A previous study in our laboratory measured the ratio of particle concentration outside (Cout)/inside (Cin) of three N95 filtering facepiece respirator (FFR) models with an N95-Companion and other aerosol instruments using a breathing manikin. Results showed that the Companion measured Cin was contributed by particle penetration through face seal leakage and not through filter media suggesting that the Cout/Cin ratio should be similar for different N95 models at any given leak size. To better understand the phenomenon, the current study analyzed the influence of factors, including filter penetration, resistance, and flow rate on the Companion ratios for two N95 FFR (N1 and N2) and one surgical mask (SM1) models using a manikin. Cout/Cin ratios were measured at 10 and 40 L breathing minute volumes with sealed condition and at artificially introduced leaks. Filter efficiency and resistance were measured at 10 and 40 L/min constant flow rates. Results showed that filter efficiency and resistance were higher for N1 than for N2 and SM1. The ratio for N1was also higher than those of N2 and SM1 at sealed condition at both breathing rates. The result can be due to the higher filtration efficiency of N1. With increasing leak sizes, however, the ratios were higher for N2 than for N1 at 10 and 40 L breathing rates. Higher ratios for N2 can be attributed to the lower resistance that allowed higher aerosol flow through the filter media and captured more negatively charged particles to produce lesser Cin or greater Cout/Cin ratio. Ratios obtained for SM1 at different leak sizes were higher than the ratios of N1 at 10 L. SM1 ratios, however, decreased dramatically at 40 L indicating that high flow rate may adversely affect the ratios. Further studies are needed to better understand the influence of filter media characteristics on the Companion measured Cout/Cin ratios.