Hypersensitivity pneumonitis is a specific immunologic lung disease characterized by inflammation of the lung parenchyma. The causative agents are numerous and diverse, and the immune pathogenesis includes formation of specific IgG antibody and formation of lung granulomas. The clinical spectrum is also diverse and ranges from recurrent, acute flu-like illnesses to a gradually increasing breathlessness. Hypersensitivity pneumonitis sometimes occurs in sporadic outbreaks, for example, when a building's ventilation system becomes contaminated. Diagnosis is often difficult because it requires a high index of suspicion, and the appropriate laboratory studies (i.e., precipitins) are sometimes unfamiliar to many physicians (see Chapter 5). Table 2-7 lists various causes of hypersensitivity pneumonitis that have been reported in nonindustrial indoor environments.
No prevalence rates are available for the general population. Clinical assessments of the type recommended by Solomon (1990) have not been utilized to obtain rate estimates or population impacts.
In 1977, it was estimated that 2,000 hospitalizations occurred in the United States in which hypersensitivity pneumonitis was the primary (50 percent) or secondary (50 percent) diagnosis. Treatment consists of corticosteroid therapy and removal from exposure. Failure to institute these measures can result in disease or disability resulting from irreversible fibrosis and respiratory failure (EPA, 1991b).
Indoor Environment Exposure to allergenic bioaerosols in residential or commercial heating, ventilation, and air-conditioning (HVAC) systems can cause hypersensitivity pneumonitis. Causative agents include thermophilic actinomycetes, Aspergillus species, Aureobasidium species, and other proteins.
Occupation/Hobby The risk of hypersensitivity pneumonitis increases markedly with exposure to allergens through hobbies and occupations, ranging from 0.5 to 10 percent of exposed populations. For example, the prevalence of hypersensitivity pneumonitis among pigeon breeders ranges from 6 to 15 percent (NIAID, 1979). Farmer's lung, a hypersensitivity pneumonitis that usually arises from exposure to thermophilic actinomycetes, probably occurs in 3–4 percent of exposed populations, with estimates ranging from 0.5 to 10 percent of farmers (NHLBI, 1982; NIAID, 1979). Occurrences among