Occupational Asthma
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- Definition and mechanism
- Incidence, risk factors and prevention
- Diagnosis
- Prognosis
- Management of occupational asthma
- Further information
- References
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Summary of practice points |
Level of evidence |
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| Consider the diagnosis of occupational asthma in all new cases of adult-onset asthma, because early diagnosis and avoidance of exposure is associated with the best prognosis. | [√] |
| In people with high-risk occupations, the presence of new-onset rhinitis is associated with increased risk for occupational asthma. | IV |
| Consistent improvement in asthma symptoms outside the work environment is a good indicator of occupational asthma. | [√] |
| Suspected cases of occupational asthma should be investigated by serial lung function measurements analysed by a validated method. | IV |
| Serial PEF testing cannot reliably rule out the diagnosis of occupational asthma. | IV |
| With continued exposure to the causal agent in the workplace, occupational asthma is unlikely to improve and may worsen. | [√] |
Generally, occupational asthma has a poor prognosis and is likely to persist and deteriorate unless identified and managed early and effectively. Where feasible, early referral to a respiratory physician gives patients the best chance of good control or cure.1 Alternatively, referral to an occupational health physician might be appropriate for some patients. Where specialist services are not readily accessible, GPs can investigate and manage suspected occupational asthma effectively by:Occupational asthma accounts for up to 15% of all adult-onset asthma.1 It is the most commonly reported occupational respiratory disorder in westernized industrial countries.2,3
- being aware of high-risk occupations
- taking a very careful history, looking for links between onset of symptoms and occupation-related exposure to potential causal agents
- understanding that the general principles of pharmacotherapy for asthma apply also to occupational asthma. See Principles of Drug Therapy.
Accurate diagnosis and documentation are essential to support a potential Workers Compensation claim. This would normally require specialist reports. For more information, including details of occupational asthma surveillance schemes in states and territories, contact the National Occupational Health and Safety Commission.
Definition and mechanism
Occupational asthma is defined as new-onset adult asthma caused by exposure to the workplace environment and not by factors outside the workplace.
Patients with occupational asthma fall into two groups:
1. Immunological (IgE-mediated) occupational asthma is characterised by a delay between exposure to a respiratory sensitiser and the development of symptoms.
2. Non-immunological occupational asthma typically occurs within a few hours of high-concentration exposure to an irritant at work. This sometimes occurs weeks or months after repeated low-concentration exposure to an irritant.
- The majority of patients with occupational asthma have immunological asthma.
- The onset of rhino-conjunctivitis prior to asthma symptoms is more strongly associated with immunological occupational asthma than the non-immunological form.1
Incidence, risk factors and prevention
Incidence
At least 31 out of every 1,000,000 workers in Australia will develop occupational asthma each year.4 The true incidence is likely to be much higher, because occupational asthma is under-reported.1
Risk factors
Occupational asthma is most frequently reported among people with the following occupations (not in order of risk):1
- animal handlers
- bakers and pastry makers
- chemical workers
- food processing workers
- nurses
- spray painters
- timber workers
- welders.
Cigarette smoking can increase the risk of developing occupational asthma in response to exposure to some sensitising agents.1
Prevention
Reducing airborne exposure to potential allergens in the workplace lowers workers' risk for becoming sensitised and developing occupational asthma.1 Respiratory protective equipment is only effective as protection against workplace airborne allergens when worn properly, removed safely, and either replaced or maintained regularly. Such equipment reduces the risk, but does not prevent occupational asthma.1
Diagnosis
| Practice points |
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Consider the possibility of an occupational asthma diagnosis in all new cases of adult asthma or rhinitis.
- Ask about exposure to airborne substances at work.
- In addition to taking a standard history, ask about the pattern of symptoms with respect to work (Table 1).
In people whose occupation is associated with a high risk for occupational asthma, the presence of new-onset rhinitis may signal increased risk for developing immunoglobulin E (IgE)-mediated occupational asthma within 12 months.1
For patients in high-risk occupations, consider referral to a specialist respiratory physician or occupational physician with expertise in occupational asthma.
For more information on taking a history in a patient with respiratory symptoms, see Diagnosis in Adults and Ongoing Care.
Table 1. Additional questions to ask patients with adult-onset asthma
Note that these questions are not specific for occupational asthma and may also identify people with asthma due to agents at home, or who experience exercise-induced asthma due to work-related activity.
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What substances do you handle at work? Are there any dusts, powers or substances in the air? |
Several hundred agents have been reported to cause occupational asthma and new causes are reported regularly in medical literature. A direct relationship between occupational asthma and allergen exposure at work has been demonstrated for acid anhydrides, cimetidine, colophony, enzymes, green coffee, castor bean, flour allergens, crab, isocyanates, laboratory animal allergens, piperazine, platinum salts, prawns, and western red cedar.1
For a more complete list, see: |
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When did the symptoms start?? Have there been any changes at work? |
Consider with respect to the timing of any changes in the patient's work environment or lifestyle. |
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Are symptoms different when not at work? Do symptoms improve on days off? |
Any improvement in symptoms when away from work is a good indicator of occupational asthma. |
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Do symptoms improve during a long holiday from work? |
Regular improvement of symptoms when away from work is a more reliable indicator of occupational asthma than increase in symptoms when at work.1 |
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Does anyone else at work have a similar problem? |
Indicates that the workforce and workplace should be assessed. |
A diagnosis may not be possible once exposure has ceased, so a specialist referral should be arranged first if possible.
Objective tests
Lung function tests and blood testing may help establish the diagnosis of occupational asthma.
Peak expiratory flow
Serial measurement of peak expiratory flow (PEF) is a useful test for occupational asthma.5-10
- Measure PEF at least four times a day over at least three weeks, at and away from work, and analyse the results by a validated method. A UK group of researchers has developed a computer program (OASYS) as a validated tool to assist in diagnosing occupational asthma from serial PEF records (See www.occupationalasthma.com).11
- Serial PEF cannot reliably rule out the diagnosis of occupational asthma. As a diagnostic test for occupational asthma, serial PEF (when performed and interpreted according to established protocols) is associated with a low rate of false positives, but approximately 30% false negatives.
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Practice tip Peak expiratory flow has a special role in the initial investigation of occupational asthma, where the frequent lung function testing required would make spirometry impractical. |
The gold standard for diagnosis of occupational asthma is a specific challenge with the suspected agent. However, such challenges are only done in specialist referral centres.
Specific immunoglobulin testing
Blood tests or skin prick tests for specific immunoglobulin E (IgE) for suspected allergens can help to identify the causal agent, when interpreted together with evidence from:
- a detailed history about symptoms
- the temporal relationship of symptoms to the workplace
- exposure to substances at work.
Where possible, the diagnosis of occupational asthma should be confirmed by a specialist in this field. Documentation of specialist confirmation will support a potential claim for Workers Compensation. For information about access to referral, contact The Thoracic Society of Australia and New Zealand.
Prognosis
| Practice point |
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With continued exposure to the causal agent in the workplace, occupational asthma is unlikely to improve and may worsen.12 (III) |
- If the diagnosis of occupational asthma is made early and the person avoids further exposure to the respiratory sensitiser/s in the workplace, symptoms may resolve completely.1
- Improvement is greatest in patients with near-normal lung function at the time of diagnosis and a short duration of symptoms (< 12 months).1
- With continued exposure to the workplace sensitiser, occupational asthma is unlikely to improve and may worsen.1
- Symptoms and functional impairment may persist for many years after avoidance of further exposure to the causative agent.1
- Death due to occupational asthma has occasionally been reported.13
Management of occupational asthma
- Ideal management includes complete and permanent avoidance of exposure to the environmental agents causing asthma. However, this is not always possible for individual patients.
- If complete avoidance of the allergen is not possible, the worker should be moved to an area of the workplace with lower exposure or occasional exposure and remain under regular specialist medical surveillance.
- Medical management should be as for other patients with asthma. See Principles of Drug Therapy and Ongoing Care.
- If the patient's employer has access to an occupational health service, the general practice can liaise with this service with the patient's consent.
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