Impact of Occupant Behavior on Indoor Air Quality in Small Single-Person Households: A Field Study Using Low-Cost Sensors

Single-person households are among the fastest-growing housing types, exceeding 35% of all households in Korea, yet field evidence on their indoor environmental quality (IEQ) remains scarce. This study monitored temperature, relative humidity, PM2.5, PM10, and CO2 at 1-min intervals for 30 days in July 2022 in four small single-person dwellings in Korea, using low-cost sensors, outdoor stations, window sensors, and an occupant survey. Air exchange rate (AER) was estimated from CO2 decay, and indoor/outdoor (I/O) ratios, peak events, and predicted mean vote/Predicted percentage of dissatisfied (PMV/PPD) were analyzed by occupancy state. Mean indoor CO2 was 942–1278 ppm, exceeding 1000 ppm for 37.5–56.2% of the period. AER remained low regardless of window state (0.11 h−1 closed vs. 0.27 h−1 open), below the 0.5 h−1 benchmark for regulated multi-family housing and among the lowest values in recent field studies. I/O ratios were about 72% higher when occupants were awake than when asleep, and indoor peak events from cooking, cleaning, and candle burning produced I/O ratios of 5.2–41.4. Thermal comfort varied by household and occupancy state. For the dwellings investigated, which fall outside Korea’s mandatory ventilation requirement, a steady-state mass balance under the assumptions adopted here yielded an estimated minimum continuous airflow of approximately 30 m3/h per dwelling.