Is the increased incidence of allergic diseases a by-product of technologic development? Can this hypothesis be supported by measurable alterations in immunoreactivity? Continuous sero-epidemiologic monitoring since 1983 in children from Grabs prove a statistically significant increase in allergic sensitization, in particular against grass pollen. An etiologic link between the increased prevalence of hay fever and growing volume of motor vehicle traffic is suggested by: 1. the temporal correlation: Prevalence of hay fever has increased simultaneously with the raise of emission of photo-oxidants. High ozone and pollen immissions do also coincide over the course of the day. 2. pathophysiology: Ozone prolongs and intensifies exposure to allergens by reducing bronchociliary clearance and increasing permeability for inhaled antigens. Children from farmers exhibit a lower morbidity for hay fever in spite of a more intense exposure to both photo-oxidants and grass pollen. They show that not only exposure but also adaptive mechanisms may be important in assessing correlations with aerial pollutants.