Exposure to nitrogen dioxide and chronic obstructive pulmonary disease (COPD) in adults: a systematic review and meta-analysis

Exposure to nitrogen dioxide (NO2) has long been linked to elevated mortality and morbidity from epidemiological evidences. However, questions remain unclear whether NO2 acts directly on human health or being an indicator of other ambient pollutants. In this study, random-effect meta-analyses were performed on examining exposure to nitrogen oxide (NOx) and its association with chronic obstructive pulmonary disease (COPD). The overall relative risk (RR) of COPD risk related to a 10 μg/m3 increase in NO2 exposure increased by 2.0%. The pooled effect on prevalence was 17% with an increase of 10 μg/m3 in NO2 concentration, and 1.3% on hospital admissions, and 2.6% on mortality. The RR of COPD cases related to NO2 long-term exposure was 2.5 and 1.4% in short-term exposure. The COPD effect related with a 10 μg/m3 increase in exposure to a general outdoor-sourced NO2 was 1.7 and 17.8% to exposure to an exclusively traffic-sourced NO2; importantly, we did observe the effect of NO2 on COPD mortality with a large majority in lag0. Long-term traffic exerted more severe impairments on COPD prevalence than long-term or short-term outdoor effect; long-term mortality effect on COPD was serious in single model from this meta-analysis. Overall, our study reported consistent evidence of the potential positive association between NO2 and COPD risk.