Multivariate structure and time-segmented exposure characteristics of indoor air quality in childcare centers

Understanding indoor air quality (IAQ) in childcare environments is critical for safeguarding young children, who are physiologically vulnerable to airborne pollutants due to higher inhalation rates and developing respiratory systems. This study characterized daypart-specific IAQ patterns and their multivariate structures by using high-resolution monitoring at seven childcare centers in Korea. Continuous 1-min measurements of PM₂.₅, PM₁₀, total volatile organic compounds (TVOC), CO₂, radon, temperature, and relative humidity were collected and aggregated into 5-min averages for analysis. PM₂.₅ (8.9-13.9 µg·m⁻³), PM₁₀ (18.4-25.8 µg·m⁻³), CO₂ (445-1,029 ppm), and radon (16-191 Bq·m⁻³) largely complied with domestic and WHO guidelines. In contrast, TVOC concentrations exceeded recommended reference ranges at all facilities and showed substantial inter-facility variability. Pollutant levels varied clearly by time of day: particles increased during active classroom periods and declined afterward; CO₂ temporarily exceeded 1,000 ppm during peak occupancy; TVOC showed activity-related peaks with evening persistence; and radon increased in one facility before morning ventilation. Exploratory principal component analysis (PCA) showed that the three components accounted for 83.5% of the total variance, reflecting increases in activity-related particulates, ventilation-dependent gaseous accumulation, and temperature-related variations in TVOC levels. K-means clustering grouped similar daypart profiles across centers, indicating recurring multivariate IAQ patterns beyond the daily average concentrations. The pollutant peaks did not occur simultaneously. Particulate matter aligned with occupancy, whereas gaseous pollutants accumulated under limited ventilation. These findings suggest that IAQ in childcare settings is better interpreted in relation to operational time segments rather than daily averages alone, and that ventilation strategies should consider both timing and pollutant-specific behavior when managing exposure in sensitive indoor environments.