Environment and development of respiratory allergy: I. Outdoors.

Respiratory allergic diseases appear to be increasing in most countries. In particular, asthma morbidity and mortality have been reported to be increasing despite the availability of effective asthma medications. It has been also observed that subjects living in urban and industrialized areas are more likely to have respiratory allergic symptoms than those living in rural areas. This increase has been linked to air pollution, rather than to a modification of the genome of the patient. The interplay of genetic and environmental factors in the development of allergic disorders remains a subject of investigation. In the outdoor environment, the most important air pollutants are sulphur dioxide, ozone and particulate matter, in particular diesel exhaust emissions. These pollutants, besides acting as irritants, increasing airways hyper-reactivity, are thought to be causal factors which act to modulate the immune response, with an adjuvant activity on immunoglobulin E (IgE) synthesis. In other words, atopic state can be upregulated by environmental influences, and some subjects develop atopic diseases in response to these environmental factors. Since airborne allergens and air pollutants are often both increased in the same areas, potentiation, either in the degree of acquired sensitization or in the degree of the response to allergens, should be considered as an important factor which might help to explain the increasing frequency of allergic respiratory disease. In the light of our present knowledge, it is evident that further investigation in human subjects, aimed at evaluating specific agents, and their concentration in the atmosphere, which can influence pulmonary function, is needed. These studies may help to explain the increasing problem of asthma morbidity and mortality.