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The Importance of Objective Testing in Occupational Asthma

By Charlene Mhangami, Vitalograph

Published 12 September 2023

What is occupational asthma?

Working is important for an individual’s health and well-being, as well as providing the employee with income, social interaction, purpose, and identity. Workplaces are required to be safe and free from risks to health and there is a legal duty for employers to maintain this. However, there are some instances where workers may develop a disease called occupational asthma (OA) in the workplace where they may become allergic to certain substances and chemicals which cause airflow obstruction, i.e. some healthcare workers may develop a latex allergy by breathing in the powder from the inner lining of the gloves or some workers may be exposed to substances like ammonia and develop asthma as the result of an irritant effect, not an allergic reaction.

Before delving deeper into occupational asthma (OA) it is important to discuss what is asthma, according to GINA (2022), “Asthma is characterized by chronic airway inflammation, there are two key defining features of asthma, which include variable respiratory symptoms such as wheezing, shortness of breath, chest tightness, and cough. Also, variable expiratory flow and airflow limitation”. Approximately 360 million people have asthma around the world, for those workers with asthma, their asthma control may be affected by factors in the workplace, especially if the asthma is related to the workplace. This leads to the development of the term ‘work-related asthma’ (WRA). It is suggested that 25% of adult-onset asthma is WRA.

WRA can be subdivided into phenotypes which include work-aggravated asthma (WAA), which affects people with pre-existing asthma or report symptoms worsened by non-specific factors in the workplace. Causes include workplace temperature or humidity extremes, exertion from manual work tasks, workplace stress, or anxiety. In contrast, occupational asthma (OA) is caused by airborne exposures in the working environment and accounts for 1 in 6 cases of adult asthma. OA can be subdivided into two categories Allergic OA due to sensitization and Irritant induced asthma as shown in Figure 1.

 

Flowchart figure1
Figure 1 BTS Clarification of Occupational Asthma1

There are over 400 known causes of OA, and most cases in the UK are mainly related to exposure to a small number of allergens, commonly flour dust, and isocyanates. Allergic OA sensitization requires a period of repeated allergen exposure and the duration of time between first exposure and symptom onset is referred to as the ‘latent period’. The asthmagens that cause Allergic OA can be divided into two categories: high molecular weight (HMW) agents, and low molecular weight (LMW) asthmagens.

HMW includes animal proteins, cereal grains, and natural rubber latex. Some examples of workers at risk of this are bakers, laboratory workers, and detergent manufacturers, to name a few.

LMW includes di-isocyanates, metals, and acrylic monomers. Workers who may be at risk of this are adhesive workers, welders, spray painters, and many others.

OA caused by exposure to HMW asthmagens is an IgE-associated response involving T-helper cells and this is a common allergic response to asthma. The immune response is also relevant to a small number of LMW, such as anhydrides and platinum salts. It is suggested the highest risk of OA is within the first year of exposure, and reported latency is variable ranging from a week to many years.

A main risk factor for the development of OA is the level of exposure to the cause. Individual susceptibility is also important, as only a proportion of similarly exposed workers become sensitized, and only some of these will ever develop OA.

Investigations in the workplace

The diagnosis of OA involves first confirming the presence of asthma and evidence that asthma is caused by workplace conditions with various investigations involved. The best diagnostic approach for OA is having a good combination of clinical history and objective diagnostic tests. The diagnostic process aims to confirm or exclude OA; no single diagnostic test exists for OA, and the exact approach used will vary based on the individual patient’s circumstances, the level of practice and expertise of healthcare professionals, and the testing facilities available. Clinical and occupational history plays a key role in making diagnosis in OA, this includes an overview of full occupational history, a fully detailed discussion of respiratory symptoms and their relationship to work. NICE recommends checking for possible OA by asking organisations to regularly monitor their employees with new adult-onset symptoms. It is important to note a diagnosis of OA cannot be made based on history alone, as some of the symptoms may improve away from work. Sensitized workers are also at risk of other related forms of occupational allergies such as rhinitis.  Therefore, cautious precautions should be taken in the investigation.

A UK study has shown the duration of symptoms prior to diagnosis is 4 years, and reasons for a late diagnosis varied. These include lack of awareness or patient engagement and in some cases, diagnostic delays resulting from missed opportunities to ask simple screening questions during health surveillance.

Investigations into OA should be carried out as early as possible, prior to the commencement of any medical interventions and while patients are still working in the job considered to be the cause of their OA. Health surveillance programs are a way of preventing occupational health diseases and can help to identify those with OA early on, reducing the severity and impact of the disease. However, few workers are identified via health surveillance programs as only 30% of workers have access to health surveillance.

“According to the BTS, annual health surveillance is generally carried out by an occupational health provider and usually consists of a respiratory symptom questionnaire and spirometry. In some special circumstances, such as the detergent and precious metal refining industries, immunological surveillance is also used”

 

Serial PEF

There are a variety of objective tests available for investigating OA. Serial PEF offers a cheap and simple first-line approach to assessing the physiological responses to inhaled agents in the workplace. However, it is important to note serial PEFs are only helpful if the patient is still exposed to the potential agents. Serial work-related PEF measurements do not usually confirm a specific cause for OA and may be seen in other conditions such as WAA and occupational hypersensitivity pneumonitis. Depending on the local protocol, occupational health professionals can initiate serial PEF recording or upon referral to secondary care. In serial PEF recording, this involves the patient learning how to use the PEF monitor and understanding the charts outputted from the data collected. There is software available for healthcare professionals that analyzes the data such as the Oasys software, which looks for pattern recognition and produces an Oasys score to show positive or negative work effects. The serial PEF data should be collected over a long period of time, and the worker should make recordings before and during work, as well as on periods of annual leave

Occupational Asthma Picture 1

Immunological testing: specific IgE and SPT

A positive SPT or high level of IgE antibodies related to the workplace allergen confirm sensitization but not the presence of OA. The diagnostic usefulness of immunological testing for OA is greater for HMW allergens with a clear immunological mechanism (enzymes, laboratory animals, latex and wheat flour). Specific IgE tests are also available for a limited number of LMW agents and when present, have high specificity for diagnosis.

Spirometry

All workers suspected of having OA should have their PEF, FEV1 and FVC measured and interpreted according to international spirometry guidelines.  Some measured values are commonly within normal limits, but in some cases, a bronchodilator responsiveness test may need to be performed to confirm reversible airflow obstruction. In addition to its diagnostic value, spirometry is important in providing a baseline measurement for monitoring throughout employment.

Monitoring FEV1 has proven beneficial in OA however, if available, the pattern of previous spirometry results (often from health surveillance) can be informative. For example, if a drop in FEV1 is followed by a marked improvement, that can clearly be linked to a reduction in exposure.

Occupational Healt Picture 2

“Workers found to have new symptoms of asthma, airflow obstruction, accelerated forced expiratory volume (FEV1) decline (≥15% from baseline, in addition to expected age-related loss) or sensitization to a workplace allergen, may be referred to an occupational physician, their General Practitioner (GP) or directly to a specialist centre”.

Fractional exhaled nitric oxide with sputum and eosinophils

A common way of measuring airway inflammation is by fractional exhaled nitric oxide (FeNO), or by induced sputum eosinophil counts. This has an established role in the assessment of patients with suspected asthma, but normal levels may be found in a proportion of patients with OA. Although limited data is available in their assessment of inflammatory response, these types of tests are more useful with OA due to HMW chemicals, as LMW chemicals are less likely to cause eosinophilic disease. Elevated FeNO levels were found in OA induced by HMW agents where an IgE-mediated mechanism was involved (e.g. baker’s asthma), as well as in OA induced by some LMW agents such as diisocyanates. It is suggested that FeNO levels were more consistently increased in patients with OA to HMW agents than in those with LMW agents.

Non-specific bronchial hyper-responsiveness testing

An option for measuring airway reactivity is by using mannitol or histamine as a specific test, although this has low sensitivity for OA diagnosis, and in some early cases of the disease the airway reactivity resolves with a few days away from work. The test is very useful in those who are symptomatic with recent exposure, where the absence of non-specific bronchial hyper-responsiveness makes OA unlikely. Nevertheless, this type of testing is performed within a secondary care setting.

Management of occupational asthma

A common consensus across this article is that correct diagnosis is essential for a favorable outcome in the management of occupational asthma. NICE lists occupational exposures as a risk factor for poor asthma outcomes. The diagnostic process should involve a combination of diagnostic tests and questionnaires. Having an objective test involved in the diagnostic process increases the confidence of the healthcare professional in their diagnosis, and the confidence of the worker who may be diagnosed with OA. Management of occupational asthma involves asthma education, control of exposure to environmental triggers, and initiation of appropriate pharmacotherapy. If OA is not diagnosed correctly, and the worker is not protected or able to avoid exposure to the triggers, permanent changes can present themselves in the lung.

References:

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.

Global strategy for asthma management and prevention (2016 update) – gina. Available at: https://ginasthma.org/wp-content/uploads/2016/04/GINA-2016-main-report_tracked.pdf 

Asthma: Diagnosis, monitoring and Chronic Asthma Management. Available at: https://www.nice.org.uk/guidance/ng80/resources/asthma-diagnosis-monitoring-and-chronic-asthma-management-pdf-1837687975621

Tiotiu, A.I. et al. (2020) ‘Progress in occupational asthma’, International Journal of Environmental Research and Public Health, 17(12), p. 4553. doi:10.3390/ijerph17124553.

Occupational asthma (2022) Mayo Clinic. Available at: https://www.mayoclinic.org/diseases-conditions/occupational-asthma/symptoms-causes/syc-20375772

 

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