Indoor biological pollutants — that is, invisible airborne living organisms or particles released from living organisms — can originate from the outdoor air or from internal sources such as building occupants and building materials that host microbiological growth (e.g. mould on walls or other damp surfaces, or in poorly maintained heating, ventilation and air-conditioning systems). Biological pollutants, or bioaerosols, include viruses (such as the common flu virus), bacteria, fungi and spores, and allergenic particles including pollens, dust mites, pet fur and dander (a mixture of dead skin cells and hair or feathers). The larger the particles the faster they settle from the air onto furniture and floors, mostly attached to dust. They are very common in the indoor air and, depending on the characteristics of the biopollutant, can potentially cause infectious diseases, allergic reactions or toxic health effects. Safe levels to ensure good air quality are defined for many chemical compounds and particulate matter, but there is little awareness and no such limits with respect to all airborne contaminants of biological origin. It is therefore good to be informed about them, to understand the importance of damp conditions for mould growth, and to put in place basic hygiene measures to avoid the spread of germs.