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Understanding occupational asthma

Lorraine Smith
HND MPPM
Senior Respiratory Physiologist

Vicky Moore
BSc(Hons) MSc
Clinical Scientist

Rahul Mukherjee
MBBS DTM&H FCCP FRCP
Consultant Respiratory Physician
Department of Respiratory Medicine and Physiology
Birmingham Heartlands Hospital

Occupational asthma is caused by exposure to specific agents in the workplace. This involves exposure over a period of time; the condition may appear in weeks, months or could take years to develop, as this article explains

Asthma can occur at any age, most frequently in childhood or young adulthood. Adult asthma should always be followed up by investigations about workplace exposure, in case this is the cause. Atopic asthma (extrinsic asthma) is when you have a genetic disposition to develop an allergic disease. If both parents are atopic their children will have a 70% chance of being atopic. Late onset asthma (intrinsic asthma) can occur in middle age; the cause can be often unknown or may not be linked to allergens at all. Adult onset asthma may also be due to changes in the workplace. Figure 1 shows the various types of asthma.

[[Occ asthma Figure 1]]

The prevalence of asthma in the general population is approximately one in 12 - around 5.2 million people in the UK. Occupational asthma (asthma caused in the workplace) occurs in 9-20% of all adult asthmatics and is the most common occupational lung disease in the Western world.1-3 In the UK up to 3,000 new cases of occupational asthma are diagnosed each year. For each 10-year period the Health and Safety Executive (HSE) estimates the cost to our society to be over £1.1bn.4 In 2000, the HSE agreed to a package of measures to reduce the incidence of occupational asthma caused by exposures to substances in the workplace by 30% by 2010.

Occupational asthma is caused by exposure to specific agents in the workplace over a period of time; the condition may appear in weeks, months or could take years to develop.
Early diagnosis of occupational asthma would be of great benefit to the patient's long-term health in terms of control and therapy management. Delays in diagnosis and prolonged exposure may lead to persistent asthma and long-term disability. Patients may have to take early retirement, be redeployed, or change their job to avoid further exposure.

The best outcome for the patient is to be diagnosed within a year of the first work-related symptom and remain employed without further exposure to the agent, which leads to a better prognosis.5-7 Unfortunately, removal from exposure may lead to unemployment. It is, therefore, very important to have the correct diagnosis of occupational asthma, as if incorrect, advising patients to be removed from exposure may lead to financial loss and social consequences.8,9

Primary care
The first point of contact for the patient is their GP and practice nurse in a primary care setting. Ideally, all practices should be screening patients for occupational asthma if they present with new, recurrent or deteriorating symptoms of asthma, rhinitis or conjunctivitis. Practitioners should enquire about the patient's place of work, the materials they use and whether their symptoms improve on days away from work or on holiday.

Management plans should be in place, and guidelines for this are available from a variety of sources, including the British Thoracic Society (BTS) and the British Occupational Health Research Foundation (BOHRF).10-12

Baseline tests of spirometry and bronchodilator response, plus serial measurements of peak expiratory flow (PEF) should be performed. Once performing spirometry, care should be taken in interpreting the spirometry results because normal spirometry and a non-bronchodilator response does not exclude occupational asthma. Significant inhaler response is >15% or >200 ml increase in FEV1 according to the BTS asthma guidelines.10-13 PEF is defined as the maximum flow achieved during expiration delivered with maximum force, starting from the level of maximum inflation (total lung capacity (TLC)).14 The minimum data required for serial measurements of PEF is four readings a day for a three-week period when using a computer programme such as Oasys.15,16 PEFs are an essential tool in assisting the diagnosing occupational asthma, to see whether PEF varies between periods at work and away from work.

If the patient is suspected to have occupational asthma, the BOHRF guidelines recommend they should then be referred to a specialist in this field. Currently, there are five main specialist centres for occupational lung disease in the UK: London, Birmingham, Sheffield, Manchester and Liverpool. Figure 2 shows the journey the patient can take to referral and diagnosis.

[[Occ asthma Figure 2]]

Health surveillance
Health surveillance within the workforce plays an important role in identifying occupational asthma. It can detect the disease from early onset, thus improving the worker's outcome. Health surveillance is usually performed by an occupational health department within a workplace, but many smaller workplaces will not have access to this. For those with access, the occupational health physician may be the one referring on to the specialist centre.

Specialist centres               
Once referred to a specialist clinic a detailed medical history is taken, asking the patient their current symptoms and when they started, including any improvement in symptoms on days away from work or on holiday, plus any family history of asthma or atopy and their smoking history.

Their occupational history must include specific details of present employment and all previous jobs, their tasks and all potential and known exposures. The diagnostic pathway for the worker is shown in Figure 3.

[[Occ asthma Figure 3]]

Assessment of patients with possible occupational asthma
The specialist consultant will ask the patient to perform serial measurements of PEF, taking readings every two hours from waking up in the morning until going to bed at night, for four weeks. It is recommended to take at least three readings at each two-hourly interval and the best two should be within 20 l/min of each other. The highest reading from these three is then recorded on a specialist card.

The record card must also include time starting work, time stopping work, time waking up and going to bed, jobs done throughout the day, treatment (if any) and any colds or chest problems. If two-hourly measurements are difficult to achieve, they can be fitted in with the worker's day, taking reading on waking, before the start of work, during each break at work, upon leaving work, before their evening meal and before bed. Similar measurement times should be performed on days off work.

On completion, the PEF measurements can be analysed visually by an expert, or by a validated method such as Oasys (Occupational Asthma SYStem). Oasys is a computerised programme that can plot and interpret serial PEF readings.
Although PEF measurements are generally used as firstline investigation for occupational asthma, it does not find the specific cause. Other tests can also be used for confirmation of the diagnosis or to find the specific agent responsible. These tests are:

  • Immunology blood test - the measurement of sensitisation to specific immunoglobulin E or (IgE) antibodies.
  • Skin prick tests to specific allergens such as latex and special metals (eg, cobalt). Skin tests for common environmental allergens, eg, house dust mite, moulds, cat, dog and grass pollen are also usually performed to assess general allergy.
  • Non-specific reactivity measurements to agents such as methacholine, histamine and mannitol. This measures airway hyper-responsiveness and is useful when for occupational asthma when performed during exposure and repeated again after at least one week away from exposure.
  • Specific inhalation challenge testing, which is considered as the gold standard for diagnosing occupational asthma.17-19 In this test, the worker is challenged to the causative agent they are exposed to in the workplace. The process is time-consuming as workers can only be exposed to one substance each day, so are admitted for at least a week. The tests are performed in specialist centres, due to the risk of severe asthmatic reactions.
  • Exhaled nitric oxide (FENO) measurement is a marker of asthma that monitors airway inflammation. It can be used to assess the response to anti-inflammatory medications. This is not currently used for the diagnosis of occupational asthma but may vary between periods of exposure and non-exposure.

Once a worker has been diagnosed with occupational asthma and lets their workplace know, the workplace should report the case to the HSE under the RIDDOR scheme.

Surveillance schemes
Workers thought to have occupational asthma are reported to various schemes within the UK. The SWORD (surveillance of work-related occupational respiratory diseases) scheme is a group of respiratory physicians who report monthly on cases and suspected outbreaks within a workforce, as occupational asthma can tend to occur in clusters.

In the UK, the most frequently reported agents causing occupational asthma include: isocyanates, flour and grain dust, colophony and fluxes, latex, animals, aldehydes and wood dust. The workers most commonly reported to surveillance schemes are paint sprayers, bakers and pastry makers, nurses, chemical workers, animal handlers, welders, food process workers and timber workers.20

Conclusion
Occupational asthma is complex, usually requiring investigation by a specialist centre if the specific cause is to be found. Serial PEF measurements do not require a specialist as any healthcare team can carry them out. It is a non-invasive test and is a cheap firstline investigation.

If the diagnosis of occupational asthma is delayed and the worker remains in the workplace, their symptoms and lung function may worsen. Relocation away from the agent/removal of the agent within a year of their first symptom is the best scenario, as this will allow them to have the best prognosis and hopefully prevent them from losing out financially. New causative agents are constantly being discovered and old ones often turn up in new locations.

The main message is to always consider the possibility of occupational asthma if an adult presents with symptoms of asthma and/or rhinitis.

References
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