In Mold, Housing and Wood, building materials experts at the Western Wood Products Association write:
Other conditions can increase the amount of mold spores in the indoor air of buildings. Homes with exposed-dirt crawl spaces and basements tend to have more airborne mold spores than homes without (Lumpkins, 1973; Su, 1992). With the right humidity conditions, some molds can grow on house dust. It is not surprising, then, that poor housekeeping and high indoor humidity are both associated with increased levels of airborne mold spores (Solomon, 1975; Kozak, 1979).
The biggest source of indoor mold spores is often the outdoor air (Solomon, 1975). Higher levels of indoor mold spores tend to be found in homes with yards having dense and overgrown landscaping (Kozak, 1979).
Indoor mold levels are generally lower in buildings with forced-air heating systems (as opposed to window ventilation) and lower still when these systems include a well-maintained and properly functioning air conditioning system.
Mold spore levels outdoors vary with the season and weather. They may be very high in the growing season or approach zero when snow covers the ground (Solomon, 1975). Except for the snow-cover situation, mold spores in normal indoor environments are usually between 20-50 percent of the outdoor levels.