“…The PM concentrations in metro systems whether on platforms or inside trains are generally higher compared to those in outdoor air . Levels of PM 10 , PM 2.5 or particle number (PN) have been measured on platforms or inside trains in many metro systems, including those in London (Pfeifer et al, 1999;Sitzmann et al, 1999;Adams et al, 2001;Seaton et al, 2005), Berlin (Fromme et al, 1998), Tokyo (Furuya et al, 2001), Boston (Levy et al, 2002), Hong Kong (Chan et al, 2002a), Guangzhou (Chan et al, 2002b), Stockholm (Johansson and Johansson, 2003), New York (Chillrud et al, 2004), Helsinki (Aarnio et al, 2005, Asmi et al, 2009, Prague (Braniš, 2006), Rome (Ripanucci et al, 2006), Budapest (Salma et al, 2007), Beijing (Li et al, 2006;Li et al, 2007), Mexico , Taipei (Cheng et al, 2008;Cheng et al, 2009;Cheng and Yan, 2011), Seoul (Kim et al, 2008;Park and Ha, 2008), Paris (Raut et al, 2009), and Los Angeles (Kam et al, 2011). The widely varying PM levels and their sources reported in the different metro systems throughout the world indicates that PM may originate from the outside atmosphere or may be generated internally in the underground portion of the metro system (Pfeifer et al, 1999;Sitzmann et al, 1999;Johansson and Johansson, 2003;Chillrud et al, 2004;Aarnio et al, 2005;Braniš, 2006;Ripanucci et al, 2006;Salma et al, 2007;…”