In 1995 NIOSH issued new regulations for respirators. All nine new respirator categories are permitted in health-care facilities for the prevention of the transmission of tuberculosis. The N95 respirator is the most frequently used. It has been evaluated as to its efficiency for collecting TB-size bacteria. The study results show that N95 respirators collect 0.1 to 0.3 um particles with efficiencies of 95% or higher. TB-size bacteria of 0.8 um and larger, however, are collected with much higher efficiencies: 99.5% or higher. Furthermore, the study results show that no bacteria are reentrained during normal exhalation. Only at low humidity flow conditions, corresponding to violent coughing or sneezing, has reaerosolization of 0.1% or less been observed. The survival and potential growth of several bacteria were investigated on polypropylene respirator filters which are the most common among the NIOSH-certified respirators. Mycobacterium smegmatis, a biochemical simulant of Mycobacterium tuberculosis (Mtb), and the other test bacteria did not grow on the tested respirators, even when the respirators were stored at temperature, humidity and nutrition conditions most favorable for microbial growth. However, Mycobacterium segmatis could survive on respirators for up to three days during storage. The culturability of Mycobacterium segmatis was not affected by the air flow simulating respirator wear under medium work load conditions. It is concluded that all of the new NIOSH-certified respirators, including the N95 respirators, offer good protection against TB bacteria. A significant number of bacteria can enter the respirator wearer's breathing space only through face seal leaks. Bacteria collected by the respirator filter will not reaerosolize during normal exhalation. This study shows that Mtb surrogate bacteria collected on a respirator can remain viable only under conditions that are not usually encountered in hospital wear. Reaerosolization of a very small fraction of collected bacteria can occur only at low humidity under extreme conditions of violent sneezing or coughing. Thus, minimal infection risk is expected from the reaerosolization of Mtb collected on respirators. Emphasis for respiratory protection in environments that may contain airborne Mtb, thus, needs to focus on appropriate fit of the respirator to the wearer's face.