Mold spores are much smaller than pollen grains, allowing many of them to bypass the normal filtering function of the nose effectively.  Inhalation of mold spores into the lung is a common cause of asthma attacks in people allergic to molds. 
Mold spores come from soil and decaying vegetation, and are ubiquitous.  Mold counts increase near irrigated farm land, golf courses, artificial lakes and high water use vegetation.  In the semi-arid southwest, atmospheric mold spore counts are much lower than in regions that have a higher rainfall.  Mold growth and spore counts increase with increased rainfall and high humidity, after the summer rains and  in the fall.  Dry and cold conditions tend to inhibit mold growth. 
In houses with evaporative cooling and old carpets, mold is a problem.  Growth of mold in houses increases after a plumbing or roof leak onto carpet. 
Many types of mold are found in soil and house dust.   The most common types in the southwest include Alternaria, Cladosporium, and Helminthosporium.