[Allergic diseases as environmental diseases]

Respiratory allergic diseases belong to atopic diseases. Their prevalence has steadily increased over the past decades. Recently, a plateau effect seems to occur. This increased prevalence should be related to environmental changes. However, "classical" aerocontaminants, such as aero-allergens and air pollutants cannot account for such a drastic increase in prevalence. Change in diet can account for biological changes but the relation to allergic diseases seems questionable. Use of contraceptive pills and caesarean section had been put forward as hypotheses but their implication seems unlikely. Among the classical risk factors, 2 only should remain in the primary prevention of atopic diseases, namely, prolonged breast feeding and avoiding passive smoking. The major input of epidemiological research over the past decades lies in the "hygienic hypothesis" and the protective effect of early exposure to farm animals and raw cow milk. Progress to come in this field should lie in a better knowledge of in utero exposure on the foetal immune system.