Workers exposed in Iowa swine production units were known to be significantly exposed to grain dust, endotoxin, animal danders and other proteins, ammonia and hydrogen sulfide gas, and a variety of microorganisms including fungi and thermophilic actinomycetes. Cross-sectional epidemiological surveys had documented a high prevalence of respiratory symptoms including chronic cough, phlegm and wheezing, but also prominent constitutional symptoms including fever, myalgias and malaise. This study was one of several projects supported by a Specialized Center of Research in Occupational and Immunologic Lung Disease.