Mortality associations with long-term exposure to outdoor air pollution in a national English cohort

Abstract

Background: Long-term exposure to ambient levels of fine particulate matter has been associated with increased mortality, particularly from cardiovascular disease. Relatively few studies have investigated these relationships in nationally representative population samples, in European populations or included gaseous pollutants. Aim: To investigate the relationship between long-term exposure to a range of pollutants and mortality in a national English cohort. Methods: 835,607 patients aged 40-89 years, registered with 205 general practices were followed from 2003-7. Annual average concentrations in 2002 for particulate matter with a median aerodynamic diameter <10 and <2.5 (PM2.5) microns, nitrogen dioxide (NO2), ozone and sulphur dioxide (SO2) at 1km2 resolution, estimated from emission based dispersion models, were linked to patient residential postcode. Deaths were ascertained from linkage to death certificates, and hazards ratios (HR) for all- and cause-specific mortality for pollutants were estimated from Cox models adjusting for age, sex, smoking, body mass index and area-level socioeconomic status (SES). Results: All pollutants (except ozone) were positively associated with all-cause mortality (83,103 deaths), between 2% and 6% increase in risk per inter quartile range increase in pollutant concentrations. Associations with PM and NO2 were larger for respiratory deaths (e.g. HR, 1.09; 95% confidence interval, 1.05-1.13, per 1.9 µg/m3 of PM2.5) but nearer to unity for cardiovascular deaths (for PM2.5 HR=1.00; 0.97-1.03). SO2 was positively associated with all causes of mortality. Conclusions: These results strengthen the evidence linking long-term ambient air pollution exposure to increased all-cause mortality. However, the stronger associations with respiratory mortality are not consistent with the majority of US studies in which associations with cardiovascular causes of death tend to predominate.