One of the main factors that determine indoor air quality are Microbes
Microorganisms such as fungi and viruses may play a role in the development of asthma and allergies involving the airways. This is for instance a problem in damp buildings or indoor environments where there is mold because many fungi release substances that cause allergies. Virus infections may also be transmitted by indoor air and some of them can lead to an increase in asthma and allergies.
(GreenFacts- Facts on Health and the Environment)