The winds not only generate aerosols but also scatter preformed particles, including pollen grains, spores, bacteria, viruses, algae, fungi, nematodes, protozoa, and fragments of plant and animal tissues. The concentrations of certain kinds of biological aerosols are monitored for allergy sufferers through the familiar air quality indices of fungal spores and pollen. More generally, however, investigations of biological aerosols have been limited despite their relevance for studies of air quality, climate, chemical cycles, and so forth. Biological aerosols span a large range in size, from radii of <0.1 |im for viruses to hundreds of micrometers for large pollen grains and spores. Evolution has shaped certain types of pollen and spores to favor their dispersal through the atmosphere, and thus even though they are geometrically quite large, such particles can be transported over long distances and to great heights. For example, culturable fungi have been recovered from the atmosphere at altitudes between 57 and 77 km (Imshenetsky et al., 1978).
Fungi are among the most abundant of the viable biological aerosols (Duce et al., 1983), and their numbers vary strongly with location, season, meteorology, and diurnal cycle. Worldwide, fungi of the genus Cladosporium are the most abundant, and in temperate regions fungal spores from this genus are especially abundant in summer and early fall. Spores and hyphal fragments from other genera of fungi, including Alternaría, Drechslera, Epicoccum, Aspergillis, and Penicillium, are commonly collected in samples of particulate matter in air. Under some circumstances, such as crop harvesting or mowing, the numbers of airborne fungal spores from local sources can reach impressive numbers, up to 109 spores per cubic meter of air (Levetin, 1995). Many species of fungi contain substances called allergans that trigger allergic reactions in humans, and various types of fungi cause respiratory and opportunistic infections. Leathers (1981) reported that each year several hundred thousand persons are infected with airborne, disease-causing fungi in the United States alone. Among the pathogenic fungi, Coccidiodes imitis, which is endemic to the southwestern United States and causes "valley fever," presents particularly serious health and economic problems. Each year several hundred persons require hospitalization because of this fungus, which is spread via spores transported through the atmosphere from the desert regions to metropolitan areas.
Bacteria are patchily distributed in the atmosphere, and they are released from both natural and anthropogenic sources by various mechanical processes including wind abrasion, agricultural activities, etc. Typical concentrations of culturable bacteria range from 10 to 1000 colony-forming units per cubic meter of air, but the numbers of bacteria in the atmosphere can reach 109 per cubic meter under disturbed conditions (Muilenberg, 1995). Bubbles rising in the oceans scavenge bacteria and viruses from the water column (Blanchard, 1983; Baylor et al., 1977, respectively) in the same way organic carbon and trace elements are scavenged, and bacterial enrichments of several 100-fold have been observed in the aerosol relative to seawater. Airborne bacteria produced in the operation of wastewater treatment plants pose potential health hazards (Hickey and Reist, 1975), but the best known case of a health problem associated with biological aerosols is Legionnaires' disease caused by bacteria (Legionella pneumophilia) growing in air-conditioning cooling towers (Dondero et al., 1980).
Pollen grains contain genetic material from male seed plants, and one group of pollen-producing plants has been classified as anemophilous because they entrust pollination of the female flowers to the wind rather than insects (Muilenberg, 1995). Pollen is produced in flowers, and the amount of airborne pollen is governed by the life cycles of plants, which in turn are influenced by extrinsic factors such as temperature, the photocycle, and precipitation. The walls of many pollen grains are composed of a resistant outer layer made of a polymer called sporopollenin and an inner wall of cellulose. Allergans associated with pollen most commonly affect the upper respiratory tract as hay fever and related maladies, but pollen exposure also can lead to asthma. The pollen from anemophilous plants, which are more common in temperate areas and less so in the tropics, tends to be smaller than from the entomophilous (insect-pollinated) plants. Studies of pollen grains in marine sediments have been used in paleoclimate reconstructions, particularly those involving paleowinds (e.g., Hoogheimstra, 1987).
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