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Safety in the workplace: occupational asthma

Sue Cross
RN BSc(Hons)
NPDip PGCE
International Project Manager
Respiratory Education Resource Centres
University Hospital Aintree
Liverpool
E:sue@respiratory erc.com

Occupational asthma is defined as asthma caused largely or wholly by the work environment, usually associated with exposure to a respiratory allergen or sensitiser. Once an agent encountered in the workplace has initiated asthma, all the standard agents known to aggravate nonoccupational asthma (eg, perfumes, cold air and dust) may also aggravate the patient with occupational asthma.
Occupational asthma falls into two major categories:

  1. Asthma caused by sensitisers or other agents in the workplace, with a time lag (latent period) between first exposure and the development of symptoms (occupational asthma).
  2. Asthma caused by a single (usually) high exposure to a known irritant. Asthma symptoms commence rapidly after this event. This is known as reactive airways dysfunction syndrome, or RADS.(2)

Epidemiology
Occupational asthma is common. A recent meta-analysis recorded a median value for the attributable risk of occupationally associated asthma of 9%. Therefore a substantial proportion of asthmatic patients in all healthcare settings have occupational asthma.(3)

Major causes
There are well over 400 known causes. Isocyanate asthma is the most common cause in this country. Isocyanate asthma is largely confined to paint sprayers using two-pack isocyanate systems for car body repair work. Other common causes in the UK include latex, flour and biocides, such as glutaraldehyde.
 
Clinical aspects

History
History taking is critical to the diagnosis of occupational asthma. All healthcare professionals must have heightened awareness concerning a possible diagnosis. If it is not suspected when it is truly present, the patient will undergo unnecessary risk at work, continuing to expose themselves to an agent causing their asthma.
Generally, a good history for occupational asthma would include clear work-related respiratory symptoms, normally defined as either worse at work or better on rest days. When occupational asthma is chronic or symptoms are well established, two rest days over a weekend may be insufficient to allow symptoms to improve. In this case, symptoms normally improve over holidays. It is important to realise that, once initiated, occupational asthma may be provoked by other stimuli, so that even on rest days and during holidays asthma symptoms may worsen.
Even when the history is highly suggestive of occupational asthma (work-related symptoms suggestive of asthma), the positive predictive value of a good history is only between 60 and 70%, which means that only 60-70% of those with a good history will have occupational asthma, and clearly a proportion of those without a good history will also have the condition.
It is important therefore to increase diagnostic accuracy by the other means listed below. The exact combination will depend on local expertise and availability, but history alone is generally a poor indicator of the presence or absence of a definitive diagnosis of occupational asthma. Other important aspects to ask about include: nasal symptoms - allergic rhinitis is common in workers with occupational asthma and may precede the onset of respiratory symptoms; hobbies and potential exposure; pets (especially birds); previous asbestos exposure; medication and response to treatment; family history of respiratory disease; other possible environmental exposures to noxious agents; a preceding (childhood, or at least previous relevant employment) diagnosis of asthma (although this does not exclude occupational asthma in addition); and atopy or a family history of allergy.

Occupational history
A detailed occupational history should be recorded for all patients where a diagnosis of occupational asthma is being considered. The best approach to this is as follows:

  • A chronological list of jobs should be recorded, starting with the first employment after school.
  • Tasks performed and agents worked with should then be recorded for each job, allowing the patient to talk freely about possible exposures. Some patients will be able to list all exposures accurately, while others will have less knowledge. It is essential that patients are allowed to talk without interruption about work tasks performed, as occasionally the diagnostic clue is found in the history (eg, a cook who occasionally bakes using flour, or a postman who uses spray-painting equipment during a Saturday job). Do not be put off a potential work effect if the worker is not working with a "known" respiratory sensitiser. The list of causative agents is constantly growing, and you may be seeing the first case caused by a previously unrecognised agent (see Table 1).
  • The health of coworkers should be recorded - that is, is your patient the first in the workplace with respiratory problems, or are other work colleagues also symptomatic?

[[NIP11_table1_18]]

Other information from history
Workers often have access to occupational health records and material datasheets. Employers are required by law to keep such information as part of the COSHH (Control of Substances Hazardous to Health) assessment at each worksite. You can ask the worker to send you copies of these documents. Occasionally, these will clearly state that the worker is exposed to a known sensitiser that you had not expected from the history.

Investigations
Lung functions

If a clinical diagnosis of asthma is suspected from history taking, it is important to investigate the work effect further using serial measures of lung function. This seems a sensible general approach and is adopted by most centres dealing with cases of occupational asthma.

Twice-daily PEF recordings
Peak expiratory flowrate (PEF) recordings are not reliable for diagnosing occupational asthma. Increased diurnal variation consistent with asthma may be seen. There are insufficient recordings to be able to distinguish work periods from rest periods, particularly taking into consideration all the other factors that potentially influence airway calibre (such as pollution, viral infection, exposure to irritants and chemicals, and so on), irrespective of the exposure seen at work.(4)

Two-hourly PEF measures at work
Two-hourly peak flows are measured for a period of four weeks. It is suggested that a period of three days rest is included within this month. This information is then analysed to assess work-rest-work and rest-work-rest periods, according to a predetermined set of gold-standard individuals with proven occupational asthma. This will give the individual a work-effect index, which will allow the investigator to comment on whether work is an important cause of their current problem.

Immunological tests
Immunological investigations may aid the diagnosis, particularly if the clinical probability of occupational asthma or allergy is high. Common tests performed are:

  • Skin prick testing to common allergens to diagnose atopy.
  • Skin prick testing to relevant occupational allergens (eg, wheat flour skin testing solution in a bakery).
  • The measurement of specific IgE by ImmunoCAP. IgE are allergic antibodies responsible for the immediate, type 1, hypersensitivity reactions. They are often found in workers with occupational asthma exposed to high-molecular-weight occupational allergens. Examples would be laboratory workers with rat urine IgE or carpenters with IgE to wood dusts. The presence of specific IgE is not diagnostic of occupational asthma, as these antibodies also occur in atopic (and to a lesser degree, nonatopic) workers who are exposed but have no respiratory symptoms. Therefore their presence may suggest occupational allergy and asthma, although the result has to be interpreted in light of the other clinical information.

Ethical issues
Complex ethical issues arise due to the interaction between the worker, fellow workers, the employer, the GP, other doctors and nurses, and the Health and Safety Executive (HSE). Medical confidentiality considerations are paramount in such situations.
Any health professional who believes that the workplace is a possible cause of a patient's condition must be conscious that the patient may not wish this information to be divulged to another person, such as their employer. Before they consent to disclosure, the implications of such communication must be explained to the patient. For example, the patient could fear job loss. Additionally, advice may be sought from HSE doctors or nurses.

References

  1. Bernstein DI, et al. Definition and classification. In: Bernstein IL, et al, editors. Asthma in the workplace. New York: Marcel Dekker; 1993.
  2. Brooks SM, et al. Reactive airway dysfunction syndrome (RADS); persistent asthma syndrome after high level irritant exposures. Chest 1985;88:376-84.
  3. Meyer JD, et al. SWORD '98 surveillance of work related and occupational respiratory disease in the UK. Occup Med 1999;49:485-9.
  4. Burge PS, et al. Development of an expert system for the interpretation of serial measurements in the diagnosis of occupational asthma. Occup Environ Med 1999;56:758-64.