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FAQ air pollution Frequently Asked Questions

The question library on air related issues

What is air pollution?

Air pollution means the presence of one or more unwanted substances in air. Air pollutants have a negative impacts on humans, animals and plants, and on air quality.
The most frequently present categories of air pollutants are sulphur oxides, nitrogen oxides, Volatile Organic Compounds (VOC) and small dust particles (aerosols).

What causes air pollution?

The main sources of air pollution are the industries, agriculture and traffic, as well as energy generation. During combustion processes and other production processes air pollutants are emitted. Some of these substances are not directly damaging to air quality, but will form harmful air pollutants by reactions with other substances that are present in air.
Examples of large-scale air pollutants are VOC (Volatile Organic Compounds) and small dust particles. When large concentrations of these substances are emitted this negatively affects ecosystems, materials and public health.

Emissions of nitrous oxide (N2O) mainly stem from agriculture, because nitrogen in soils can easily be denitrified by bacteria. Nitrous oxide is emitted during the denitrification process. Additionally, the application of (artificial) fertilizers causes emissions of ammonia (NH3), nitrogen oxides (NOx) and methane CH4), a greenhouse gas.
The agricultural sector is known for its extensive use of pesticides. This application causes emissions of many toxic chemicals.

Industrial processes vary greatly and as a result there are many different chemical wastes. The industries are responsible for emissions of carbon monoxide, carbon dioxide, sulphur dioxide, nitrogen oxides, small dust particles, VOC, methane, ammonia and radioactive radiation.
During energy generation chemicals such as methane are released into the air as a result of oil and natural gas extraction. The combustion of coal and natural gas for electricity production causes the release of sulphur dioxide, nitrogen oxides and carbon dioxide into the air.

Traffic is held responsible for one-third of the greenhouse gas emissions. Emissions caused by traffic are mainly those of carbon dioxide, carbon monoxide, nitrogen oxides, VOC and small dust particles.

Consumers are also partly responsible for air pollution. Firstly because the products they use have caused air pollution during their production and distribution and secondly because heating of houses and offices causes chemicals release into the air. When people use paints or cosmetics VOC is released and perspiration, pet fertilizer use and cleanser use cause ammonia emissions. Last but not least, many chemicals (carbon dioxide, carbon monooxide) are emitted during smoking.

How does air pollution form?

Air pollution can form in various ways. Chemicals are emitted during many different human activities. In the atmosphere these chemicals can react with other chemicals to more dangerous substances. Air pollutants often have properties that are harmful to the environment.
The weather plays an important role in the formation and disapearance of air pollution. This is mainly influenced by wind and temperatures. Air pollutants can be transported by wind, causing a pollution to spread widely. Rain can remove pollutants from air, causing soil and water pollution. Sunlight can aid the convertion of air pollutants to different substances.

Chemicals can come from various sources, and are formed during different processes. Air pollution can be divided in categories according to the source it is derived from:

- Biological air pollution, such as pollens, small insects and microrganisms (bacteria, fungi, yeasts and algae)
- Physical air pollution, such as sound, smell, thermal pollution and radioactive radiation
- Chemical air pollution, such as ozone, aerosols and ammonia

Air pollution is caused by both human and natural sources. Human sources are traffic, agriculture or industry, as was mentioned before. Natural sources are be dust storms, volcanic eruptions and emissions from plants.

Air pollution undergoes a number of processes:
- Emission (contaminants are released into the air)
- Transport (contaminants are transported to different locations through air)
- Exchange (compounds react with other compounds in air)
- Distribution (contaminants are distributed in air)
- Immission (contaminants maintain in a certain area)
- Deposition (contaminants are deposited in a certain area, on the soil or on objects)

What types of air pollution are there?

Air pollution consists of gases and/ or particles. These have a distinct chemical or physical structure, or a distinct effect on human health.

The main air contaminants are:

- Sulphur dioxide (SO2). This contaminant is mainly emitted during the combustion of sulphur-containing fossil fuels, such as crude oil and coal. Sulphur dioxide concentrations in air have decreased in the past two decades, mainly because we use more non-sulphur-containing fuels for the generation of energy. Sulphur dioxide is a stinging gas and as a result it can cause breathing problems with humans. In moist environments, sulphur dioxide may be transferred to sulphuric acid. This acid causes acidification and winter smog.
- Nitrogen oxides (NOx). These contaminants are emitted by traffic, combustion installations, such as power plants, and the industries. Nitrogen oxides are also released from farmland in the agricultural sector. Using catalysers in car exhausts can prevent emissions of nitrogen oxides. Nitrogen oxides are gasses that react with other air pollutants when they are present in air. For example, nitrogen oxides play an important rolein the formation of ozone in the lower atmosphere, and in acidification and eutrophication processes. They can deeply penetrate the lungs and damage human lung functions.
- Ammonia (NH3). Ammonia forms during agricultural activities. Ammonia plays an important role in acidification and eutrophication.
- VOC (Volatile Organic Compounds). VOC can be a range of different contaminants, such as carbohydrates, organic compounds and solvents. These compounds usually derive from petrol and gasoline reservoirs, industrial processes and fuel combustion, paint and cleanser use, or agricultural activities. VOC play an important role in ozone shaping in the lower atmospheric layer, the main cause of smog. VOC can cause various health effects, depending on the kind of compounds that are present and their concentrations. Effects can vary from smell nuisance to decreases in lung capacity, and even cancer.
- Methane (CH4). The main source of methane pollution is agriculture. But smaller amounts of methane can also be released during waste combustion and natural gas extraction. Methane is a greenhouse gas that contributes to the greenhouse effect and ozone loss.
- Carbon monoxide (CO). This gas consists during incomplete combustion of fuels. When we let a car engine run in a closed room, carbon monoxide concentrations in the air will rise extensively. Carbon monoxide contributes to the greenhouse effect, smog and acidification. The gas can bind to haemoglobin in blood, preventing oxygen transport through the body. This results in oxygen depletion of the heart, brains and blood vessels, eventually causing death.
- Dust particles. Dust particles form a complex of organic compounds and minerals. These can derive from natural sources, such as volcanoes, or human activities, such as industrial combustion processes or traffic. Particles are categorised according to particle size. The smallest particles have the ability to transport toxic compounds into the respiratory tract. Some of these compounds are carcinogenic. The upper respiratory tract stops the larger dust particles. When they are released into the environment, dust particles can cause acidification and winter smog.
- Ozone (O3). Ozone is created through photochemical transfer of oxygen. This process takes place under the influence of ultra violet sunlight (UV), aided by pollutants in the outside air. Ozone causes smog and contributes to acidification and climate change. Ozone is an aggressive gas. Which can easily penetrate the respiratory tract, deeply. When humans are exposed to ozone, the consequences may be irritation of the eyes and the respiratory tract.
- Radioactive radiation. Radioactive radiation and radioactive particles are naturally present in the environment. During power plant incidents or treatments of nuclear waste from a war where nuclear weapons are used, radioactive radiation can enter the air on account of humans. When humans are exposed to high levels of radioactive radiation, the chances of serious health effects are very high. Radioactive radiation can cause DNA alteration and cancer.

What are the main environmental effects of air pollution?

Here, we will sum up and briefly explain the main environmental effects of air pollution:

Acid deposition

Acid deposition is not merely characterized as acid rain; it can also be snow and fog or gas and dust. Acid deposition mainly forms during fossil fuel combustion. When emissions of sulphur dioxide and nitrogen oxides come in contact with water, they will become sulphuric acid and nitric acid.

When acidifying agents, such as sulphur dioxide, nitrogen oxides and ammonia, end up in plants, surface water and soils, this has a number of consequences:
- Availability of nutrients and metal spores is likely to decrease.
- When acidity is high more metals will dissolve in water. This can cause surface water to become polluted, which has serious health effects on aquatic plants and animals. For example, high aluminum (Al) concentrations can complicate nutrients uptake by plants. This makes aluminum one of the prior causes of forest decay. Mercury can be dispersed by transport through surface water, causing it to accumulate in fish. Mercury can bio magnify up the food chain, to be taken up by humans eventually.
- Buildings and monuments may be damaged through erosion. Sulphur dioxide breaks down limestone by reacting with calcium carbonate, causing limestone to absorb water during rainfall. Limestone will than fragment.


Eutrophication is caused by an increase in plant nutrients in water. The higher availability of nutrients causes certain water plants, such as algae and duckweed, to grow so extensively. This blocks sunlight supplies to water. The plants also use all available oxygen supplies, which will not be renewed because heterotrophic plants and bacteria need light to perform photosynthesis. Eutrophication causes ecosystem disruption. Nitrogen pollutants such as nitrogen oxides and ammonia contribute to this problem.

For further information about eutrophication click here.


Smog is a combination of the words smoke and fog. We can distinguish two separate types of smog, summer smog and winter smog.

Photochemical smog, or summer smog, mainly consists of ozone. It is a brown, oxidising fog. The causes of photochemical smog are nitrogen oxides and VOC, which stem from traffic and industries. Ozone is formed according to the following chemical reactions:

NO2 + uv --> NO + O
O + O2 --> O3

The reverse reaction is:

NO + O3 --> NO2 + O2

The best circumstances for the creation of high ozone concentrations are summer temperatures, direct sunlight and standing air layer, which enable dilution of contaminants.
Humans cannot adapt to smog. Health effects of smog greatly depend upon the concentrations of ozone and other photochemical oxidants. These contaminants cause eye and respiratory irritations. Plants are extremely vulnerable to ozone. Even in low concentrations it can cause serious damage.

Winter smog is also referred to as acid smog; it mainly consists of foggy elements. Winter smog is found in areas where vertical dispersion of air pollutants is not possible. Usually temperatures decrease during the day in high air layers.
Heated air layers near the earths' surface rise, causing the air pollutants to be dispersed vertically, and to be diluted. In winter ground temperatures are sometimes lower than those of the upper atmospheric layers, causing the air to stay near the ground, so that pollutants will not spread. This causes winter smog.
Winter smog can form when temperatures are low and sulphur dioxide concentrations increase consequential to central heater emissions from houses.
The cold outside air will cause moist to condensate into fog. Aerosols in air play a part in this process, because they serve as condensation cores for water vapour.
Humidity aids the transfer of sulphur dioxide to sulphuric acid, causing the smog to be acid. Acid smog causes breathing problems and eye irritations.

Ozone loss

Ozone is created everywhere in the atmosphere through chemical reactions under the influence of UV-light. Ozone is decomposed under the influence of visible light and UV-A light. During ozone decomposition an oxygen-poor molecule is released, which aids the breakdown of ozone. There are a number of compounds that catalyse ozone decomposition. Examples are hydroxide (OH), nitrogen oxides, chlorine (Cl) and bromine (Br).
Chlorine mainly aids the decomposition of ozone when it is a part of CFC-bonds (Chloro-Fluor-Carbohydrates). These compounds are not lost during the chemical reaction, causing them to decompose ozone multiple times.
The decomposition and production of ozone is a natural process. However, human activities have caused large concentrations of ozone-decomposing chemicals to enter the atmosphere, causing the natural balance to be disrupted.
Ozone is very important for all life on earth, because it absorbs harmful UV-B radiation from the sun. The highest concentrations of ozone are located in the atmospheric layer between twenty and forty kilometres above the earth. When the ozone concentrations in this layer decrease, UV-B radiation may reach the earth. This radiation damages DNA and causes skin cancer. The radiation can also damage the human immune system, causing humans to become more susceptible to infections. UV-B radiation also causes cataract and nearsightedness. The radiation can decrease growth and photosynthesis activity in a number of plants. Primary crops, such as rice, corn and sunflower are very susceptible to this. Trees are also susceptible to the radiation.
UV-B radiation can affect aquatic life up to twenty metres under the water surface. It is damaging to species, such as plankton, fish larvae, shrimps, crabs and seaweeds. Phytoplankton forms the basis of the aquatic food chain. When radiation causes phytoplankton to decrease in number it will affect entire ecosystems.

The Greenhouse effect (GE)

Air pollution impacts

In the following table we can see which air pollutants are involved in various environmental problems.

Atmospheric problems







Photochemical smog





Winter smog










Climate change





How does air pollution spread and how can we handle this?

The dispersion of air pollutants mainly depends on physical processes is air; those of wind and weather. How far air pollutants are transported mainly depends upon particle size of the compounds and at which height the pollution was emitted into the air. Fumes that are emitted into air through high smoke stags will mix with air so that local concentrations are not very high. However, wind will transport compounds and the pollution will become very disperse. Rain can remove pollutants from air. This causes precipitation and consequentially soil and water pollution.
For environmental agencies it is very important to determine exactly how an air pollutant spreads. Air is not a very complex medium. This enables us to predict the dispersion of air pollutants with computer models. In a computer model dispersion is calculated by means of different parameters, such as wind speed, wind direction, temperature, air humidity and cloudiness. These predictions are of great significance when we are dealing with toxic clouds or radioactive radiation, because these are a danger to human health and because inhabitants of polluted areas need to be warned.

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