Fungi and bacteria are an essential part of the earth environment. In man-made interiors, however, they cause inappropriate decay, and human disease, which, on SSF, could be devastating. Three human diseases caused by environmental microorganisms are of particular concern. Legionnaires' disease is a serious pneumonia, resistant to many antibiotics, and carrying a 15% fatality rate. The organism (Legionella pneumophila) inhabits hot water systems where biofilms have developed. Although normal healthy people are not generally at risk, any impairment of lung defenses allows a few hundred organisms per cubic meter of air to be dangerous. Continuous halogen treatment and maintenance of adequately elevated temperatures combined with monitoring should be effective controls.Hypersensitivity pneumonitis (HP) is an acute pneumonitic response to inhaled antigens. There may be a synergistic effect between antigen and endotoxin, lowering necessary antigen dose, and increasing attack rates. Any small fungal spore or bacterial endospore, including those from common environmental fungi such as Penicillium and Cladosporium and bacteria such as Bacillus and the thermophilic actinomycetes can probably cause HP. Human risk factors are unknown. Preventing condensation, halogen treatment of water reservoirs, and monitoring will contribute to control.Fungi produce the most potent known toxins, including carcinogens, immunosuppressants, and central nervous system poisons. Effective doses for inhaled toxins are unknown, but are probably low. Airborne spores are known to contain these toxins. Human risk factors do not apply--all are affected. Toxigenic fungi should not be allowed on SSF. Prevention of contamination and monitoring for these specific organisms is essential.