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Rh For some infectious diseases, the barrier of immunity can be erected through vaccination of susceptible persons. Vaccination against tuberculosis using BCG vaccine has not been recommended for health-care workers or other adults at high risk for acquiring tuberculosis infection (60). The AMA has reported that even should BCG vaccine be recommended for certain health-care workers, "the latter should be aware that the vaccine may not afford significant protection against tuberculosis" (4).

NIOSH strongly supports the concept of a hierarchy of controls, which is the foundation of current practice for preventing exposures to hazards in the workplace. Substitution, administrative controls and work practices, and engineering controls because of their greater reliability should receive the highest priority. However, when the effectiveness and reliability of other control measures are not known, cannot completely control the hazard, or cannot be assured under all conditions that can be reasonably anticipated, personal respiratory protection is an essential addition to the armamentarium of control. This is why surgical masks for patient-care personnel have been traditionally indicated for infectious diseases such as Lassa fever, Marburg virus disease, smallpox, and tuberculosis in combination with special-ventilation private rooms (52). The purpose of masks for patientcare personnel have been explained as follows (52):

At the present time, the exposure of workers to aerosolized TB droplet nuclei cannot be completely controlled at the infectious source nor is it plausible that exposures can be completely prevented by interrupting the pathway of contagious emissions between a person with infectious tuberculosis and workers nearby in the same room. Also at present, it appears impossible to determine the quantitative efficacy and reliability of each available control method. Hence it is impossible to assure that health- care -facility workers will not