Hooper LG, Young MT, Keller JP, Szpiro AA, O’Brien KM, Sandler DP, Vedal S, Kaufman JD, London SJ. Environ Health Perspect. 2018 Feb 6;126(2):027005. doi: 10.1289/EHP2199
Not many people would guess that air pollution is good for their health, but measuring the exact impact is a challenging feat. Assessing the pollutant exposure levels and health outcomes of a group of people large enough to draw meaningful conclusions is no small task. Now, new research suggests that exposure to air pollution is associated with chronic lung inflammation, or ‘bronchitis.’
What is Chronic Bronchitis?
Chronic bronchitis is defined as long-term cough and production of sputum (mucus coughed up from the lower airways) for at least three months in two or more consecutive years. Smoking is the primary risk factor for chronic bronchitis, but long-term air pollution exposure has long been suspected as a contributor.
Researchers at the University of Washington addressed this knowledge gap by analyzing advanced air pollution exposure assessments and specific outcome definitions. They leveraged data collected from participants in the National Institute of Environmental Health Sciences Sister Study, an initiative which has studied over 50,000 U.S. women with a sister diagnosed with breast cancer, but no personal breast cancer diagnosis at baseline. The comprehensive health data collected on these women includes self-reported data on the presence of cough and phlegm, and the duration of each. Women who met the definition of chronic bronchitis at baseline (between 2003-2009) were classified as having ‘prevalent’ chronic bronchitis, while those who met the definition during follow-up visits (which occurred every 2 to 3 years) were classified as having ‘incident’ chronic bronchitis.
How do we Estimate Air Pollution Exposure?
Air pollution exposure was estimated using a validated national model of year 2000 annual mean concentration levels of pollutants. Using the home addresses of the Sister Study participants and data from the U.S. Environmental Protection Agency (EPA) Air Quality System, researchers predicted participant exposure to pollutants PM2.5, PM10, (particulate matter <2.5 micrometers (μm) and <10 μm in diameter, respectively) and nitrogen dioxide (NO2).
After controlling for factors that might impact chronic bronchitis independently of air pollution (like age, ethnicity, BMI, education, household income, occupational exposure to dust or vapors/fumes, smoking status, and years of secondhand smoke exposure), there were 47,357 participants left with complete data. Analyses showed that exposure to higher concentrations of PM10 (as measured in 2000) was significantly associated with prevalent chronic bronchitis, chronic cough, and chronic phlegm (as reported at baseline surveys from 2003-2009).
How do we Make Sense of the Findings?
The correlation of these variables does not necessarily imply causation; indeed, incident chronic bronchitis (i.e. bronchitis which developed later) should be more informative than prevalent chronic bronchitis for making causal inferences. This study’s power to detect associations between air pollutants and incident chronic bronchitis was somewhat limited by the relatively short follow-up duration (average: 5.7 years). However, the association is bolstered by the fact that it aligns with other findings in the literature, by the strength of the association, and because it is biologically plausible.
The relatively larger size of PM10 particles impacts where they are generally deposited within the lungs. The location of deposition then informs the physiological response to the intrusion of these foreign particles. Particulate matter triggers an inflammatory response in the epithelial cells of the airway. This chronic airway epithelial inflammation is the pathologic foundation of chronic bronchitis.
Like most studies, this one too had limitations. The first was that this research relies on self-reported data of lung symptoms, which can be subject to the inaccuracies of memory. However, the researchers conducted an assessment in a subset of participants reporting an actual diagnosis of chronic bronchitis from a doctor and found that the association was preserved, which helps make the finding more plausible. Another limitation is that the study only looked at women; it’s possible that men may not experience the same physiological response to PM10 as women.
Despite limitations, this research suggests that long-term air pollution exposure, particularly PM10, is a risk factor for chronic bronchitis and the respiratory symptoms (chronic cough and phlegm) that characterize it. These results may have policy implications in the regulation of PM10, if regulators are striving to lower the burden of chronic respiratory conditions on public health.