The possibility that asthma may be caused by a reaction to a substance or substances present at the patient's place of work will only be identified if either the patient, or the patient's doctor, is aware of the possibility and prepared to consider it.  Because this is not always the case a  proportion of occupational asthma cases get missed.  This is unfortunate since it is a potentially preventable condition, although complete cure may only be possible if the responsible occupational agent is recognised and avoided early, before there is irreversible damage to the patient's airways.

How is occupational asthma diagnosed?

The symptoms of occupational asthma may have a clear time relationship either with the time of starting a new job – or with the time of changing exposures within a job.  More importantly, symptoms may tend to improve when the patient is away from work, although the weekend might not be sufficiently long for this to be obvious. The clinical history of the disease is therefore important in establishing the diagnosis, as is a detailed knowledge of occupational processes and exposures.  It can be very helpful if the patients keeps a daily peak flow chart (i.e. takes regular measurements through a peak flow meter), tell-tale improvements either at the weekend or during holiday times can then give the game away.  Otherwise, the necessary investigations are similar to those required for ordinary asthma and for other allergic diseases.  

In the UK and across western Europe, it is believed that about 10% of adult asthma is of the occupational type although most of these cases probably go unrecognised. Although this figure is as precise as there is, it results from an analysis of several separate epidemiological studies, and may represent two rather distinct types of case.

Although the precise numbers are not know, it is likely that true occupational asthma is likely to be less common than work-related asthma.  The distinction is an important one, not least because there is likely to be a greater need for career change and there may well be medico-legal consequences. 

So whilst it is difficult to be sure about the number of cases, a study from the University of Manchester shows that reported cases are declining, the exact reasons for this (it may partly be due to a laxity in reporting) are unclear.

Whatever the true situation, the number of new cases that are reported each year in the UK are likely to represent a considerable underestimation of the true incidence. 

What causes occupational asthma? 

Over 300 different items have been identified as capable of causing asthma, although three-quarters of all the cases identified in the UK are caused by around a dozen compounds.  These are listed below:

Is it under-diagnosed?

Although in many cases the association between the asthma and the occupation is fairly obvious, in many cases the asthma is so indistinguishable from standard asthma that the association can easily be overlooked.  Even more confusingly, patients may not fully improve until all exposure to the causative agent is prevented and in many cases this may require a change of occupation.  Such are the social and economic consequences of occupational and work-related asthma, patients may be reluctant to take this bold step and may therefore be denied the only fully satisfactory way of establishing the completeness of the relationship beyond doubt.  

Around one case in four is diagnosed and managed within the occupational health service.  Not surprisingly these these cases tend to be diagnosed earlier and are less severely affected than those identified by chest physicians.