Asthma is a chronic disease of the lungs that affects millions around the world, particularly in industrialized countries. The symptoms of asthma include shortness of breath, chest tightness, wheezing, and coughing; nighttime symptoms that interfere with sleep can be particularly troublesome. These symptoms are caused by inflammation, swelling, and constriction of muscles that surround the breathing tubes in the chest, the airways connecting the lungs to the mouth and nose. The hallmark of asthma is variability and reversibility in the inflammation and constriction of the airways.

Number of Persons with Asthma in the United States, National Health Interview Survey, 1980-1996

Although researchers have identified a number of chemicals in the workplace that can cause asthma, the cause of most cases of asthma is unknown. Genetic risk factors are important; adults who have asthma are more likely to produce children with asthma than adults who do not suffer from...