Over land surfaces a quarter of the total airborne particulate may be made up of biological material in the form of pollen, fungal spores, bacteria, viruses, or fragments of plant and animal matter. Meteorological variables affect the initial release of this material and its dispersal once airborne. Temperature and water availability will affect the size of the source and will control the release of some actively released fungal spores. Inertly released material will become airborne when the drying of the surface reduces bonding forces, and when the material is disrupted by sufficiently strong air movement or by mechanical disturbance. The wind speed necessary to disrupt material is noted to be less on a plant surface than on the ground surface. Measurements of the concentrations of airborne material near dominant sources are reviewed for both area sources, and for point sources such as sewage and waste treatment works, agricultural practices, and diseased animals. The concentration of airborne material remote from sources is considered along with the effects of on and off shore winds and some examples of long distance transport of material. The vertical concentration of bacteria is noted to decline less rapidly than that of fungal spores. The short-term variation of pollen, fungal spore, and bacterial concentrations are also considered.

• fungi: spore, aerosol