The mediating role of lung function on air pollution-induced cardiopulmonary mortality in elderly women: The SALIA cohort study with 22-year mortality follow-up.

BACKGROUND

Air pollution exposure is associated with reduced lung function and increased cardio-pulmonary mortality (CPM).

OBJECTIVES

We analyzed the potential mediating effect of reduced lung function on the association between air pollution exposure and CPM.

METHODS

We used data from the German SALIA cohort including 2527 elderly women (aged 51-56 years at baseline 1985-1994) with 22-year follow-up to CPM. Exposures to PM, PM, PM absorbance, NO and NO were assessed by land-use regression modelling and back-extrapolated to estimate exposures at baseline. Lung function (FVC, FEV) was measured by spirometry and transformed to GLI z-scores. Adjusted Cox proportional hazards and causal proportional hazards mediation analysis models were fitted.

RESULTS

The survival analysis showed that reduced lung function (z-scores of FVC or FEV below 5% predicted) reflected significantly lower survival probability from CPM (p < 0.0001). Longterm exposures to NO and NO were associated with increased risks of CPM (eg. HR = 1.215; 95%CI: 1.017-1.452 for IQR increase in NO and HR = 1.209; 95%CI: 1.011-1.445 for IQR increase in NO) after adjusting for reduced lung function and additional covariates. The associations of PM absorbance and CPM remained significant in models adjusted for FEV/FVC, but the associations with PM and PM were not significant. The mediation analysis showed significant indirect effects of NO and NO on CPM mediated through reduced FEV and FVC. The largest indirect effects were found for exposures to NO (HR = 1.037; 95%CI: 1.005-1.070) and NO (HR = 1.028; 95%CI: 1.004-1.052) mediated through reduced FVC. The mediated proportion effect ranged from 13.9% to 19.6% in fully adjusted models.

DISCUSSION

This study provides insights into the mechanism of reduced lung function in association between long-term air pollution exposure and CPM. The mediated effect was substantial for exposure to nitrogen oxides (NO and NO), but less pronounced for PM and PM.