Investigating suspected work-related asthma

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Definition and prevalence of work-related asthma

Work-related asthma includes work-exacerbated asthma (worsening of asthma control due to workplace factors) and occupational asthma (new-onset asthma).

An estimated 25% of adults with asthma have work-related asthma.³

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Work-exacerbated asthma

Work-exacerbated asthma refers to worsening of asthma control due to workplace factors (e.g. inhalation of airborne substances, physical exertion or exposure to cold air). It occurs in an estimated 22% of adults with asthma.⁴

People with work-exacerbated asthma experience symptoms on more days, use more healthcare resources and have lower quality of life than people with asthma that is unrelated to work.⁴ Persistent exposure to workplace factors that worsen asthma control might result in greater loss of lung function than would occur if the person were not exposed to these factors, but this has not been clearly demonstrated.⁴

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Occupational asthma

Occupational asthma refers to new-onset asthma caused by exposure to an airborne substance in the workplace (sensitiser or irritant).

An estimated 16–20% of adult-onset asthma is caused by workplace exposure to sensitisers or irritants.^{5, 6} Hundreds of substances are known to cause occupational asthma and more are identified each year.² Many of these are found in Australian workplaces.⁷

Occupational asthma is preventable if exposure to sensitisers and irritants is identified and controlled.

Undiagnosed occupational asthma results in ongoing poor asthma control, increasing dose requirements for asthma medicines, and possibly irreversible decline in lung function.^{2, 8} Deaths from occupational asthma have been reported.^{2, 9}

There are two forms of occupational asthma:

• sensitiser-induced occupational asthma (allergic mechanism)
• irritant-induced occupational asthma (non-allergic mechanism).

Sensitiser-induced occupational asthma

Sensitiser-induced occupational asthma develops after a period of exposure (a few days to several years) to a specific substance (e.g. inhaled protein or other chemical) in the workplace.²

People with a history of atopy have higher risk of developing sensitiser-induced occupational asthma when exposed to some antigens (e.g. animal proteins, flour). Symptoms of rhinitis (e.g. sneezing, nasal congestion) often occur at work before occupational asthma develops.³

Employers should be aware of the risk associated with the use of sensitising agents and should control workers’ exposure, as far as is practical.⁷ Comprehensive lists of identified sensitising agents are available from the OASYS research group, Midland Thoracic Society, UK and the UK government Health and Safety Executive.

Table. Examples of common sensitising agents and occupations associated with exposure

Agent	Occupations
Low molecular weight agents
Wood dust (e.g. western red cedar, redwood, oak)	Carpenters Builders Model builders Sawmill workers Sanders
Isocyanates	Automotive industry workers Adhesive workers Chemical industry Mechanics Painters Polyurethane foam production workers
Formaldehyde	Cosmetics industry Embalmers Foundry workers Hairdressers Healthcare workers Laboratory workers Tanners Paper, plastics and rubber industry workers
Platinum salts	Chemists Dentists Electronics industry workers Metallurgists Photographers
High molecular weight agents
Latex	Food handlers Healthcare workers Textile industry workers Toy manufacturers
Flour and grain dust	Bakers Combine harvester drivers Cooks Farmers Grocers Pizza makers
Animal allergens (e.g. urine, dander)	Animal breeders Animal care workers Jockeys Laboratory workers Pet shop workers Veterinary surgery workers

Source: Adapted from Hoy R, Abramson MJ, Sim MR. Work related asthma. Aust Fam Physician 2010; 39: 39-42. Available from: http://www.racgp.org.au/afp/201001/35841

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• Go to: OASYS research group's information on Causative agents
• Go to: UK government Health and Safety Executive's Asthma information

Irritant-induced occupational asthma

Irritant-induced occupational asthma is due to an inflammatory (non-immunological) response of the lower respiratory tract to exposure to an irritant in the workplace. Respiratory symptoms typically occur within minutes to hours of exposure.² Reactive airways dysfunction syndrome (a type of irritant-induced occupational asthma) occurs after a single massive exposure to an irritant.²

Recurrent low-level exposure might also result in irritant-induced occupational asthma, but this has not been clearly demonstrated.²

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Investigation of work-related asthma symptoms

Asking all workers with asthma whether their asthma symptoms improve when away from work is a useful screening tool. Those who answer 'yes' need investigation for work-related asthma.¹

Investigation of suspected work-related asthma is complex. It involves confirming the diagnosis of asthma, identifying the workplace as the cause of asthma symptoms, and identifying the specific causal agents. This involves taking a detailed history, further investigations (e.g. serial peak expiratory flow measurement, skin prick tests, bronchial provocation [challenge] testing) and sometimes worksite visits.^2, 3

People with suspected work-related asthma should ask their employer to provide a safety data sheet containing information about the constituents and properties of substances used at the workplace. Information about safety data sheets (previously called Material Safety Data Sheets) is available from Safe Work Australia.

Accurate diagnosis and documentation are essential to support a potential Workers Compensation claim. This normally requires a report from a specialist with experience investigating work-related asthma.

• Go to: Safe Work Australia

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Definition of variable expiratory airflow limitation

Most of the tests for variable expiratory airflow limitation are based on showing variability in FEV₁. While reduced FEV₁ may be seen with many other lung diseases (or due to poor spirometric technique), a reduced ratio of FEV₁ to FVC indicates airflow limitation.¹⁰ Normal FEV₁/FVC values derived from population studies vary,^11, 12 but are usually greater than:¹¹

• 0.85 in people aged up to 19 years
• 0.80 in people aged 20–39 years
• 0.75 in people aged 40–59 years
• 0.70 in people aged 60–80 years.

In children, it is less useful to define expiratory airflow limitation according to a specific cut-off for FEV₁/FVC ratio, because normal values in children change considerably with age.¹²

Some spirometers provide predicted normal values specific to age group. If these are available, a FEV₁/FVC ratio less than the lower limit of normal (i.e. less than the 5th percentile of normal population) indicates airflow limitation.

Variable expiratory airflow limitation (beyond the range seen in healthy populations) can be documented if any of the following are recorded:

• a clinically important increase in FEV₁ (change in FEV₁ of at least 200 mL and 12% from baseline for adults, or at least 12% from baseline for children) 10–15 minutes after administration of bronchodilator
• clinically important variation in lung function (at least 20% change in FEV₁) when measured repeatedly over time (e.g. spirometry on separate visits)
• a clinically important reduction in lung function (decrease in FEV₁ of at least 200 mL and 12% from baseline on spirometry, or decrease in peak expiratory flow rate by at least 20%) after exercise (formal laboratory-based exercise challenge testing uses different criteria for exercise-induced bronchoconstriction)
• a clinically important increase in lung function (at least 200 mL and 12% from baseline) after a trial of 4 or more weeks of treatment with an inhaled corticosteroid
• clinically important variation in peak expiratory flow (diurnal variability of more than 10%)
• a clinically important reduction in lung function (15–20%, depending on the test) during a test for airway hyperresponsiveness (exercise challenge test or bronchial provocation test) measured by a respiratory function laboratory.

Notes

Patients referred to a respiratory function laboratory may be asked not to take certain medicines within a few hours to days before a spirometry visit.

A clinically important increase or decrease in lung function is defined as a change in FEV₁ of at least 200 mL and 12% from baseline for adults, or at least 12% from baseline for children, or a change in peak expiratory flow rate of at least 20% on the same meter.^13, 10 A clinically important increase in FVC after administering bronchodilator may also indicate reversible airflow limitation, but FVC is a less reliable measure in primary care because FVC may vary due to factors such as variation in inspiratory volume or expiratory time.

The finding of ‘normal’ lung function during symptoms reduces the probability that a patient has asthma, but a clinically important improvement in response to bronchodilator or inhaled corticosteroid can occur in patients whose baseline value is within the predicted normal range.

The greater the variation in lung function, the more certain is the diagnosis of asthma. However, people with longstanding asthma may develop fixed airflow limitation.

Reversibility in airflow limitation may not be detected if the person is already taking a long-acting beta₂ agonist or inhaled corticosteroid.

Airflow limitation can be transient and does not necessarily mean that the person has asthma (e.g. when recorded during a severe acute infection of the respiratory tract). Ideally, airflow limitation should be confirmed when the patient does not have a respiratory tract infection. Reduction in lung function during a respiratory tract infection with improvement in lung function after its resolution, commonly occurs in people with asthma, but can also be seen in patients with COPD or in healthy people without either asthma or COPD.^14,15

• Go to: National Asthma Council Australia’s Spirometry Resources
• Go to: National Asthma Council Australia and Woolcock Institute Peak Flow Chart

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Referral options for investigation of work-related asthma

To identify a specialist with experience investigating work-related asthma, consult the Thoracic Society of Australia and New Zealand or the Australasian Faculty of Occupational and Environmental Medicine.

• Go to: Thoracic Society of Australia and New Zealand 
• Go to: Australasian Faculty of Occupational and Environmental Medicine

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