Recent studies suggest that a wide range of human health effects could result from exposure to carbon nanotubes (CNTs). A National Institute for Occupational Safety and Health survey of the carbonaceous nanomaterial industry found that 77% of the companies used respiratory protection, such as filtering facepiece respirators (FFRs). Despite CNT studies in some occupational settings being reported, the literature for mass-based penetration of CNTs through FFRs is lacking. The aim of this study was to conduct a quantitative study of single-walled CNT (SWCNT) and multiwalled CNT (MWCNT) penetration through FFRs. A CNT aerosol respirator testing system was used to generate charge-neutralized airborne SWCNTs and MWCNTs for this study. The size distribution was 20-10000nm, with 99% of the particles between 25 and 2840nm. Mass median diameters were 598 and 634nm with geometric standard deviations of 1.34 and 1.48 for SWCNTs and MWCNTs, respectively. Upstream and downstream CNTs were collected simultaneously using closed-face 3.7-cm-diameter filter cassettes. These samples were subsequently analyzed for organic carbon and elemental carbon (EC), with EC as a measure of mass-based CNTs. The mass-based penetration of SWCNTs and MWCNTs through six FFR models at constant flow rates of 30 l min-1 (LPM) was determined. Generally, the penetrations of SWCNTs and MWCNTs at 30 LPM had a similar trend and were highest for the N95 FFRs, followed by N99 and P100 FFRs. The mass-based penetration of MWCNTs through six FFR models at two constant flow rates of 30 and 85 LPM was also determined. The penetration of MWCNTs at 85 LPM was greater compared with the values of MWCNTs at 30 LPM.