 not only for particle indoor sources.

The lowest ratio I/O was obtained for the S4 school. This ratio close to 1 is because the location is constantly exchanging air from outside, which causes the concentrations obtained are similar inside and outside. If a window is open the ratio I/O is close to 1 for all particle sizes ([@bib51]). Perhaps, the fact that school facilities are situated on the first floor makes the access to the class is not straight from the street and thus the number of particles introduced by shoes is less. And finally it should be noted that this is a nursery school (not all children walk and do not present the same activity as students of an infant school) and thus the effect of resuspension of particles is minimized at this location.

On the other hand, must be stated that S5 school had the highest rate I/O for PM10 of all locations. These high concentrations were associated with school characteristics, such as the orientation and morphology of the streets surrounding it. This school is located next to an unpaved park that all students of children entering the centre have to cross, which significantly increases the contribution of particles due to transport through the shoes and clothing ([@bib29]). Then these particles are resuspended and an increase of the concentration is produced. [@bib50] in their study in different schools in Korea obtained average ratios I/O of 2.06 for PM~10~ in classrooms and [@bib45], found ratios of up to 8.8 for PM~2.5~. The results are explained in both cases by the student activities that cause resuspension of particles. In S5 school also produces a large accumulation. In the study area there is a large effect of particle accumulation indoor and in ambient air mainly due to climatic characteristics ([@bib18]; [@bib16]). There are not many days during the year in which the wind speed is significant, i.e. days in which there is an effective exchange of indoor air by natural ventilation (type of ventilation used by all schools). The dominant regimes of winds in