Abstract

Epidemic versus occupational asthma: similarities and differences
We here consider the similarities and differences between epidemic and occupational
asthma and the way in which these could affect previous comparisons. Among the similarities the most important relates to the novelty of exposure. In both epidemic and occupational asthma a given population is exposed to a novel allergen from a particular point of time. This important time sequence is usually well established in studies of epidemic and occupational asthma and allows identification of a causal relationship. Studies of occupational and soybean epidemic asthma have been able to assess aetiological models which have included risk factors such as atopy and smoking, providing a more complete knowledge of the aetiology. In addition, other important features such as the latency interval and the intensity of exposure are amenable to observation in both occupational and soybean epidemic asthma.
A proportion of patients with epidemic asthma experienced asthma attacks during outbreaks in areas far from the soybean point source, similar to occupational asthma
where a worker may develop asthma without being directly exposed. Clearcut avoidance of exposure has allowed investigation of the subsequent evolution of the disease in occupational and soybean epidemic asthma; sensitisation, airways hyperresponsiveness, and symptoms may persist for long periods of time, if not
indefinitely, after avoidance of exposure. The fact that soybean epidemic asthma and most cases of occupational asthma are associated with specific IgE adds biological coherence to the comparative review presented here.
In contrast, some relevant differences should be highlighted. Soybean epidemic asthma involved the exposure of a total community, whereas occupational asthma reflects the exposure of a selected working population. At least two consequences arise from this difference. Firstly, the important finding that children were only rarely affected by soybean epidemic asthma cannot be tested for occupational asthma and, similarly, it is difficult to test whether older workers are at a higher risk of occupational asthma as was the case in the soybean epidemic asthma because of the
limited age range of working populations and their possible assignation to lower exposures.
Secondly, the healthy worker effect may reduce the risk of developing occupational asthma, whereas in epidemic asthma those with previous asthma may well have been more susceptible to soybean allergy. Finally, allergens are not inhaled as isolated particles but as complex aerosols which may differ both between occupational and epidemic asthma and between different occupational exposures.

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