On the interrelationship between outdoor air pollution and respiratory allergy

Respiratory allergic diseases appear to be increasing in both prevalence and severity in most countries, and 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, among various factors, to air pollution and to the westernized lifestyle. In the outdoor environment, the most important air pollutants are sulphur dioxide, nitrogen dioxide, ozone and particulate matter. Particulate diesel exhaust emissions, besides acting as irritant, are thought to modulate the immune response, with an adjuvant activity on IgE synthesis, thereby facilitating allergic sensitization in predisposed subjects. In other words, atopic state can be upregulated by environmental influences, and some subjects develop atopic disease in response to these environmental factors when they are inhaled in combination with aeroallergens. Moreover, air pollutants produce greater responses in asthmatic subjects. Since airborne pollen allergens and air pollutants are often increased contemporaneously, an enhanced IgE-mediated response to aeroallergens and enhanced airway inflammation could account for the increasing frequency of allergic respiratory diseases, in particular those induced by pollen allergens, in urban areas.