Asthma which includes an important bronchial hyper-reactivity to several substances is closely linked to environmental factors. Atmospheric allergens are among the best known of these factors. Some and the most numerous, like pollen, are natural. Others like isocyanates are artificial and mostly the cause of professional accidents. The noxious effect of air pollution on airways has been clearly established by the numerous observations compiled over half a century. As far as asthma is concerned, the bronchoconstricting effect of pollutants inhaled at high doses is well known. Numerous epidemiological studies and tests performed in laboratories have also revealed the action of different pollutants inhaled at "realistic" concentrations. The relationship between asthma and climatic environment appears more difficult to define and the objective facts which could play a part in these relations are few and contradictory.