Substances from the air can be bound to the surface of a material through adsorption. This process can be used to purify air, e.g. using activated carbon as the adsorbent.



General term for mixtures of gases (e.g. air) with small solid or liquid particles dispersed in them. The particles have diameters of around 1 nm to 10 µm.



Committee for Indoor Guide Values

In 1993, the so-called "ad-hoc working group" Indoor Guidelines was established at the initiative of the Conference of Health Ministers. It quantitatively assesses indoor air pollutants and sets nationwide guideline values for individual substances in indoor air. In 2015, the working group was renamed the "Committee for Indoor Air Guide Values (AIR)".

The committee consists of experts from the Indoor Air Hygiene Commission (IRK) and experts from the Indoor Air Working Group of the Environmental Hygiene Committee of the Working Group of the Supreme State Health Authorities (AOLG). The AIR office is located at the German Environment Agency in Dessau.



Ventilation system

Technical device for supplying outdoor air to indoor rooms and removing polluted ("used") indoor air. Depending on the operating mode, either the air can be supplied or extracted exclusively in a controlled manner or a combined supply and exhaust air system can be operated. A ventilation system should always be adapted to the building structure and individual needs.



Benzene is a ring-shaped compound of carbon and hydrogen ("hydrocarbon"). At room temperature, benzene is present as a liquid, but evaporates very easily due to its low boiling point (80 °C). Benzene has a typical "aromatic" odor. Compounds derived from benzene are therefore also known as "aromatics". Benzene has been proven to be carcinogenic to humans. For this reason, the benzene concentration in indoor air should be kept as low as possible.



In indoor spaces, emissions are the release of chemical substances from building products and furnishings into the indoor air. An example of emissions is the evaporation and release of a paint solvent from a paint film.


Endocrine disruptors

Endocrine disruptors are hormone-active substances that can alter the hormone system in an effective dose and consequently damage health.

Further information on the definition can be found in the information portal of the German Environment Agency on the European Chemicals Regulation "REACH".




The sum of environmental influences or a specific environmental factor to which a person is exposed and which affects them. The uptake of foreign substances into the human body occurs through exposure pathways.


Exposure assessment

The contact of humans with chemical, biological, or physical influences with regard to health effects. The estimates relate to a specific time and concentration (dose).


Exposure pathways

The possible ways in which the human body can take in foreign substances: (a) inhalation: through breathing, (b) dermal: through skin contact, (c) oral: through ingestion.


Fine particels




Indoor air guideline values

The so-called indoor air quality guidelines are used for the quantitative assessment of at what concentration an individual substance present in indoor air can pose a health risk. The indoor air guideline values are derived nationwide in Germany by the so-called ad hoc working group.


Indoor Air Hygiene Commission

The Indoor Air Hygiene Commission (IRK) of the German Environment Agency advises its President on all issues relating to indoor air hygiene. The members of the commission are appointed on an honorary basis for a period of three years and are predominantly from scientific institutions and the relevant state authorities. The IRK draws up recommendations and statements on indoor air hygiene.


Air condition system

A technical device used to create and maintain a comfortable indoor air quality independent of external environmental influences. Air conditioning systems can heat or cool, humidify or dehumidify, filter or exchange the air supplied into the room from outside. The term "air conditioning" is often used to refer exclusively to the cooling of indoor air.


Carbon dioxide

Carbon dioxide (CO2) is a colorless and odorless gas that is naturally present in the Earth's atmosphere. In recent years, carbon dioxide emissions have increased significantly due to human activities, particularly the burning of fossil fuels such as coal, oil and natural gas. These emissions contribute to the greenhouse effect, trapping heat in the atmosphere and leading to global warming and climate change.

Excessive levels of carbon dioxide in enclosed spaces can be harmful to human health. Carbon dioxide is exhaled by humans as a metabolic product. It is therefore often used as an "indicator gas" for indoor air quality. Monitoring and controlling carbon dioxide levels in indoor environments, such as homes, offices, and classrooms, is important to ensure good indoor air quality and the well-being of occupants. 



Carbon monoxide

Carbon monoxide (CO) is a gas that is produced during incomplete combustion, e.g. when burning wood, coal or gas without a sufficient supply of air. Carbon monoxide is odorless and tasteless and is similar in weight to air. It binds about 300 times more strongly to the red blood cells than oxygen, thus blocking oxygen transport in the blood. In addition to the level of exposure, the duration of exposure also plays a role in toxicity.

Concentrations below 7 mg/m³ of carbon monoxide are not associated with health hazards, even with prolonged exposure. Fatal poisoning can occur from a concentration of around 1000 mg/m³.


Guide value

Hygienically based assessment value of a substance at which the probability of complaints and adverse health effects increases with increasing concentration. The guide values are not based on toxicologically, but are based on empirical values.


Air exchange

Air exchange is the replacement of indoor air with outdoor air, either naturally (through leaky buildings or open windows) or artificially (through ventilation systems). The air change rate is the speed at which this occurs. It is usually expressed in units of 1/h (number per hour). An air change rate of 0.2 per hour therefore indicates that the room air is replaced once every 5 hours on average, or that 20% of the room volume is replaced by outside air every hour.

The air change rate is of great importance for indoor air quality - as there are many sources of air pollutants indoors, the air change rate determines the extent to which these are removed from the room.

In cities with high levels of outdoor air pollution, the introduction of air pollutants from outdoor air can be problematic - for details, see the indoor and outdoor air concentrations of air pollutants topic page.


Air pollutants

Substances (airborne compounds) that have the potential to harm humans or the environment at elevated concentrations in the air.


Ventilation traffic light

Technical aid that indicates the need for ventilation using colors similar to the traffic light. In most cases, these traffic lights also have a display for online monitoring of the CO2 concentration in the room air.


Air change rate

Air change rate is the rate at which indoor air is replaced with outdoor air. For example, with an air change rate of 1 per hour, the air volume in the indoor space is exchanged once within an hour.



Climatic conditions that prevail in a small, precisely defined and therefore clearly limited area.


Minimum air change rate

Minimum required supply air volume flow of fresh air to comply with minimum hygiene requirements and prevent damage to buildings. The minimum air change rate depends on the number of users/occupants in a room and ensures that the exhaled carbon dioxide cannot accumulate. In naturally ventilated buildings, an air change rate of 0.5 per hour is considered the minimum hygienic air change rate.


Natural ventilation

Unregulated air exchange that is influenced by the wind pressure against the building and the temperature differences/uplift between indoor and outdoor air due to leaks in the building envelope (e.g. open joints, cracks, gaps in the masonry).







Unit for nanometer.

1 nanometer (nm) = 1 billionth of a meter = 10-9



Ozone is a natural component of the Earth's atmosphere and plays a crucial role in the ozone layer, which is located in the stratosphere. The ozone layer helps to protect the earth from harmful ultraviolet (UV) radiation from the sun. While ozone in the stratosphere is beneficial, ground-level ozone can be harmful to human health and the environment.

Indoor ozone can be a concern if it exceeds safe levels. High ozone levels can cause respiratory problems, especially for people with asthma or other respiratory conditions.


Polychlorinated aromatic hydrocarbons (PAH)





Unit of pressure. The standard pressure is 101,325 Pa = 1,013.25 hPa = 101.325 kPa.



Indoor particles are mostly liquid or solid particles in the gas phase (aerosols). Particles are divided into the following categories according to their size: 

  • Coarse particles (particle diameter > 10 µm)
  • Inhalable particles, PM10 (particle diameter < 10 µm)
  • Respirable particles, PM2.5 (particle diameter < 2.5 µm)
  • Ultrafine particles, UFP (particle diameter < 0.1 µm)



Penetration of outdoor air into the indoor environment through various barriers (e.g. building envelope).




Photoacoustic spectroscopy

A physical method based on the photoacoustic effect that can be used to detect air pollutants. When light pulses encounter a gas mixture, the light is absorbed by the molecules upon impact. The resulting measured sound waves provide information about the type and concentration of the molecule under investigation.



Chemical reactions that are initiated by the effect of light.






Abbreviation for "parts per million".

1 ppm = 10-6 = 0.0001%


Product loading factor

The loading factor in product emission testing refers to the ratio of the product surface area to the volume of air being sampled in the test chamber during the test. It is an important parameter used to determine the emission rate of pollutants or chemicals from a product into the surrounding air.


Test chamber measurement

In a test chamber measurement, a building product or furnishing item is tested for emissions under controlled test conditions (temperature, humidity, air exchange). A distinction is made between small test chambers (volume from 20 liters) and large walk-in test chambers (volume up to 50 m³). 


Sulfur dioxide

Sulphur dioxide (SO2) is typically an outdoor air pollutant, but it can also be present indoors in certain situations. Indoor sources of sulphur dioxide including combustion processes and building materials. Exposure to sulphur dioxide indoors can cause respiratory irritation, especially in individuals with pre-existing respiratory conditions. It is important to ensure proper ventilation in indoor spaces and avoid potential sources of sulphur dioxide to maintain good indoor air quality.


Sick building syndrome

Symptoms that cannot be attributed to a specific cause, but which are associated with the person concerned spending too much time indoors. A characteristic example is health or mood disorders as a result of spending time in air-conditioned rooms. 


Nitrogen dioxide



SVOC is the abbreviation for "semivolatile organic compounds". These compounds generally have a boiling point between 250 and 400 °C.


Dew point

The dew point is the temperature below which the air must fall at constant pressure for it to reach saturation and for water vapor to subsequently condense as dew or mist.










TVOC is the abbreviation for "total volatile organic compounds". It represents the overall concentration of volatile compounds that are measured as a total value using a specific determination method.


Ultrafine particles (UFP)





German Chemical Industry Association (German name: Verband der Chemischen Industrie)



VOC is the abbreviation for "volatile organic compounds". These air pollutants generally have a boiling point between 60 and 250 °C.



VVOC is the abbreviation for "very volatile organic compounds". These compounds generally have a boiling point between 0 and 60 °C.


Thermal bridges

A thermal bridge is the part of a building or component through which heat is conducted faster to the outside than through adjacent components. Thermal bridges can be made visible with the help of thermography.

Thermal bridges are not only problematic because of the energy loss: in the area of the thermal bridge, the interior wall is cooler than other parts of the affected room, which can lead to condensation of air humidity (falling below the dew point) and mold growth.



Unit for micrograms.

1 microgram (µg) = 1 millionth of a gram = 10-6 g







Unit for micrometer.

1 micrometer (µm) = 1 millionth of a meter = 10-6 m