“…Thus, outdoor particle number concentrations may not adequately reflect indoor concentrations, particularly in winter, when the ventilation rates are diminished (Weichenthal et al 2007). Nevertheless, previous studies also showed that the majority of indoor UFPs are originated from indoor sources and that higher particle number concentrations may be associated with longer occupation times and, therefore, it is important to assess UFP number concentrations both from indoor and outdoor sources (Beko et al 2013, Cavaleiro Rufo et al 2015, Isaxon et al 2015, Vinzents et al 2005, Wallace & Howard-Reed 2002, Weichenthal et al 2007). Certain features that may influence outdoor UFP number concentrations such as the different characteristics of urbanization and traffic intensity, industrial activity and densely packed housing are characteristically present in urban environments and rarely seen in rural environments (Kumar et al 2010, Matson 2005, Yoon et al 2011.…”