2020 No https://www.sciencedirect.com/science/article/abs/pii/S0048969719356682?via%3Dihub Pacitto, A Europe Experimental Elementary (K-5) Ventilation rates School gym Air purifier Particle number concentration Black carbon Particulate matter (PM) Indoor air quality The study revealed that the use of air purifiers with windows kept closed (natural ventilation) can lead to a significant reduction in terms of indoor-to-outdoor concentration ratios. In the smaller gym (air changes per hour of the purifiers, ACH, equal to 9.2 h−1) the I/O ratios were reduced by 93% and 95% in terms of particle number and PM1–10, respectively; whereas in the larger school gym (ACH = 1.7 h−1) the corresponding reductions were 70% and 84%. For manual airing scenarios, the effect of the air purifiers on outdoor-generated sub-micron particles is reduced; in particular, for low ACH values (i.e. ACH = 1.7 h−1), the reduction is quite negligible (6%).