Occupational asthma

Asthma is 'work-related' when there is an association between symptoms and work. The different types of work-related asthma should be distinguished, since the implications to the worker and the occupational health management of the disease differ

Work-related asthma includes two distinct categories:

• work aggravated asthma, i.e. pre-existing or coincidental new onset adult asthma which is made worse by non-specific factors in the workplace, and
  
  
• occupational asthma i.e. adult asthma caused by workplace exposure and not by factors outside of the workplace. Occupational asthma can occur in workers with or without prior asthma
• occupational asthmacan be further subdivided into:
  
  
  • sensitiser-induced occupational asthmacharacterised by a latency period between first exposure to a respiratory sensitiser at work and the development of immunologically-mediated symptoms
  • irritant-induced occupational asthma that occurs typically within a few hours of a high concentration exposure to an irritant gas, fume or vapour at work (1)
• workplace agents that induce asthma through an allergic mechanism can be broadly divided into those of high and low molecular weight
  • the former are usually proteins and appear to act through a type I, IgE associated hypersensitivity.
  • some low molecular weight chemicals are associated with the development of specific IgE antibodies, this is not the case for the majority
  • almost 90% of cases of occupational asthma are of the allergic type

Occupational factors account for about 1 in 10 cases of asthma in adults of working age (4)

• Health and Safety Executive (HSE) estimate that 1,500 to 3,000 people develop occupational asthma each year. This rises to 7,000 cases a year if work-aggravated asthma is included
  • it is thought that thereported incidence of occupational asthma is underestimated by about 50% (3)
• it is the commonest industrial lung disease in the developed world with over 400 reported causes (2)
  • most frequently reported agents include isocyanates, flour and grain dust, colophony and fluxes, latex, animals, aldehydes and wood dust
  • workers most commonly reported to surveillance schemes of occupational asthma include paint sprayers, bakers and pastry makers, nurses, chemical workers, animal handlers, welders, food processing and timber workers
  • high risk work includes (2)
    • baking
    • pastry making
    • spray painting
    • laboratory animal work
    • healthcare
    • dentalcare
    • food processing
    • welding
    • soldering
    • metalwork
    • woodwork
    • chemical processing
    • textile, plastics and rubber manufacture
    • farming and other jobs with exposure to dusts and fumes
    • smoking has been identified to increase the risk of occupational asthma in workers exposed to: isocyanates, platinum salts, salmon and snow crab

Occupational rhinitis and occupational asthma frequently occur as co-morbid conditions (1)

• epidemiological evidence from the general population of a strong association between the development of asthma and a previous history of either allergic or perennial rhinitis. Occupational rhinitis is purported to be a risk factor for the development of occupational asthma, especially for high-molecular-weight sensitisers
  • rhino-conjunctivitis is more likely to appear before the onset of IgE associated occupational asthma
  • risk of developing occupational asthma is highest in the year after the onset of occupational rhinitis

Diagnosis of occupational asthma

• occupational asthma should be suspected in all workers with symptoms of airflow limitations (2)
• the following screening questions could be useful in patients with airflow obstructions:
  • are you better on days away from work?
  • are you better on holiday?
  • patients with a positive answer should be considered as having occupational asthma and should be investigated (2)
• made most easily before exposures or treatments are modified
  • serial measurement of peak expiratory flow is the most available initial investigation
  • minimum standards for diagnostic sensitivity >70% and specificity >85% are:
    • at least three days in each consecutive work period
    • at least three series of consecutive days at work with three periods away from work (usually about three weeks)
    • at least four evenly spaced readings per day (2)
  • when done and interpreted to validated standards there are very few false positive results, but about 20% are false negatives
  • skin prick tests or blood tests for specific IgE are available for most high molecular weight allergens, and a few low molecular weight agents but there are few standardised allergens commercially available which limits their use. A positive test denotes sensitisation, which can occur with or without disease
  • the diagnosis of occupational asthma can usually be made without specific bronchial provocation testing, considered to be the gold standard diagnostic test

Work-related asthma and rhinitis: case finding and management in primary care (4):

1 in 10 recurrences of asthma in adults are due to occupational asthma, so take a detailed history if important. If a patient has rhinitis which is worsened by being at work, they have a higher risk of asthma starting in the 1st year of symptoms. Occupational asthma has a worse prognosis if there is continued exposure, so prompt diagnosis is important. Advise serial PEF readings (at least 4 a day) and prompt referral to a respiratory specialist. The following chart is helpful:

Open Image in Modal

management principles:

• primary prevention aims to prevent the onset of disease, often by reducing or eliminating exposure to the agent in the workplace
  • is the most effective measure
  • relocation away from exposure should occur as soon as diagnosis is confirmed, and ideally within 12 months of the first work-related symptoms of asthma (2)
  • reduction in airborne exposure will result in a reduction the number of workers who become sensitised and who develop occupational asthma (1)
• secondary prevention aims to detect disease at an early or presymptomatic stage for example by health surveillance
• tertiary prevention aims to prevent worsening symptoms by early recognition and early removal from exposure and is considered later under the management of an identified case of occupational asthma
• referral from primary care:
  • if possible work-related asthma
  • refer quickly to a chest physician or occupational physician
  • arrange serial PEF measurements
  • if possible work-related rhinitis
    • refer to an allergy specialist or occupational physician
    • monitor for the development of asthma symptom

For more information then consult the British Occupational Health Research Foundation http://www.bohrf.org.uk/

Reference:

• (1) Guidelines for the prevention, identification & management of occupational asthma: Evidence review & recommendations. British Occupational Health Research Foundation. London. 2010
• (2) British Thoracic Society (BTS)/Scottish Intercollegiate Guidelines Network (SIGN) 2011. British Guideline on the Management of Asthma. A national clinical guideline
• (3) Fishwick D et al. Standards of care for occupational asthma. Thorax. 2008;63(3):240-50.
• (4) British Thoracic Society (BTS)/Scottish Intercollegiate Guidelines Network (SIGN) 2019.. British Guideline on the Management of Asthma. A national clinical guideline