BACKGROUND: Short-term exposure to air pollution has adverse effects among patients with asthma, whether long-term exposure to air pollution is a cause of adult-onset asthma is unclear. OBJECTIVE: To investigate the association between air pollution and adult onset asthma. METHODS: Asthma incidence was prospectively assessed in six European cohorts. Exposures studied were annual average concentrations at home addresses for nitrogen oxides assessed for 23,704 participants (including 1,257 incident cases) and particulate matter assessed for 17,909 participants through ESCAPE land-use regression models, and traffic exposure indicators. Meta-analyses of cohort-specific logistic regression on asthma incidence were performed. Models were adjusted on age, sex, overweight, education and smoking and included city/area within each cohort as a random effect. RESULTS: In this longitudinal analysis, asthma incidence was positively, but not significantly, associated with all exposure metrics, except for PMcoarse. Positive associations of borderline significance were observed for NO2, (adjusted OR = 1.10; 95% CI: 0.99, 1.21 per 10 mug/m3; p=0.10) and NOx (1.04; 95% CI: 0.99, 1.08 per 20 mug/m3; p=0.08). Non-significant positive associations were estimated for PM10 (1.04; 95% CI: 0.88, 1.23 per 10 mug/m3), PM2.5 (1.04; 95% CI: 0.88, 1.23 per 5 mug/m3), PM2.5absorbance (1.06; 95% CI: 0.95, 1.19 per 10-5/m), traffic load (1.10; 95% CI: 0.93, 1.30 per four million vehicles x m/day on major roads in a 100m buffer) and traffic intensity (1.10; 95% CI: 0.93, 1.30 per 5,000 vehicles/day on the nearest road). A non-significant negative association was estimated for PMcoarse (0.98; 95% CI: 0.87, 1.14 per 5 mug/m3). CONCLUSIONS: Results are suggestive of a deleterious effect of ambient air pollution on asthma incidence in adults. Further research with improved personal-level exposure assessment (versus residential exposure assessment only) and phenotypic characterization is needed.