Indoor/outdoor air concentrations of air pollutants

The exchange of air between indoor and outdoor environments plays an important role in terms of air hygiene. Ventilation processes ventilate out pollutants emitted indoors, while pollutants in the outside air enter the building. Indoor air therefore contains air pollutants that are released by sources both outdoors and indoors.

The ratio of the pollutants in the indoor air to the outdoor air (I/O ratio) provides an indication of where the predominant source of the corresponding compounds is located: indoors or outdoors.

An I/O ratio of <1 usually indicates a main source outdoors. Since many air pollutants can be generated both indoors and outdoors, the I/O ratio does not necessarily have to be highly significant. Therefore, an I/O ratio of, for example, ~1 does not allow for conclusions to be drawn.

A significant source of outdoor air pollutants is the combustion of fossil fuels by traffic and industrial plants. Particles are also frequently carried in with the outdoor air.

Particles and other air pollutants can also be produced by combustion processes indoors, e.g. smoking, cooking activities (frying, grilling, baking, heating water, toasting, ...) and the burning of candles and incense. Other indoor sources include gas and electric stoves, photocopiers, printers (laser and 3D printers) and consumer products such as hairspray, cleaning products and furniture polishes. For example, furniture polishes can contain terpenes, which contribute to the formation of particles through chemical reaction processes in the air in the presence of ozone (O₃).

Household activities such as vacuuming and sweeping also release particles into the indoor air. The same applies to walking on floor coverings, especially carpets, using upholstered furniture and handling textiles (making beds, folding clothes, etc.). Particles are released by resuspension during all these activities.

The German Environment Agency provides answers to frequently asked questions on the subject of ultrafine particles (UFP) on a dedicated homepage. The site is regularly updated to reflect the growing state of knowledge.

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