Authors’ reply to Kolstad and colleagues

Kolstad and colleagues suggest that the results of our study on aircraft noise and cardiovascular disease may reflect confounding rather than causal associations,1 2 citing largely negative results of a large Danish study of occupational noise exposure, hypertension (based on registry data),3 and stroke.4 Occupational studies are subject to the “healthy worker” bias. For example, people who are prone to ill health related to noise exposure are more likely to leave or never start working in jobs with high exposure to noise, and although the study adjusted for early leavers some residual …