There is no fixed period of time in which asthma can develop. Asthma as a disease may develop from a few weeks to many years after the initial exposure. Studies carried out on some platinum refinery workers show that in most cases asthma develops in 6 to 12 months. But it may occur within 10 days or be delayed for as long as 25 years.

Analysis of the respiratory responses of sensitized workers has established three basic patterns of asthmatic attacks, as follows:

Immediate - typically develops within minutes of exposure and is at its worst after approximately 20 minutes; recovery takes about 2 hours.

Late - can occur in different forms. It usually starts several hours after exposure and is at its worst after about 4 to 8 hours with recovery within 24 hours. However, it can start 1 hour after exposure with recovery in 3 to 4 hours. In some cases, it may start at night, with a tendency to recur at the same time for a few nights following a single exposure.

Dual or Combined - is the occurrence of both immediate and late types of asthma.

The frequency of occupational asthma is unknown, although various estimates are available. In Japan, 15 percent of asthma in males is believed to be occupational. In the United States, two percent of all cases of asthma are thought to be of occupational origin. The number of cases of occupational asthma varies from country to country and from industry to industry. About six percent of animal handlers develop asthma due to animal hair or dust. Between 10 and 45 percent of workers who process subtilisins, the "proteolytic enzymes" like "Bacillus subtilis" in the detergent industry develop asthma. However, preparations of the enzymes in granulated form, which is less readily inhaled, have reduced the likelihood of asthma. Approximately five percent of workers exposed to such chemicals as isocyanates and certain wood dusts develop asthma.

Some workplace conditions seem to increase the likelihood that workers will develop asthma, but their importance is not fully known. Factors such as the properties of the chemicals, and the amount and duration of exposure are obviously important. However, because only a fraction of exposed workers are affected, factors unique to individual workers can also be important. Such factors include the ability of some people to produce abnormal amounts of IgE antibodies. The contribution of cigarette smoking to asthma is not known. But smokers are more likely than nonsmokers to develop respiratory problems in general.

Sufferers from occupational asthma experience attacks of difficult breathing, tightness of the chest, coughing, and breath sounds such as wheezing, which is associated with air-flow obstruction. Such symptoms should raise the suspicion of asthma. Typically these symptoms are worse on working days, often awakening the patient at night, and improving when the person is away from work. While off work, sufferers from occupational asthma may still have chest symptoms when exposed to airway irritants such as dusts, or fumes, or upon exercise. Itchy and watery eyes, sneezing, stuffy and runny nose, and skin rashes are other symptoms often associated with asthma.

Lung function tests and skin tests can help to confirm the disease. But some patients with occupational asthma may have normal lung function as well as negative skin tests.

The diagnosis of work-related asthma needs to be confirmed objectively. This can be done by carrying out pulmonary function tests at work and off work. Specific inhalation challenges can demonstrate the occupational origin of asthma and may identify the agents responsible when the cause is uncertain. Specific inhalation challenge tests require breathing in small quantities of industrial agents that may induce an attack of asthma. But they are safe when performed by experienced physicians in specialized centres.

Although there are drugs that may control the symptoms of asthma, it is important to stop exposure. If the exposure to the causal agent is not stopped, treatment will be needed continuously and the breathing problems may become permanent. People may continue to suffer from occupational asthma even after removal from exposure. For example, a follow-up study of 75 patients with asthma caused by red cedar dust showed that only half the patients recovered. The remaining half continued to have asthmatic attacks for a period of 1 to 9 years after the termination of exposure.

Dust masks and respirators can help to control workplace exposure. However, these protective devices, in order to be effective, must be carefully selected, properly fitted and well maintained. Preventing further exposure might involve a change of job. If a job change is not feasible, relocation to another area of the plant with no exposure may be essential.

The best way to prevent occupational asthma is to replace dangerous substances with less harmful ones. Where this is not possible, exposure should be minimized through engineering controls such as ventilation and enclosures of processes.

Education of workers is also very important. Proper handling procedures, avoidance of spills and good housekeeping reduce the occurrence of occupational asthma.

Some of the occupations where asthma has been seen are listed in the following tables. It should be noted that the lists of occupational substances and microbes which can cause asthma are not complete. New causes continue to be added. New materials and new processes introduce new exposures and create new risks.