Nighttime CO₂ Accumulation and the Limits of Satisfaction-based Screening in Energy-Vulnerable Older-Adult Dwellings During Summer

Abstract Under-ventilation and a mismatch between occupant perception and measured indoor air quality are documented in low-income housing, but evidence for hot-humid, cooling-constrained summers in energy-vulnerable older-adult dwellings is limited, and recent Korean measured air-change data have not addressed this subgroup or occupant perception. In summer 2025 we monitored 109 energy-vulnerable older-adult households in Seo-gu, Daejeon, Republic of Korea, using low-cost multi-parameter sensors (CO₂, PM₂.₅, PM₁₀; 10-minute interval), and estimated per-household air-change rate from CO₂ tracer decay. Median nighttime CO₂ (513 ppm) and PM₂.₅ (3 µg m⁻³) were low, yet nighttime CO₂ exceeded 1000 ppm in 23.2% of readings, and estimated lower-rate air change was low (median 0.37 h⁻¹ with outdoor CO₂ assumed at 420 ppm). Exceedance was concentrated in a minority of dwellings: 20% exceeded 1000 ppm on at least half of nighttime readings. Estimated air change was inversely associated with nighttime CO₂ exceedance, so exceedance may serve as a first-stage indicator of ventilation inadequacy. Treated as a screen, occupant ventilation satisfaction did not identify these high-accumulation dwellings in this cohort: a dissatisfied response missed most of them (false-negative rate 0.80–0.83), and more-satisfied households tended to be the more-exposed. This screening evaluation was post hoc and in-sample, and the satisfaction gradient was observed on the exceedance metric but not on median nighttime CO₂, so it is reported as a trend within the present study population. Low-cost objective monitoring may therefore complement complaint-based screening for prioritising ventilation assessment in vulnerable dwellings.