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Although on the increase overall, the actual prevalence of occupational asthma is difficult to assess because of variations seen according to periods, countries, jobs, and the agents responsible. In the authors' experience, this prevalence is slightly less than 10%. Three principal mechanisms, sometimes present together, not always elucidated: immunological, reflex and irritative, are responsible for such asthma. Whilst a positive diagnosis raises few problems, the aetiological diagnosis is difficult and involves two stages: recognition of the occupational characteristic of the asthma and attribution of this asthma to an allergen or substance in the occupational environment. Skin tests and in vitro tests, but above all exposure tests, make an important diagnostic contribution. Personal examples are used to illustrate the principal aetiological factors in occupational asthma of animal, vegetable or chemical origin.

A prospective study of clinical, cutaneous and biological sensitivity to the common air-borne allergens was conducted over a 16-year period in a total of 3,962 patients who reported to a hospital consultation for respiratory allergy. The study confirmed certain widely recognized semiological features, but it did it on the basis of a real population with a large number of subjects and therefore was a true textbook of allergology with a live material. In addition, the study benefited from the major advantage of computer data processing: an accurate and rapid analysis. Yet some uncertainties and omissions remained concerning answers to certain questions, but such gaps are inherent in the natural subjectivity of allergic patients and their doctors.