The authors' reply: We would like to thank Dr Joseph for his interest and comments on our editorial 1 accompanying the excellent systematic review by Bhaskaran et al.2 As Dr Joseph points out, one of the systematic review's findings was a weak negative association between ozone exposure and the incidence of myocardial infarction.

As Bhaskaran et al indicate, a reduction in ozone concentrations is likely to be a marker of increased combustion-derived fine particulate matter (PM_2.5) from vehicle exhaust. Sarnat similarly found that ambient concentrations of O₃, NO …