Rationale: Indoor pollutants have been associated with chronic obstructive pulmonary disease morbidity, but it is unclear whether they contribute to disease progression. Objectives: We aimed to determine whether indoor particulate matter (PM) and nitrogen dioxide (NO₂) are associated with lung function decline among current and former smokers. Methods: Of the 2,382 subjects with a history of smoking in SPIROMICS AIR, 1,208 participants had complete information to estimate indoor PM and NO₂, using individual-based prediction models, in relation to measured spirometry at two or more clinic visits. We used a three-way interaction model between time, pollutant, and smoking status and assessed the indoor pollutant-associated difference in FEV₁ decline separately using a generalized linear mixed model. Measurements and Main Results: Participants had an average rate of FEV₁ decline of 60.3 ml/yr for those currently smoking compared with 35.2 ml/yr for those who quit. The association of indoor PM with FEV₁ decline differed by smoking status. Among former smokers, every 10 μg/m³ increase in estimated indoor PM was associated with an additional 10 ml/yr decline in FEV₁ (P = 0.044). Among current smokers, FEV₁ decline did not differ by indoor PM. The results of indoor NO₂ suggest trends similar to those for PM ⩽2.5 μm in aerodynamic diameter. Conclusions: Former smokers with chronic obstructive pulmonary disease who live in homes with high estimated PM have accelerated lung function loss, and those in homes with low PM have lung function loss similar to normal aging. In-home PM exposure may contribute to variability in lung function decline in people who quit smoking and may be a modifiable exposure.

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