Protection factors (PF) calculated from measurements of particle concentration by PortacountH devices were reported as the ratio of particle concentrations outside and inside the mask. This is a similar concept to the fit factor as used by the US Occupational Safety and Health Administration (http://www.osha.gov/pls/ oshaweb/owadisp.show_document). Therefore, a higher PF is better and PF = 1 means complete absence of protection. For statistical analysis, the following transformation was used: