Asthma is a disorder of the respiratory system. It occurs when exposure to a substance creates an allergic response in the human body. Substances known to cause an allergic response include latex, peanuts, isocyanates, moulds, household dust mites, wood dust, epoxy resins, cement and coal-tar products; they are often referred to as sensitisers. Some people will react at very low exposure levels whilst other may not show any symptoms at all. Substances that trigger asthma in some people may give others symptoms similar to hay-fever, a runny or stuffy nose and watery or prickly eyes. Asthma can be related to both work and leisure activities. When an attack takes place the lining of the airways swells and the surrounding muscles become tight. This reduces the amount of air that can be breathed, causes loss of breath and makes the chest ‘wheezy’. Asthmatic attacks can last for minutes to hours and will be separated by symptom free periods. Asthma can be aggravated by work or be caused as a direct result of workplace exposure. People who suffer work aggravated asthma normally suffer from asthma while away from the workplace, are likely to have suffered since childhood and have a record of medical treatment. At work they may get wheezy or show other symptoms when exposed to irritant dust, fumes or vapours, cold temperatures or excessive exertion. For these people the problem can often be eliminated by improving the working environment or avoiding exposure to the irritant. Occupational asthma is the direct result of exposure to a substance used at work. It may develop after a single massive exposure to a sensitising agent, the result of an accident or a major failure of control measures. Symptoms become apparent within 24 hours of exposure and normally improve and go away within six months. If they last beyond six months they are likely to become persistent. People at work can also become allergic to a substance used in the workplace over a period of time. They may show no symptoms at their first exposure, but these develop over a period of time from as little as a few weeks to many years; usually the symptoms will appear during the first two years of exposure. Employers are under a duty to protect their workforce from the risk of occupational asthma. Medical surveillance of the workforce may be required in workplaces using known sensitisers. See also: Asthmagens, Occupational Health Surveillance.