Fungal Aerobiology

The atmosphere is filled with abundant airborne spores. These are produced by fungi in the natural environment. These may be fungi growing in the soil, on leaf litter, on leaf surfaces, or other parts of living plants. Although the majority of these fungi are saprobes, existing on dead or non-living organic material, other fungi may be plant or animal pathogens. Although the news media often refer to fungal spores as "mold spores", many of the airborne spores are not molds. They are spores of mushrooms, bracket fungi, and other fleshy fungi. By contrast, molds are microfungi, so "fungal spores" is the correct terminology. 

In many parts of the world, fungal spores occur in the atmosphere year-round. In most temperate climates, spores will be present in the air unless the ground is covered with snow and ice. Highest spore levels typically occur in the summer and fall on warm windy days.  Spores called the dry air spora, are dispersed into the air passively by simple air movement. These spores are easily detached from the fungal mycelium by the wind and can be deposited locally or carried long distances depending on meteorological variables. Common members of the dry air spora include Cladosporium, Alternaria, Epicoccum, Pithomyces, Curvularia, Drechslera-type spores as well as smut spores and others. The image below, on the left, is the typical appearance of the atmosphere in Tulsa during the fall. Most of the spores pictured here are members of the dry air spora.  

Although many spore types are washed from the atmosphere during rainfall, other airborne spore types require moisture and occur during or soon after rainy weather or during times of high humidity. Ascospores and basidiospores are typical of the what can be referred to as the wet weather air spora. Ascospores may be released during light rain or shortly after rain has ended. The image below on the right shows abundant ascospores after rainfall. Some ascsospores only require high humidity from spore release, and these are found in the atmosphere whenever the humidity is high, usually late night or early morning. Rain from spring to fall will often stimulate the development of mushrooms and other fleshy fungi in an area. Therefore, a few days after rainfall, there may be a peak in airborne basidiospores that are produced by these fleshy fungi. The release of basidiospores also requires moisture and their atmospheric levels are usually during late night and early morning hours when the humidity is high.

Airborne spores also occur in most indoor environments. Some are introduced from outside through doors and windows, while other spores are produced by fungi growing on indoor substrates.