Low level air pollution and exacerbation of existing copd: a case crossover analysis

Exacerbations of chronic obstructive pulmonary disease (COPD) contribute greatly to increased morbidity, mortality and diminished quality of life. Recent studies report moderately strong positive associations between exposures to several air pollutants and COPD-related emergency department (ED) visits and hospital admissions (HA). Studies that use clinically defined exacerbations rather than counting ED visits and HA may be more sensitive to environmental triggers like air pollution, but very few such studies exist. Participants in a COPD disease management group living in an area of low air pollution and who were followed closely for the earliest signs of an exacerbation provided an opportunity to study associations between air pollution and COPD exacerbation.

Associations between short term exposures to air pollutants, including sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and particulate matter < 2.5 microns (PM_2.5), and COPD exacerbation were assessed among 168 patients residing in central Massachusetts, a region with air pollution levels well below USEPA National Ambient Air Quality Standards (NAAQS). Case-crossover analyses and multivariate conditional logistic regression were used to estimate associations between 7-day average concentrations of each air pollutant, as measured at central site monitors, and COPD exacerbation experienced in the patients' homes during the period 2012-2013, while controlling for temperature and self-reported influenza.

We found that short-term exposures to SO₂ were associated with an increase in COPD exacerbation risk (odds ratio (OR) = 2.45, 95 % CI: 1.75-3.45 per 1 ppb increase) after adjustment for PM_2.5. Short-term exposures to NO₂ concentrations showed a weaker association, (OR = 1.17, 95 % CI: 1.05-1.30 per 1 ppb increase) after adjustment for PM_2.5. An unexpectedly modest negative association was seen for short-term exposures to PM_2.5.

Despite living in an area with air pollution concentrations below current USEPA NAAQS, these COPD patients appeared to suffer increased risk of COPD exacerbation following short-term exposures to increased concentrations of SO₂ and NO₂. An unexpected negative association with PM_2.5 may result from the complex air chemistry of low level PM in this region.