Air pollution is becoming an important environmental threat to human health. Owing to many regulatory efforts to control air pollution, ambient air quality has been markedly improved during the last few decades, especially in in high-income countries. However, about 90% of the world’s population are still living in areas with substantial levels of air pollutants. There is mounting evidence that chronic exposure to air pollutants can lead to increased morbidity and mortality, mainly from cardiovascular and respiratory diseases.

COPD is currently the most common cause of chronic respiratory disease-attributable deaths worldwide. Considering that airway and lung parenchyma are directly exposed to air pollution, it is easy to assume that air pollution can be a cause of COPD. However, the importance of air pollution in the development of COPD remains unclear because of inconsistent results of previous epidemiologic studies. Furthermore, there is a lack of studies addressing the effect of air pollution on the incidence of COPD, making it more difficult to make a conclusion in this regard. This may be because cigarette smoking plays an incomparable role in the pathogenesis of COPD. Also, it is exceedingly difficult to perform a long-term longitudinal study to investigate the chronic health effects of air pollution.

With this in mind, we and our colleagues performed a systematic review and meta-analysis to pool the currently available evidence on the effect of air pollution on the incidence of COPD and published our results in Environmental Research. PubMed, Embase, and Cochrane Library searches identified seven eligible studies including 2.6 million participants. Because each study focused on different kinds of air pollutants, there were six, three, and five studies with data on particulate matter with an aerodynamic diameter of 2.5 μm or less (PM2.5), PM with an aerodynamic diameter of 10 μm or les (PM10), and nitrogen dioxide (NO2), respectively.

A Significant Association

In our meta-analysis, a 10 μg/m3 increase in PM2.5 was shown to be significantly associated with increased incidence of COPD (pooled hazard ratio [HR], 1.18; 95% CI, 1.13-1.23; Figure). We also noted that a 10 μg/m3 increase in NO2 was marginally associated with increased incidence of COPD (pooled HR, 1.07; 95% CI, 1.00-1.16). However, PM10 did not show a significant association with incidence of COPD (pooled HR, 0.95; 95% CI, 0.83–1.08).

Although the hazard ratio of 1.18 for COPD incidence with a 10 μg/m3 increase in PM2.5 may not seem significant, some areas in the world experience levels of PM2.5 that reach approximately 100 ug/m3. This is especially the case in countries that are rapidly industrializing in recent years; in such areas, the effect of PM2.5 on COPD may be substantial. In contrast, we could not find significant impact of PM10, Although only there three studies with data on PM10 met our inclusion criteria, our finding of no significant impact of PM10 exposure on COPD incidence is in line with previous studies consistently reporting that PM2.5 has more detrimental health effects than PM10, as smaller particles can penetrate deeper into the lungs.

Air Pollution Control Required

Following the submission of our paper for publication, results of another important study on this topic were published in the American Journal of Respiratory and Critical Care Medicine that evaluated the relationship between long-term exposure to air pollution and new-onset COPD with 15-year follow-up. The results are similar to those of our meta-analysis. Therefore, we concluded that long-term exposure to air pollutants, especially PM2.5, can cause the development of COPD. Given the healthcare burden of COPD, both medically and financially, not only are efforts to lower smoking rates required to reduce the incidence of COPD, so too is more thorough control of air pollution.