16th January 2024

Occupational Asthma (OA)

Background

It is estimated that occupational asthma (OA) accounts for 9-15% of adult-onset asthma and is recognised as the most common cause of occupational lung disease worldwide. There are numerous recognised OA allergens. The two most common causes of OA in the UK are flour dust and isocyanates. High-risk occupations include cleaning, chemical processing, spray painting, metalworking, woodworking, plastic and rubber manufacturing and baking.

OA is defined as asthma secondary to immunological or non-immunological stimuli found in the work environment. Causative allergens are categorised as low molecular weight (LMW) and high molecular weight (HMW) proteins. The two recognised forms of OA include allergic OA and irritant-induced asthma (non-allergic). In allergic OA, repeated exposure to HMW proteins produces an IgE-mediated immune response. Level of exposure serves as the greatest risk factor for developing OA. Individual factors such as atopy, smoking history, and genetic factors predispose individuals to developing OA. Non-allergic OA is a result of bronchial hyperresponsiveness following exposure to an irritant substance and does not involve immunological sensitisation.

Presentation 

Allergic OA features:

• There is normally a history of atopy
• There is a recognised interval period between first exposure and onset of symptoms, which can range from several weeks to several years
• The presence of asthma-related symptoms such as rhinitis and conjunctivitis over a period whilst at work
• Improvement of symptoms when away from the workplace (e.g., on weekends and holidays)

Irritant-induced (non-allergic) OA features:

• Rapid onset of asthma symptoms following exposure to a substance at work
• Time of symptom onset is typically minutes to hours and can persist for longer periods of time (no interval period)
• Symptoms may last up to three months
• There is no association with smoking or atopy

Diagnosis

The diagnosis of OA relies on careful history-taking in order to help identify key risk factors such as atopy, smoking and exposure to sensitising agents and irritants. The following investigations help to support a diagnosis of OA.

Serial peak expiratory flow rate (PEFR) measurements

• This is the first-line investigation for OA
• Standard guidelines recommend PEFR measurements at least four times daily (equally spaced) over a period of at least three weeks
• PEFR measurements should be recorded alongside work tasks and any exposures
• In work-related diseases, there will be an increase in diurnal variability of PEFR.

Spirometry (pre- and post-bronchodilator)

• Values may be normal; however, this is useful as it provides a baseline for any future investigations.
• An increase in FEV1 and/or FVC of over 12% following bronchodilation confirms reversible airflow obstruction. This confirms the diagnosis of asthma.

Blood tests

• Specific serum IgE assays- These detect specific IgE antibodies to workplace allergens and can thus confirm sensitisation.

Management 

The management of OA requires a holistic approach that considers medical and workplace factors. This includes:

• Removing the precipitating agent from the work environment
• Redeployment within the same occupation
• OA may persist despite the above measures, developing into chronic asthma
  • Pharmacological management is the same as non-occupational asthma

References

• Karr, R. M. et al. (1978) “Occupational asthma,” The Journal of Allergy and Clinical Immunology, 61(1), pp. 54–65. doi: 10.1016/0091-6749(78)90474-8.
• Reid, P. A. and Reid, P. T. (2013) “Occupational lung disease,” The Journal of the Royal College of Physicians of Edinburgh, 43(1), pp. 44–48. doi: 10.4997/JRCPE.2013.111.
• Feary, J., Pinnock, H. and Cullinan, P. (2016) “Occupational asthma,” BMJ (Clinical research ed.), 353, p. i2658. doi: 10.1136/bmj.i2658.
• Kalra, P. A. (ed.) (2014) Essential Revision Notes for MRCP. 4th ed. Knutsford, England: Pastest.
• Tan J, Bernstein JA. Occupational asthma: an overview. Curr Allergy Asthma Rep. 2014 May;14(5):431. doi: 10.1007/s11882-014-0431-y. PMID: 24633615.
• Barber, C. M. et al. (2022) “British Thoracic Society Clinical Statement on occupational asthma,” Thorax, 77(5), pp. 433–442. doi: 10.1136/thoraxjnl-2021-218597.
• Nicholson, P. J. et al. (2005) “Evidence based guidelines for the prevention, identification, and management of occupational asthma,” Occupational and environmental medicine, 62(5), pp. 290–299. doi: 10.1136/oem.2004.016287.