Air quality

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Particulate matter (PM)

Particulate matter

Particulate matter denotes all solid and liquid particles that float around in the atmosphere. They can reside there for hours to months depending on their properties and on meteorological conditions. The behaviour of particles in an aerosol is determined by the properties of the particles (dimensions, form, density) and those of the gas (velocity, turbulence, composition). The term "aerodynamic diameter" has been developed to describe the behaviour of particles. This behaviour is determined by the dimensions, form and density of the particles. The aerodynamic diameter is defined as the diameter of a spherical particle whose behaviour in ambient air is identical to that of the particle under consideration, hereby assuming that the spherical particle has the same density as water. The PM10 particle fraction has an aerodynamic diameter less than 10 micrometer (μm), PM2.5 has a diameter of less than 2.5 μm. Particles with a diameter less than 0.1 µm are called ultrafine particles.


Particles end up in the atmosphere through a natural cause or through human activities of which building heating, industrial pocesses transport and agriculture are the main sources. In both cases, they can be categorised as primary and secondary particles in terms of the way in which they are formed. Primary particles are emitted directly into the atmosphere or formed by mechanical fragmentation of coarser material. Secondary particles are formed in the atmosphere by oxidation and transformation from gaseous components such as NH3, SO2, NOx or from organic compounds such as volatile organic compounds (VOCs).

EU and WHO standards


standard protection objective averaging period value max. number of exceedances
EU limit value  human health 1 day 50 µg/m³ 35
EU limit value human health 1 year 40 µg/m³  
WHO guideline human health 1 day 50 µg/m³ 3
WHO guideline human health 1 year 20 µg/m³  



standard protection objective averaging period value max. number of exceedances date by which value to be met
EU limit value human health 1 year 25 µg/m³   1 january 2015
EU limit value human health 1 year 20 µg/m³   1 january 2020
WHO guideline human health 1 day 25 µg/m³ 3  
WHO guideline human health 1 year 10 µg/m³    


Nitrogen oxides

Nitrogen oxides and its sources

Nitrogen oxide (NOx) is the generic name for a mixture consisting mainly of nitrogen monoxide (NO) and nitrogen dioxide (NO2). Nitrogen oxides are primarily emitted by human activities during high-temperature combustion processes in which dinitrogen (N2) is oxidized. The major sources of NOx are (road) transport, energy production, industry and building heating. NOx from traffic is mainly emitted as NO and to a lesser extent as NO2. However, NO has a very short lifetime of a few minutes before it undergoes photochemical reactions with other substances, including ozone and volatile organic compounds (VOCs), to form NO2 which has a longer atmospheric life time of a few hours to days. NOx is also involved in the secondary formation of PM and the formation of ozone.


Exposure to very high NO2 concentrations can cause immediate adverse health effects due to the toxicity of the gas. Limit values have been set by the European Commission and the World Health Organisation for the outdoor environment.

EU and WHO standards

standards protectin objective averaging period value max. number of exceedances
EU limit value human health 1 hour 200 µg/m³ 18
EU limit value human health 1 year 40 µg/m³  
WHO guideline human health 1 hour 200 µg/m³ 0
WHO guideline human health 1 year 40 µg/m³  



Formation of ozone

Ozone (O3) is a very reactive gas that is formed in the atmosphere by various photochemical reactions. Ozone is a secondary pollutant and is therefore not emitted directly into the air. Ground-level ozone is formed through the effect of UV light on air pollutants on hot summer days. The ozone precursors are NOx, VOC and CO. Apart from the ozone that is produced on hot and sunny days, a global background concentration is present at all times.

NOx (=NO + NO2) emissions have a double effect on ozone. On the one hand, NO breaks down O3, thereby forming NO2, and, on the other hand, NO2 aids the formation of O3. In the atmosphere, these pollutants are always in chemical equilibrium. NO has a short lifetime in the atmosphere, so that ozone is mainly broken down in places where much NO is emitted. During this reaction NO2 is formed, which has a longer lifetime in the atmosphere. This ozone-forming substance can be transported over greater distances, and contribute to the formation of ozone in more distant places. This is why ozone concentrations are generally higher in rural areas than in an urban environment with many sources of NOx.


Because of its strong oxidising power, ozone can cause harmful effects to humans, animals, plants and materials. Exposure to high ozone concentrations can cause acute health problems such as irritation to eyes, nose and throat, irritant cough and oversensitivity of the lungs. When high ozone levels are present, everyone in outdoor physical activity experiences reduced lung function and is at risk of inflammatory reactions in the airways. 

EU and WHO standards

standard protection objective averaging period value max. number of exceedances
EU information threshold human health 1 hour 180 µg/m³  
EU alert threshold human health 1 hour 240 µg/m³  
EU target value human health max. daily 8 hours 120 µg/m³ 25 (average over 3 years)
EU long term objective human health max. daily 8 hours 120 µg/m³ 0
WHO guideline human health max. daily 8 hours 100 µg/m³ 0
EU target value vegetation May to July (between 8:00 and 20:00 CET) 18000 (µg/m³).hours (average over 5 years)  
EU long term objective vegetation May to July (between 8:00 and 20:00 CET) 6000 (µg/m³).hours  


Carbon monoxide

Carbon monoxide

CO is a colorless, odorless gas that can be harmful when inhaled in large amounts. CO is released when something is burned. The greatest sources of CO to outdoor air are cars, trucks and other vehicles or machinery that burn fossil fuels. A variety of items in your home such as unvented kerosene and gas space heaters, leaking chimneys and furnaces, and gas stoves also release CO and can affect air quality indoors. At very high levels, which are  possible indoors or in other enclosed environments, CO can cause dizziness, confusion, unconsciousness and death.

Limit values

standard protection area averaging period value environment
WHO guidelines human health max. 8-hours average 10 mg/m³ outdoor


Carbon dioxide



Sulphur dioxide


The largest source of SO2 in the atmosphere is the burning of fossil fuels by power plants and other industrial facilities. Smaller sources of SO2 emissions include: industrial processes such as extracting metal from ore; natural sources such as volcanoes; and locomotives, ships and other vehicles and heavy equipment that burn fuel with a high sulfur content.;

Volatile organic compounds

Volatile organic compounds (VOCs) are emitted as gases from certain solids or liquids. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. Concentrations of many VOCs are consistently higher indoors (up to ten times higher) than outdoors. VOCs are emitted by a wide array of products numbering in the thousands, for example household products, including:

  • paints, paint strippers and other solvents
  • wood preservatives
  • aerosol sprays
  • cleansers and disinfectants
  • moth repellents and air fresheners
  • stored fuels and automotive products
  • hobby supplies
  • dry-cleaned clothing
  • pesticide

and other products, including:

  • building materials and furnishings
  • office equipment such as copiers and printers, correction fluids and carbonless copy paper
  • graphics and craft materials including glues and adhesives, permanent markers and photographic solutions.

Hazardous air pollutants

Hazardous air pollutants, also known as toxic air pollutants or air toxics, are those pollutants that are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or adverse environmental effects. US Environmental Protection Agency has compiled a list of hazardous air pollutants.