• What is occupational asthma?
• How does asthma develop?
• How long does asthma take to develop?
• How common is asthma?
• What factors increase the chances of developing asthma?
• How does the doctor know if a worker has asthma?
• How can we control occupational asthma?
• How can we prevent occupational asthma?
• What occupations are at risk for asthma?

What is occupational asthma?

Asthma is a respiratory disease. It creates narrowing of the air passages that results in difficult breathing, tightness of the chest, coughing, and breath-sounds such as wheezing.

Occupational asthma refers to asthma that is caused by breathing in specific agents in the workplace. An abnormal response of the body to the presence of an agent in the workplace causes occupational asthma.

The abnormal response, called "sensitization," develops after variable periods of workplace exposure to certain dusts, fumes or vapours.

This sensitization may not show any symptoms of disease or it may be associated with skin rashes (urticaria), hay fever-like symptoms, or a combination of these symptoms.

Not all workers react with an asthmatic response when exposed to industrial agents. Asthma strikes only a fraction of workers. Asthmatic attacks can be controlled either by ending exposure to the agent responsible or by medical treatment.

How does asthma develop?

Asthma is triggered in several ways and most of them are not completely understood. For simplicity, we categorize them into two groups: allergic and non-allergic.

Allergic Asthma

Allergic asthma involves the body's immune system. This is a complex defense system that protects the body from harm caused by foreign substances or microbes. Among the most important elements of the defense mechanism are special proteins called "antibodies." These are produced when the human body contacts an alien substance or microbe. Antibodies react with substances or microbes to destroy them. Antibodies are often very selective, acting only on one particular substance or type of microbe.

But antibodies can also respond in a wrong way and cause allergic disorders such as asthma. After a period of exposure to an industrial substance, either natural or synthetic, a worker may start producing too many of the antibodies called "immunoglobulin E" (IgE). These antibodies attach to specific cells in the lung in a process known as "sensitization."

When re-exposure occurs, the lung cells with attached IgE antibodies react with the substance. This reaction results in the release of chemicals such as "leukotrienes" that are made in the body. Leukotrienes provoke the contraction of some muscles in the airways. This causes the narrowing of air passages which is characteristic of asthma.

Non-Allergic Asthma

Following repeated exposure to an industrial chemical, substances such as leukotrienes are released in the lungs. Again, the leukotriene causes narrowing of air passages typical of asthma. The reasons for such release are still not clear because no antibody reaction seems to be involved.

Other Types of Asthma

In certain circumstances, symptoms of asthma may develop suddenly (within 24 hours) following exposure to high airborne concentrations of respiratory irritants such as chlorine. This condition is known as reactive airways dysfunction syndrome (RADS). The symptoms may persist for months or years when the sensitized person is re-exposed to irritants. RADS is controversial because of its rarity and the lack of good information on how the lungs are affected and the range of substances which cause it.

How long does asthma take to develop?

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.

How common is 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.

What factors increase the chances of developing 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.

How does the doctor know if a worker has asthma?

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.

How can we control occupational asthma?

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.

How can we prevent occupational asthma?

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.

What occupations are at risk for 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.

Table 1 Causes of Occupational Asthma - Grains, flours, plants and gums
Occupation	Agent
Bakers, millers	Wheat
Chemists, coffee bean baggers and handlers, gardeners, millers, oil industry workers, farmers	Castor beans
Cigarette factory workers	Tobacco dust
Drug manufacturers, mold makers in sweet factories, printers	Gum acacia
Farmers, grain handlers	Grain dust
Gum manufacturers, sweet makers	Gum tragacanth
Strawberry growers	Strawberry pollen
Tea sifters and packers	Tea dust
Tobacco farmers	Tobacco leaf
Woollen industry workers	Wool

 

Table 2 Causes of Occupational Asthma - Animals, insects and fungi
Occupation	Agent
Bird fanciers	Avian proteins
Cosmetic manufacturers	Carmine
Entomologists	Moths, butterflies
Feather pluckers	Feathers
Field contact workers	Crickets
Fish bait breeders	Bee moths
Flour mill workers, bakers, farm workers, grain handlers	Grain storage mites, alternaria, aspergillus
Laboratory workers	Locusts, cockroaches, grain weevils, rats, mice, guinea pigs, rabbits
Mushroom cultivators	Mushroom spores
Oyster farmers	Hoya
Pea sorters	Mexican bean weevils
Pigeon breeders	Pigeons
Poultry workers	Chickens
Prawn processors	Prawns
Silkworm sericulturers	Silkworms
Zoological museum curators	Beetles

 

Table 3 Causes of Occupational Asthma - Chemicals/Materials
Occupation	Agent
Aircraft fitters	Triethyltetramine
Aluminum cable solderers	Aminoethylethanolamine
Aluminum pot room workers	Fluorine
Autobody workers	Acrylates (resins, glues, sealants, adhesives)
Brewery workers	Chloramine-T
Chemical plant workers, pulp mill workers	Chlorine
Dye weighers	Levafix brilliant yellow, drimarene brilliant yellow and blue, cibachrome brilliant scarlet
Electronics workers	Colophony
Epoxy resin manufacturers	Tetrachlorophthalic anhydride
Foundry mold makers	Furan-based resin binder systems
Fur dyers	Para-phenylenediamine
Hairdressers	Persulphate salts
Health care workers	Glutaraldehyde, latex
Laboratory workers, nurses, phenolic resin molders	Formalin/formaldehyde
Meat wrappers	Polyvinyl chloride vapour
Paint manufacturers, plastic molders, tool setters	Phthalic anhydride
Paint sprayers	Dimethylethanolamine
Photographic workers, shellac manufacturers	Ethylenediamine
Refrigeration industry workers	CFCs
Solderers	Polyether alcohol, polypropylene glycol

 

Table 4 Causes of Occupational Asthma - Isocyanates and metals
Occupation	Agent
Boat builders, foam manufacturers, office workers, plastics factory workers, refrigerator manufacturers, TDI manufacturers/users, printers, laminators, tinners, toy makers	Toluene diisocyanate
Boiler cleaners, gas turbine cleaners	Vanadium
Car sprayers	Hexamethylene diisocyanate
Cement workers	Potassium dichromate
Chrome platers, chrome polishers	Sodium bichromate, chromic acid, potassium chromate
Nickel platers	Nickel sulphate
Platinum chemists	Chloroplatinic acid
Platinum refiners	Platinum salts
Polyurethane foam manufacturers, printers, laminators	Diphenylmethane diisocyanate
Rubber workers	Naphthalene diisocyanate
Tungsten carbide grinders	Cobalt
Welders	Stainless steel fumes

 

Table 5 Causes of Occupational Asthma - Drugs and enzymes
Occupation	Agent
Ampicillin manufacturers	Phenylglycine acid chloride
Detergent manufacturers	Bacillus subtilis
Enzyme manufacturers	Fungal alpha-amylase
Food technologists, laboratory workers	Papain
Pharmacists	Gentian powder, flaviastase
Pharmaceutical workers	Methyldopa, salbutamol, dichloramine, piperazine dihydrochloride, spiramycin, penicillins, sulphathiazole, sulphonechloramides, chloramine-T, phosdrin, pancreatic extracts
Poultry workers	Amprolium hydrochloride
Process workers, plastic polymer production workers	Trypsin, bromelin

 

Table 6 Causes of Occupational Asthma - Woods
Occupation	Agent
Carpenters, timber millers, woodworkers	Western red cedar, cedar of Lebanon, iroko, California redwood, ramin, African zebrawood
Sawmill workers, pattern makers	Mansonia, oak, mahogany, abiruana
Wood finishers	Cocabolla
Wood machinists	Kejaat

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Document last updated on February 8, 2005

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