The overall extent of mushroom allergy is not known. It may be very slight (1%) from eating, but could, alternatively, be as prevalent as pollen and mould allergy (10-30% of an allergic population). Aerospora of mushrooms and other woodland fungi, mostly basidiospores, occur in temperature zones in June to November, reaching maximum in August and September in quantities comparable to pollen and mould spores. There are large local and annual variations in species and spore concentrations in different milieus. In SPT and BPT studies about two dozen of these species have been associated with inhalant type I allergy. All species studied so far have yielded positive results. Mushroom allergens have been explored in only two studies. These show that mushrooms are antigenically rich and that a species can have more than one allergen. The difficulties of mushroom allergen research are very substantial because one usually has to rely on naturally growing mushrooms, where allergenic contamination by other allergen sources is frequent. Choice and recognition of species is also difficult. Virtually all known allergenic mushrooms and fungi are universal, growing equally well in Europe and North America. The genus Chlorophyllum occurs only in North America, but its close relatives of the genus Macrolepiota are common also in Europe. Podaxis grows only in desert regions near the equator and is not found in Europe. The majority of the large and more common universal mushroom families has not yet been investigated. The allergenicity of families Cortinariaceae, Russulaceae, Lactariaceae and Boletaceae is totally obscure even though they produce large quantities of spores in the air, particularly in northern Europe.