How the filtration of the incoming air decreases the particle concentration within a school equipped with a balanced ventilation system

To study the impact of the filtration efficiency level on the particle concentration in a rural school equipped with a balanced ventilation system with heat recovery, measurements of indoor and outdoor particle concentrations have been carried out by using three different efficiency filters. The tested filters are respectively classed G4, F7 and F9 according to NF EN 779 (2012). Air has been sampled alternatively and continuously inside the fresh air duct at the inlet of the air handling unit, inside the delivered air duct at the outlet of the air handling unit and inside the exhaust air duct. These samples have been analyzed by an optical particle counter for particle concentration measurements in the 0.3 – 10 μm particle size range. Particle concentration values have been used for the filtration efficiency calculation as well as for the calculation of the I/O ratio of the indoors to the outdoors particle concentration. The results show that the efficiency of the G4 filters on the finest particles being in size lower than 1 μm is very low and close to zero. But with the use of the F7 filters and even more with the F9 filters the filtration efficiency is much better, respectively 47 to 55 % with the F7 filters and 82 to 87 % with the F9 filters at 0.4 μm (depending on the day of measurements). When the school is unoccupied, the I/O ratio decreases as the filtration efficiency increases. When the school is occupied, the I/O ratio is the same as when the school is unoccupied considering particles smaller than 0.5 μm. But for larger particles, the I/O ratio is higher and increases when the particle size considered increases and when the filter class decreases. The I/O ratio exceeds 1 depending on the class of the filter and the particle size considered.
In summary, the ventilation system used in the studied school is able to protect people as the I/O ratio is lower than 1 for the most harmful (diameter < 1 μm) and most abundant (99 % in number) particles recorded outdoors when fine filters (F7 or even better F9) are installed within the air handling unit.