I/O ratios were always higher than 1, meaning that PM10 indoor
concentrationswere, in average, higher than ambient levels,which is
consistent with the findings from Yoon et al. (2011) in urban preschools
in Korea and from Almeida et al. (2011) in Portuguese primary
schools. On weekdays, indoor concentrations were always at
least 2 times higher than those found outdoors. Even on weekends
indoor concentrationswere found to be until 2.65 times (in average)
higher than those found outdoors. This suggested that outdoor influence
on PM indoor concentrations was not significant when
compared with indoor sources and re-suspension phenomena. In
fact, the highest I/O ratios in N_URB1 and N_URB3 were found in
lunch rooms, which is consistent with indoor sources already stated
(cooking activities and children drives). The higher I/O ratio found in
classroomC in N_URB2, aswell as the high ratio found in classroomA
in the same nursery, were also due to indoor sources and poor
ventilation to outdoors. In fact, poor ventilation to the outdoor
turned indoor sources as the major increasing factor of indoor PM
concentrations, which was also stated by Yang et al. (2009).