be the bigger problem than classic air pollutants, especially since they are much more difficult to avert. Above all, the additional carbon dioxide, which is now being released into the atmosphere in huge quantities, accumulates there more and more and causes global warming for several centuries. There is no actual degradation of these pollutants, but only long-term uptake in the oceans, whereby their acidification causes additional problems.

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See also: Air pollution, emissions and immissions, evaporative emissions, exhaust gas, exhaust gas quality, exhaust gas catalytic converter, measurement methods for fuel consumption and exhaust gas values