settings as compared to the dusty industrial environments for which these respirators were originally produced, there should be minimal "loading-size particulates" in the air. Thus, in theory, the HEPA respirator filters could provide a useful life of weeks to months. These respirator filters would normally have to be replaced on about the same frequency as the HEPA filters in the ventilation systems. Before each use, the outside of each HEPA filter should be inspected for physical damage. Biological contamination of HEPA respirator filters should not be a concern, since once any bioaerosols impact on the filter media they are not readily reaerosolized.
Positive-pressure, air-line, halfmask respirators are recommended in Table 3 starting on page 40 as the minimal acceptable devices for a limited number of procedures where the potential for aerosolization of droplet nuclei containing tubercle bacilli is high (e.g., bronchoscopy). These devices are also referred to as pressure-demand, air-line, halfmask respirators. An example of this respirator type is given in Figure 3 on page 36. NIOSH conservatively estimates that these respirators have less than 2% face-seal leakage under routine conditions (57). Additionally, the protective reliability of these respirators is substantially higher than that of powered, HEPA-filter, halfmask respirators because these devices can deliver a higher air flow to a facepiece at a higher positive pressure. Additionally, these respirators do not depend on a battery-powered blower to force clean air into their facepiece. Since no filters are used with these respirators, there is no potential for hazardous filter leakage through the rare occurrence of a damaged or improperly manufactured filters.
Table 2 starting on page 29 summarizes the substantial differences in protection between those respirators recommended by NIOSH and those respirators which have previously been used for protection of workers in health-care-facilities with potential exposures to tuberculosis.
