The majority of fungal infections in people and animals are caused by airborne fungus. Outdoor air has a significant impact on the prevalence of fungal spore levels in interior air, making it the primary source of fungal infections in indoor environments, particularly in hospitalized persons. Airborne fungi can also cause disease by directly invading skin or lungs. Fungi are single-cell organisms that belong to the kingdom Fungi and the phylum Ascomycetes. There are several types of fungi that can grow and reproduce both inside and out-of-doors. Some fungi can be found everywhere, such as mold, while others are limited to certain places such as yeast. Some fungi are harmful while others are beneficial. For example, mushrooms are grown commercially for their liquid content which consists of sugars left over from photosynthesis (organic matter) from plants such as corn. These materials are then converted into edible substances by bacteria and other microorganisms present in the environment. Outdoor air contains many different kinds of fungi that can enter buildings through doorways, windows, and heating/air conditioning systems. The most common fungi found in outdoor air include Aspergillus spp. , Candida spp. , Cladosporium spp. , Penicillium spp. , and Scopulariosis brevicaulis.