Rationale: Few studies have assessed personal exposure to pollutants and lung function among adults with chronic obstructive pulmonary disease (COPD). Blood eosinophil level may be a biomarker of airway inflammation and pollution susceptibility. Objectives: To evaluate if daily pollutant exposures are associated with lung function and if associations are modified by eosinophil level in COPD. Methods: We recruited 30 former smokers with moderate to severe COPD living in the Boston area and followed them for up to 4 nonconsecutive months in different seasons. Participants measured morning lung function and carried a portable air quality monitor daily. Previous-day exposure to pollutants (particulate matter ⩽2.5 μm in aerodynamic diameter, nitrogen dioxide [NO₂], and ozone) were measured by portable and community monitors. We constructed multilevel linear mixed-effects models with random intercepts for person and observation month, adjusted for temperature, humidity, age, sex, race, height, weight, income, and season, to assess associations of previous-day pollutant exposure with lung function and effect modification by eosinophil count (<150/μl vs. ⩾150/μl). Results: A total of 3,314 observations with exposure and lung function data were collected. Each interquartile range (5.1 parts per billion [ppb])-higher previous-day personal exposure to NO₂ was associated with an 11.3 ml (95% confidence interval [CI], -18.7 to -4.0) lower forced expiratory volume in 1 second (FEV₁) and an 18.0 ml (95% CI, -32.0 to -4.2) lower forced vital capacity. Personal and community-level exposure to particulate matter ⩽2.5 μm in aerodynamic diameter and community-level NO₂ were negatively associated with FEV₁ among the 55.2% of participants with the higher eosinophil level (P_interaction < 0.05). Conclusions: Our study highlights the need to address air pollution exposure among patients with COPD. Future research is needed to verify if eosinophil level is a biomarker for susceptibility to air pollution in COPD.

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