Hourly variation in fine particle exposure is associated with transiently increased risk of ST segment depression

Objectives: To evaluate whether hourly changes in fine particle (PM2.5, diameter, <2.5 mu m) exposure or outdoor particle concentrations are associated with rapid ischaemic responses. Methods: 41 non-smoking elderly people with coronary heart disease were followed up with biweekly clinic visits in Helsinki, Finland. The occurrence of ST segment depressions >0.1 mV was recorded during submaximal exercise tests. Hourly variations in personal PM2.5 exposure and outdoor levels of PM2.5 and ultrafine particles (<0.1 mu m) were recorded for 24 h before a clinic visit. Associations between particulate air pollution and ST segment depressions were evaluated using logistic regression. Results: Both personal and outdoor PM2.5 concentrations, but not outdoor ultrafine particle counts, were associated with ST segment depressions. The odds ratio (per 10 mu g/m(3)) for personal PM2.5 concentration during the hour preceding a clinic visit was 3.26 (95% CI 1.07 to 9.99) and for 4 h average outdoor PM2.5 it was 2.47 (95% CI 1.05 to 5.85). Conclusions: Even very short-term elevations in fine particle exposure might increase the risk of myocardial ischaemia. The precise mechanism is still unknown but could involve changes in autonomic nervous control of the heart.