farmers have decreased markedly with changes in methods of harvesting and baling hay.

Other occupations also present opportunities for development of hypersensitivity pneumonitis. Office workers are estimated to develop hypersensitivity pneumonitis at rates of from 1.2 to 4 percent (EPA, 1991b). Banaszak and colleagues (1970) reported that 15 percent of workers in one office displayed pulmonary disease from thermophilic actinomycetes; their exposure had occurred through a contaminated air-conditioning system. Sauna takers disease (caused by Aureobasidium pullulans) occurs infrequently. However, lifeguards at an indoor swimming pool reportedly experienced an extremely high rate of attack of a hypersensitivity pneumonitis-like condition; the causative agent of the disease remains unidentified (Rose and King, 1992).

Climate/Season In Japan, 74 percent of the cases of hypersensitivity pneumonitis reported by hospitals (a total of 835 cases) were associated with predominantly hot, humid climates and occurred in the summer. These cases were attributed to Trichosporon cutaneum on the basis of detection of antibody in the blood.

Immunologic Sensitization In some studies, up to 20 percent of exposed populations will have IgG against the allergen in their serum. Because more than 90 percent of affected individuals will have a specific antibody, these individuals are thought to constitute a subset of the exposed population who are at increased risk of developing the disease. These general figures, however, may vary markedly across individual circumstances.

HUMIDIFIER FEVER

Humidifier fever is an illness with influenza-like symptoms that develops shortly after exposure to aerosols from microbiologically contaminated humidifiers. Recovery can occur within days, even with continuing exposure. It often recurs on the first day of reexposure following a period of no exposure.

Inhalation challenge with extracts from contaminated water can produce symptoms of humidifier fever, but the causative agent is still unknown. Experimental exposure of symptomatic workers to humidifier allergens can induce headache, rhinitis, and lethargy as well as asthma and alveolitis; similar exposure does not cause symptoms in previously unexposed individuals. These findings have led to a presumption that a specific immunologic mechanism is operative. A World Health Organization report, for example, listed endotoxin as the leading suspect (WHO, 1990). Finnegan and others (1987) identified allergens of amebae in contaminated humidifiers but failed to correlate their presence with humidifier fever or with work-related



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