Because the study used hospital admissions for COPD to assess incidence, it is likely that the true incidence was underestimated, and that the cases represented severe COPD, as mild and moderate COPD does not often require hospitalization. This means that the reported increase in risk associated with air pollution is probably an underestimate of the true increase in risk for COPD in general. Furthermore, while smoking is known to be the primary cause of COPD in developed countries, and majority of COPD cases were smokers or previous smokers, the effect of pollution exposure was also observed in the group of non-smokers. "This result refutes the possibility that the observed effect of air pollution was due to inadequate adjustment for smoking in our data and supports the idea that air pollution affects COPD risk, irrespective of smoking status," said Dr. Andersen.

The enhanced association between increased risk of COPD and air pollution in asthmatics and diabetics suggests the possibility of an underlying link. "It is plausible that airflow obstruction and hyper-responsiveness in people with asthma, or systemic inflammation in people with diabetes, can lead to increased susceptibility of the lung to air pollution, resulting in airway inflammation and progression of COPD, but more research is needed in this area." said Dr. Andersen.

"In any case, sufficient data, including the results of this study, provide evidence that traffic-related urban air pollution contributes to the burden of COPD and that reductions in traffic emissions would be beneficial to public health."

SOURCE American Journal of Respiratory and Critical Care Medicine