The types of fungi typically found indoors are the same as those found outdoors, albeit at lower levels. If higher levels and different distributions of fungal species are found indoors than out, it is likely there is an indoor fungal source. Implicit in this observation is a moisture problem in the building. Aside from obvious water leaks, fungal growth sustaining moisture can enter a building in some subtle ways. For example, hot humid air carries a considerable amount of moisture.
Condensation can happen on interior surfaces or behind walls next to the exterior of the building.
Moist air enters the building via inadequate de-humidification by the HVAC system or infiltration through the building shell as a result of negative internal pressures. A balanced ventilation system, which provides uniform positive pressures, will minimize moist air infiltration. In addition, chilling incoming air to 50 - 55 deg. Finally, recognize that drier climates can also have problems with moisture condensing indoors if dehumidification is inadequate - air at 100 deg. It is critical to consider in the design phase how a building will be maintained.