Some 10%-20% of patients with asthma will have features of chronic obstructive pulmonary disease (COPD). The asthma/COPD overlap syndrome (ACOS) is increasingly recognized as an important airway disease phenotype and may be associated with worse outcomes than asthma or COPD alone.[2-7]
Many factors predispose a patient with asthma to developing the clinical features of COPD. According to a new study published in the American Journal of Respiratory and Critical Care Medicine (AJRCCM), exposure to air pollution is one of these factors.
The study was a retrospective, observational look at patients with asthma in the Canadian province of Ontario. The investigators started with the Ontario Asthma Surveillance Information System, a database of information on every person in the province with asthma (a total of 2.1 million people). The investigators were able to identify patients with asthma who were later listed as having a diagnosis of COPD. For the purpose of analysis, these patients were labeled as having ACOS.
After adjustment for multiple covariates, including socioeconomic status, obesity, comorbid disease, and tobacco use, the study found that air pollution increases the chances that a person with asthma will later be diagnosed with COPD. The air pollution monitors used to determine the exposure levels measured fine particulate matter (PM2.5) and ozone (O3). Each of these measurements showed significance when entered into the regression models alone, but when entered together, only PM2.5 was significantly associated with the development of COPD. For every 10-µg/m3 increase in PM2.5, the risk for COPD was increased by a factor of 2.78. The authors concluded that air pollution, PM2.5 in particular, increases the risk for ACOS.
Clinical Implications for Patients With Asthma
This study is important for several reasons. It shows the power that comes from analyzing data at the population level. The authors were able to link information from several large, provincial-level databases. They started with more than 2 million patients with asthma, and adjusted for such variables as socioeconomic status and tobacco use.
As the accompanying editorial points out, the data aren't perfect. That said, the information within is incredibly valuable, and as a researcher and academic, I envy the Canadian system that was able to produce it.
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This report is also important to clinicians. Very few studies have examined ACOS by incident development of COPD among patients with asthma. Most are cross-sectional and estimate the prevalence of ACOS among patients with COPD or asthma at a given point in time.
Assuming that the AJRCCM study is generalizable to your population (and thanks to the demographic details provided, this can be checked), approximately 10% of your patients with asthma will develop COPD after about 5 years. This number will vary by such known risk factors as tobacco use and environmental PM2.5 levels. A recent review in the Annals of the American Thoracic Society estimated PM2.5 levels across the United States. Clinicians can use these data to predict the development of ACOS in their patients with asthma, and then adjust treatment and follow-up accordingly.
Finally, there are issues for policy-makers to consider. The authors of the AJRCCM study recommend a personalized application that allows patients to monitor air pollution levels in their area. They state that outdoor activity can be curtailed at times when PM2.5 is expected to be at its highest—during rush hour, when many automobiles are on the road. They cite data from a study that used mobile applications to provide this information.
The editorial takes issue with the personalized approach and points out that mitigating the effects of air pollution will require legislation to curb emissions. Until such legislation is proposed and passed, avoiding being outdoors on days and during times when PM2.5 is high is wise for patients with asthma. The AJRCCM study can be used as justification to push mandates that limit emissions to improve the health of the population.
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