A diversity of airborne dusts, gases, fumes, and vapors can induce dose-related respiratory symptoms in individuals exposed in the workplace. These agents can cause annoyance reactions, irritational effects, sensitization, or the induction of corrosive changes in the respiratory tract, depending on their composition, concentration, and duration of exposure. The prevalence of occupational asthma (OA) ranges from 9% to 15% of the asthmatic population. Factors that might influence the development of OA include the work environment, climatic conditions, genetic proclivities, tobacco and recreational drug use, respiratory infection, bronchial hyperresponsiveness, and endotoxin exposure. Pathogenetically, new-onset OA can be allergic or nonallergic in origin. The allergic variants are usually caused by high-molecular-weight allergens, such as grain dust and animal or fish protein. Selected low-molecular-weight agents are also capable of inducing allergic OA. Symptoms ensue after a latent period of months to years. Nonallergic OA can be precipitated by a brief high-level exposure to a potent irritant. Symptoms occur immediately or within a few hours of the exposure. Once the diagnosis of allergic OA is established, the worker should be removed from further exposure in the workplace. In nonallergic OA the worker can return to work if the exposure was clearly a nonrecurring event. If the diagnosis is made in a timely fashion, most workers experience improvement. Prevention is the best therapeutic intervention.