Statistical analysis of parameters influencing the relationship between outdoor and indoor air quality in schools

Under the French national research program PRIMEQUAL, measurements of outdoor and indoor pollution have been performed in eight school buildings in La Rochelle (France) and its suburbs. The school buildings were either naturally ventilated by opening the windows or mechanically ventilated with minimum fresh air, and demonstrated various permeabilities. Ozone, nitrogen oxides (NO and NO2), and particulate matter (PM) (15 size intervals ranging from 0.3 to 20 μm) concentrations were monitored continuously indoors and outdoors for two 2-week periods. The indoor relative humidity, temperature, CO2 concentration (room occupancy), window openings and permeability of the building were also measured. Principal component analysis (PCA), a multivariate observation-based statistical method, was used to determine the parameters influencing the relationship between the outdoor and indoor concentration levels. After a brief description of the experimental data and methodology, the paper presents a detailed analysis of the PCA diagrams. This analysis leads to distinguish between positively correlated, negatively correlated and non-correlated variables. The main conclusions arising from the study are: (1) the influence of the room occupancy on the particle concentrations indoors changes with different particle sizes, (2) the building air-tightness and the outdoor concentration level greatly influence the indoor/outdoor (I/O) concentration ratios of ozone, and (3) indoor ozone and particles concentrations are negatively correlated, which may be the result of complex homogeneous and/or heterogeneous processes.