Hypersensitivity pneumonitis and related conditions in the work environment.

Hypersensitivity pneumonitis, also known as extrinsic allergic alveolitis, is an uncommon non–immunoglobulin E (IgE), T-helper cell type 1 (Th1)–mediated inflammatory pulmonary disease with systemic symptoms resulting from repeated inhalation and subsequent sensitization to a large variety of aerosolized antigenic organic dust particles. The exaggerated immune response to repeated inhalation of these particles leads to infiltration and proliferation of activated pulmonary macrophages and lymphocytes, resulting in lymphocytic alveolitis and bronchiolitis with noncaseating granulomas. Fibrosis may occur with chronic exposure. Recurrent or chronic cough and/or dyspnea with or without systemic symptoms should alert the physician to the diagnosis. The earliest forms of hypersensitivity pneumonitis were related to farming and, each year, new antigens causing occupational disease are described. Hypersensitivity pneumonitis was originally described in 1713 as an occupational lung disease in grain workers and later, in 1932, in farmers inhaling moldy hay contaminated with thermophilic actinomyces, hence the term farmer’s lung. With this recognition, modernization of farming methods has resulted in the reduction in farmer’s lung prevalence estimated at 0.5% to 3% of exposed farmers in studies spanning from 1980 to 2003. Definite conclusions on prevalence and incidence of farmers lung are elusive because of methodological issues in study design and definitions of disease, fewer farmers in general, and erroneous diagnoses. However, farming continues to represent a major source of exposure to antigens capable of causing occupational