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Objectives: To estimate concentration-response relationships for particulate matter (PM) and black carbon (BC) in relation to mortality in cohorts from three Swedish cities with comparatively low pollutant levels.

Setting: Cohorts from Gothenburg, Stockholm and Umeå, Sweden.

Design: High-resolution dispersion models were used to estimate annual mean concentrations of PM with aerodynamic diameter ≤10 μm (PM10) and ≤2.5 μm (PM2.5), and BC, at individual addresses during each year of follow-up, 1990-2011. Moving averages were calculated for the time windows 1-5 years (lag1-5) and 6-10 years (lag6-10) preceding the outcome. Cause-specific mortality data were obtained from the national cause of death registry. Cohort-specific HRs were estimated using Cox regression models and then meta-analysed including a random effect of cohort.

Participants: During the study period, 7 340 cases of natural mortality, 2 755 cases of cardiovascular disease (CVD) mortality and 817 cases of respiratory and lung cancer mortality were observed among in total 68 679 individuals and 689 813 person-years of follow-up.

Results: Both PM10 (range: 6.3-41.9 μg/m 3) and BC (range: 0.2-6.8 μg/m 3) were associated with natural mortality showing 17% (95% CI 6% to 31%) and 9% (95% CI 0% to 18%) increased risks per 10 μg/m 3 and 1 μg/m 3 of lag1-5 exposure, respectively. For PM2.5 (range: 4.0-22.4 μg/m 3), the estimated increase was 13% per 5 μg/m 3, but less precise (95% CI -9% to 40%). Estimates for CVD mortality appeared higher for both PM10 and PM2.5. No association was observed with respiratory mortality.

Conclusion: The results support an effect of long-term air pollution on natural mortality and mortality in CVD with high relative risks also at low exposure levels. These findings are relevant for future decisions concerning air quality policies.