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Rh 20% (57,58) inward face-seal leakage even after passing a fit test performed by a qualified individual. Existing standard performance tests for surgical masks have not addressed either inward or outward face-seal leakage (59). The inward face-seal leakage for these masks is assumed to be higher than 10% to 20% if the masks are not properly fitted to the wearer's face, tested for an adequate fit by a qualified individual, and then fit checked by the wearer every time these masks are donned. It is reasonable to expect at least as much for outward leakage from a masked patient.

As discussed in section IV.G starting on page 27, surgical masks and NIOSH-certified PRS cannot be reliably fit checked by their wearer before every respirator use to assure a tight face seal. Thus the amount of reduction in droplet nuclei exhaled by a masked patient is unknown. In summary, some trapping of exhaled aerosols will occur in a mask covering a patient's nose and mouth, but the extent of trapping is unknown. Correspondingly, potentially hazardous leakage will inevitably occur past a patient's mask, but the amount of leakage is also unknown.

3. Next in the hierarchy of controls are engineering controls to interrupt the pathway of hazardous emissions from the source to the worker(s) (e.g., use of negative -pressure atmospheres and other special ventilation requirements for private isolation rooms to contain the droplet nuclei in the confines of these rooms). This is the rationale for isolation precautions in hospitals (52).

Under certain circumstances, engineering controls may be neither feasible, effective, reliable, or applicable. In such cases, changes in or implementation of work practices or schedules, hazard training programs, and other administrative modifications may reduce the risk of exposure (e.g., minimizing the time a worker is in a room occupied by a potential transmitter).

4. The last, and generally least reliable control measure is to establish barriers between the worker and the hazardous work environment (e.g., personal protection equipment such as appropriate respirators used by workers in conjunction with a comprehensive personal respiratory protection program).