Long-term exposure to ambient ozone and mortality: a quantitative systematic review and meta-analysis of evidence from cohort studies

Introduction Epidemiological studies have reported associations between daily concentration of ozone and a range of adverse health outcomes. However, the evidence from narrative reviews for long-term exposures is suggestive of associations with respiratory mortality only. We conducted a systematic review and meta-analysis of the evidence from cohort studies reporting associations between long-term exposure to ozone and mortality. Methods Using search terms relating to the study design, pollutant and health outcome we identified cohort studies evaluating associations between long term exposure to ozone and mortality indexed in Embase and Medline to September 2015 and PubMed to October 2015. Articles cited in reviews/key publications were also checked. Study details and estimate information were extracted and used to calculate standardized effect estimates expressed as hazard ratios (HR) per 10 ppb increment in long-term ozone concentrations. Results 8 cohorts (14 publications) presented results for ozone and all-cause and cause specific mortality. We found no evidence of associations between long-term annual O3 concentrations and the risk of death from all-causes, cardiovascular or respiratory diseases or lung cancer. Four cohorts assessed ozone concentrations measured during the warm season. Summary HRs for cardiovascular and respiratory causes of death derived from 3 cohorts were 1.01 (95% CI: 1.00, 1.02) and 1.03 (95% CI: 1.01, 1.05) per 10 ppb respectively. Conclusions Our review found only a limited number of independent studies assessing associations between long-term exposure to ozone and mortality. Climate change and increasing anthropogenic emissions of ozone precursors may impact on future ozone levels. Hence, further studies of the long-term effects of exposure to high ozone levels are required to fully assess the risks to human health.