Alternaria are a ubiquitous fungi consisting of over 250 known species, and is considered extremely common and widespread. The spores are found in high concentrations during dry, windy conditions, which make them extremely airborne.
Where found: Alternaria are typically found outdoors, on dead plant material, wood, cereal grains, grasses and soil. The National Survey of Lead and Allergens in Housing revealed that Alternaria is frequently found in indoor environments as well (carpets, textiles, window frames, air conditioning systems, wallpaper, etc). Exposure to A. alternata is a risk factor for asthma in humans, especially those with existing pulmonary diseases, allergies, or weak immune systems.
Alternaria thrives in damp humid, dark environments and growth rate is highest in temperature ranges 68-77°F (20-25°C). It is very common in the spring and summer months.