town in the Austrian Tyrol that is built on an alluvial fan of a rock slide, had radon measurements between 2,000 and 250,000 Bq m−3 (Ennemoser et al., 1994). The incidence of lung cancers was statistically higher than expected in the town population.

INHALATION OF BIOLOGICAL CONTAMINANTS

Biological airborne contaminants—bioaerosols—which can be ingested or inhaled by humans include bacteria, viruses, and fungi of geogenic origin as well as airborne toxins (Griffiths and DeCosemo, 1994). Bioaerosol sizes range typically from 0.5 to 30 µm in diameter, and usually particles are surrounded by a thin layer of water (Stetzenbach, 2001). In other instances, the biological particles can be associated with particulate matter such as soil or biosolids (Lighthart and Stetzenbach, 1994). Bioaerosol particles in the lower spectrum of sizes (0.5–5 µm) are typically of most concern, as these particles are more readily inhaled or swallowed (Stetzenbach, 2001).

Bioaerosols generated from the land application of biosolids may be associated with soil or vegetation, depending on the type of land application, and are therefore considered an earth-derived source of pollution. The soil particles or vegetation provide a “raft” for the biological particles contained within the aerosol (Lighthart and Stetzenbach, 1994). However, for soil particles to be aerosolized, the particles need to be fairly dry, and low soil moisture contents are known to promote microbial inactivation (Straub et al., 1992; Zaleski et al., 2005).

Potentially there are three phases to the bioaerosol exposure pathway—launching of bioaerosols, transport, and deposition onto humans or interception by humans. Launching can result directly from human activity (coughing or sneezing) or indirectly from waste handling and loading of sewage, biosolids, or animal wastes. Launching can also occur from natural sources, such as the wind-blown spores released from soil fungi. Transport distances can be short, as in the case of one human sneezing and infecting a nearby person. In other cases, transport can be over hundreds of kilometers. Human interception of bioaerosols, resulting in infection or illness, can be via ingestion or inhalation.

Health Effects of Airborne Pathogens

Table 3.3 illustrates the wide variety of human pathogens that can be aerosolized. Although most of these pathogens do not originate or reside in soils, there are some significant bacterial pathogens found in soils (e.g., Bacillus anthracis, the causative agent of anthrax, although anthrax outbreaks from soil have rarely been documented). Many fungi are found in



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