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Special Interest Topic:
Occupational Asthma

There are several definitions of asthma which usually consist of variable symptoms such as chest tightness, wheeze and breathlessness accompanied by airway narrowing which is reversible over short periods of time, either spontaneously or as a result of treatment.  Another feature of asthma is bronchial hyper-responsiveness, or a heightened sensitivity to several inhaled non-specific stimuli.  People with hyper-responsive airways will bronchoconstrict in response to stimuli such as exercise or an allergen at a dosage at which a normal person will not respond.  The airways of patients with asthma are characterised by a desquamative eosinophilic bronchitis, with epithelial cell disruption and mucosal infiltration by inflammatory cells such as eosinophils and lymphocytes.

Occupational asthma is asthma which is caused by exposure to a sensitisor or irritant encountered at work.  There is usually a preliminary period of exposure without symptoms, ranging from days to years.  Sensitisation, implying an immunological response to the agent, may initially only cause minor symptoms such as sneezing or eye irritation, but once sensitisation has developed it may become permanent.  Occupational asthma usually occurs some months to years after sensitisation has occurred.  The asthma would not have occurred in the absence of development of sensitisation to an occupational agent.  Rarely, it may occur after a single large dose of a provocative agent (so-called "irritant-induced" asthma).

Not all agents encountered at work will produce sensitisation, and several will just act as irritants and merely provoke asthma symptoms by non-specific mechanisms.  These are termed provokers (in contrast to sensitisers).  Provokers may cause acute bronchospasm in individuals who have pre-existing asthma, but they do not initiate occupational asthma.

Both initiators and provokers of asthma can occur at work.  Spray painters with no pre-existing asthma may, for example, develop occupational asthma due to exposure to isocyanates which are used as hardening agents in spray paints.  In this case, isocyanates are initiators of asthma and the spray painter is said to have occupational asthma.  On the other hand, outdoor workers, for example, may be exposed to cold air in winter and also to exertion.  These stimuli may provoke asthma symptoms in individuals with a pre-existing asthma, or a pre-existing tendency to asthma, but no sensitisation has taken place.  The workers here are said to have work-provoked asthma or non-specific bronchial hyper-responsiveness.  Occasionally, it can be very difficult to distinguish between true occupational asthma (i.e. sensitisation) and work-provoked asthma.

Criteria for the diagnosis of occupational asthma include:

1. Exposure to a sensitising agent at work.

2. Variable symptoms of airflow limitation accompanied by evidence of spontaneous or drug-induced airway reversibility (>15% increase in FEV₁) with bronchial hyper-responsiveness (i.e. symptoms and sign consistent with the diagnosis of asthma).

3. A history of work-related respiratory symptoms which have developed after an initial symptom-free period of exposure.

4. Reliable self-recorded serial peak flow records which show consistent deterioration during periods at work and improvement during absences from work.

5. Where applicable, evidence of a specific immunological response to the sensitising agent.

6. Positive specific challenge test.

Irritant -induced asthma (or reactive airways dysfunction syndrome, RADS) is chronic asthma that persists after a single inhalation of a respiratory irritant in toxic concentrations.  Examples of such toxic irritants include sulphur dioxide, toluene diisocyanate and ammonia fumes.  Specific criteria for the diagnosis of irritant-induced asthma have been published (Brooks 1985).

Criteria for the diagnosis of irritant-induced asthma:

1. Onset of symptoms after a single specific exposure incidence or accident

2. Exposure to a gas, smoke, fume or vapour that was present in very high concentrations and had irritant qualities

3. onset of symptoms within 24 hours after the exposure, and persisting for at least 3 months

4. Symptoms consistent with asthma- cough, wheezing and dyspnoea predominating

5. Pulmonary function tests may show airflow obstruction

6. Appropriate challenge tests demonstrate increased airway responsiveness

7. Exclusion of other pulmonary diseases

Causes of Occupational Asthma

There are several hundred causes of occupational asthma, which are usually classified into four groups:

1. Chemicals

2. Agents originating from animals

3. Agents originating from plants

4. Agents originating from microorganisms

The European Union has established a code for several occupational substances that may cause sensitisation by inhalation (now labelled R42).  The most common sensitising substances are dusts from cereal flours, enzymes, natural rubber latex, laboratory animals and low molecular weight substances such as isocyanates and acid anhydrides.

Examples of the most commonly described sensitising agents are shown below: