Total inward leakage (TIL) is an estimate of the performance of a respirator, which is measured as the leakage of contaminants through the filter media and through the faceseal interface and exhalation valve of respiratory protective devices under laboratory conditions. Several test agents have been used to measure TIL in different countries. There is a lack of consensus on the most appropriate test method to measure TIL. The International Organization for Standardization (ISO) developed a standard (ISO, 2014) on the comparison of TIL measurement using sodium chloride (NaCl) and corn oil aerosols, and sulfur hexafluoride gas. A comparison of the TIL values between different methods will enable the selection of a relatively suitable method for measuring TIL. NIOSH’s National Personal Protective Technology Laboratory (NPPTL) investigated the TIL values for respirators worn by test subjects exposed to NaCl and corn oil aerosols in two aerosol chambers side-by-side. Four air-purifying respirator categories, made