Introduction: Studies have shown effects of ambient particulate matter ≤ 2.5 microns in diameter (PM2.5) on oxidative stress biomarkers; however, less is known of these effects indoors in patients with chronic obstructive pulmonary disease (COPD). In this study, we assessed the association between indoor PM2.5 apportioned into indoor and outdoor origin and biomarkers of oxidative stress.Methods: We recruited 82 men with COPD who ran a micro-environmental sampler in their home for 1 week up to 4 times a year (n=225 samples). Teflon filter samples were analyzed gravimetrically for indoor PM2.5, and outdoor PM2.5 was measured at a central site. Daily indoor PM2.5 was calculated and apportioned into indoor (PMin) and outdoor origin (PMout) using the indoor-outdoor sulfur ratio as a surrogate for infiltration. Urine samples were obtained for analysis of 8-OHdG and malondialdehyde (MDA). We fit mixed effects models with a random intercept for each participant and adjusted for age, race, season, time of urine collection, BMI, creatinine, diabetes, heart disease, indoor nitrogen dioxide, ambient temperature, and relative humidity.Results: We observed suggestive effects of total PM2.5 on 8-OHdG, with the strongest effect on the 1 day lag (3.6% increase per IQR, 95% CI: -0.69-8.2). For MDA, there were statistically significant effects of total PM2.5 with longer moving averages (strongest effect on the 1-7 day moving average: 6.5% per IQR, 95% CI: 0.88-12.4). Effects of PMout across time windows displayed similar yet stronger patterns compared to total PM2.5, with the greatest effects on the 1 day lag (16.7% per IQR, 95% CI: 1.8-33.7) for 8-OHdG and the 1-7 day moving average (19.3% per IQR, 95% CI: -0.9-43.6) for MDA. There were no statistically significant effects of PMin.Conclusions: Our results suggest that indoor infiltration of PM2.5 of outdoor origin is positively associated with increased lipid peroxidation (MDA) and DNA oxidation (8-OHdG) in patients with COPD.