Conclusion
With > 23,000 adults across Europe followed for 10 years, including 1,257 incident cases of asthma, this is the largest study to estimate the association between traffic-related air pollution, assessed using a standardized and validated method at the individual level, and asthma incidence in adults. Our findings provide suggestive but no firm evidence for a role of air pollution exposure on asthma incidence in adults. Further research with improved individual-level exposure assessment (taking into account, for example, time–activity patterns) and phenotypic characterization in a life-course perspective is needed to better understand the effect of air pollutants on asthma.