Comorbidity, Air Pollution, and Risk of Pneumonia

Air pollution is a risk factor for pneumonia; however, evidence on individual susceptibility is limited. To examine whether pre-existing comorbidities modify the association between long-term air pollution exposure and pneumonia incidence. We followed 3,083,227 Danish residents aged ≥ 30 years from 2000 to 2018 for first-ever hospital contact for pneumonia. Annual concentrations of fine particulate matter (PM_2.5) and nitrogen dioxide (NO₂) were estimated via hybrid land-use regression models and assigned to residential addresses. Comorbidities [myocardial infarction, heart failure, atrial fibrillation, stroke, asthma, chronic obstructive pulmonary disease (COPD), lung cancer, diabetes, dementia] were identified from prior hospital contacts. Stratified Cox models were used to assess heterogeneity in air pollution-pneumonia associations across comorbidities. Aalen additive models were used to quantify absolute risks attributable to air pollution. Results were reported as per 2 µg/m³ for PM_2.5 and 10 µg/m³ for NO₂. During a mean follow-up of 16 years, 309,092 pneumonia cases were identified. Individuals with comorbidities exhibited higher (from 70% to 221%) pneumonia incidence than the general cohort. We found associations of air pollution and pneumonia among most comorbidity groups, with hazard ratios ranging from 1.03 (diabetes) to 1.07 (heart failure) for PM_2.5 and from 1.04 (diabetes) to 1.12 (heart failure) for NO₂, except for lung cancer and dementia patients. Individuals with COPD showed the highest susceptibility, with risk differences of 426 and 578 cases per 100,000 person-years for PM_2.5 and NO₂. The association between air pollution and pneumonia risk varies by comorbidity status, with COPD patients being the most vulnerable.