The composition and viability of indoor microbial communities are determined by the characteristics and dynamic interactions of the building they inhabit, the building's occupants, and the surrounding external environment.
Air, water, and surfaces are the primary reservoirs for microbes found indoors. Microbes enter the indoor environment primarily through occupant shedding, through being carried from the outdoors through the air and water, and through microbial growth that occurs indoors.
Exposure to microbes is affected by how buildings exchange air with the environment that surrounds them. This exchange can take place through infiltration (unintentional air leakage) and other forms of ventilation, including natural (such as opening windows) and mechanical (heating, ventilation, and air conditioning [HVAC] systems). It is also strongly affected by human activity.