Danish researchers have found that long term exposure to low-level air pollution may increase the risk of severe chronic obstructive pulmonary disease (COPD). While acute exposure of several days to high level air pollution was known to be a risk factor for exacerbation of preexisting COPD, until now there had been no studies linking long-term air pollution exposure to the development or progression of the disease. The study was published in the American Thoracic Society’s American Journal of Respiratory and Critical Care Medicine.
The researchers studied data from the Danish Diet, Cancer and Health Study, which consisted of more than 57,000 individuals between ages 50 and 64 who lived in Copenhagen or Aarhus, the first and second largest cities in Denmark, between 1993 and 1997. A self-administered questionnaire provided data on smoking, dietary habits, education, occupational history, and lifestyle. They then used the unique personal identifiers to link the cohort to the Danish Hospital Discharge Register to identify hospital admissions and discharges due to COPD, and estimated pollution exposure by linking residential addresses to outdoor levels of NO2 and NOx levels, which were used to approximate the overall level of traffic-related pollutants since 1971. The researchers looking at exposures over 15-, 25-, and 35-year periods to assess the effect of different exposure lengths on COPD incidence.
“We found significant positive associations between levels of all air pollution proxies and COPD incidence,” said Zorana Andersen, PhD, post doctoral fellow at the Institute of Cancer Epidemiology of the Danish Cancer Society in Copenhagen. “When we adjusted for smoking status and other confounding factors, the association remained significant, indicating that long-term pollution exposure likely is a true risk factor for developing COPD.”
These associations were slightly stronger in men, obese patients, and those eating less than 240 grams of fruit each day. But notably, the effect of air pollution on COPD was strongest in people with preexisting diabetes and asthma.
The study used hospital admissions for COPD to assess the incidence of the disease. The cases, however, represented severe COPD, as mild and moderate COPD do not often require hospitalization. Thus, the reported increase in risk of COPD associated with air pollution is probably an underestimate of the true increase in risk for COPD in general, according to the researchers.
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 nonsmokers.
“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 Andersen.
The enhanced association between increased risk of COPD and air pollution in asthmatics and diabetics suggests the possibility of an underlying link, according to Andersen.
“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 Andersen.
Source: American Thoracic Society