Air Quality in Primary School Classrooms: Associations of PM₁₀ and PM_2.5 Levels With Outdoor and Built Environment Characteristics

Background: Particulate matter (PM) levels in school classrooms represent a significant fraction of the daily exposure to air pollution among schoolchildren. PM levels may vary over time and may be influenced by a variety of environmental factors. However, previous studies on classrooms’ PM levels relied on short-term measurements, while relevant data from Europe remain scarce. We aimed to prospectively monitor PM_2.5 (PM with aerodynamic diameter ≤ 2.5 μm) and PM₁₀ (PM with aerodynamic diameter ≤ 10 μm) levels in school classrooms and explore their relationships with outdoor and built environment characteristics, especially during teaching periods. By addressing PM exposure during teaching periods and evaluating the effects of mitigation measures, built characteristics, and environmental factors within primary school settings in Cyprus, this study fills critical gaps in understanding particulate exposure in educational environments. Methods: From March to June 2021, indoor PM_2.5 and PM₁₀ concentrations were assessed prospectively using optical particle sensors, in classrooms across nine primary schools in Nicosia, Cyprus. School hours were divided into distinct teaching and recess periods to study the variations in PM levels throughout the day, as well as the effect of outdoor sources. Indoor measurements were analyzed using 2-min intervals. Corresponding time-stamped data on outdoor air pollution were obtained from the nearest air quality station. Additionally, direct, on-site comparison of indoor versus outdoor PM concentrations was possible using weekly gravimetric measurements by low-volume Harvard samplers. Data on the built environment were collected from on-site assessments and questionnaires. A linear mixed effects model was defined by fixed effects for the exposure variables and random intercepts to account for daily variations in PM levels, with additional variability between classrooms nested within each day. Results: Teaching periods, particularly after recess breaks, showed higher PM₁₀ (+28.2%) and PM_2.5 (+13.4%) levels than teaching periods before recess (p < 0.001). Outdoor pollution had the most significant impact on indoor classroom PM₁₀ (+227.9%) and PM_2.5 (+84.3%) levels (p < 0.001). Accessibility to soil yards led to substantially higher PM₁₀ (86.1%) and PM_2.5 (89.9%) (p < 0.001). Classroom windows per square meter resulted in a 10.4% lower level of PM₁₀, but a 5.5% higher level in PM_2.5 (p < 0.05). Classrooms on upper floors were associated with 17.3% lower PM₁₀ levels (p < 0.001). Both temperature and relative humidity were positively associated with PM levels (p < 0.001). The use of air purifiers significantly reduced PM₁₀ and PM_2.5 levels by 43.5% and 30.4%, respectively (p < 0.001). Conclusion: We demonstrated the significant impact of various factors on indoor PM levels in classrooms, including outdoor pollution, soil yard accessibility, and specific classroom characteristics. While air purifiers effectively reduce PM₁₀ and PM_2.5 levels, other factors such as teaching periods, temperature, and humidity also play critical roles, emphasizing the need for comprehensive air quality management strategies in schools.