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Rh of occupational exposure. As applied to tuberculosis, this mandate is especially demanding because there is no consensus among experts as to the number, if any, of droplet nuclei containing tubercle bacilli which can be safely breathed by a susceptible worker. Hence, to assure that "no worker will suffer" occupational infection with tubercle bacillus requires the formulation of recommendations which, if implemented, would reduce to the minimum the probability of air contaminated with droplet nuclei being shared between a person with infectious tuberculosis and a worker. The recommendations in this document represent the approach to prevention which most nearly enables NIOSH to meet the directives explicit in the Occupational Safety and Health Act of 1970.

C. The Principle of Public Health Prudence—Traditionally, in addition to careful adherence to its mandates in the Occupational Safety and Health Act of 1970, NIOSH has developed its recommendations for prevention in accord with an operational philosophy which may be called "the principle of public health prudence." Loosely stated, this principle holds that "when faced with uncertainty, it is better to err in favor of human life and health than in favor of any competing value." In the context of NIOSH recommendations for the protection of workers, the principle may be restated as an informal NIOSH operating policy that "faced with scientific uncertainty, if we must err, it will always be on the side of too much protection for the worker rather than too little." This philosophy is supported in a court decision that OSHA and the Nation's courts "cannot let workers suffer while it awaits the Godot of scientific certainty" (14).

NIOSH fully accepts that the evidence available is not adequate to confidently assess both the efficacy and reliability of various currently recommended procedures for preventing the transmission of tuberculosis in health-care facilities. Given the absence of definitive data, particularly for the particulate respirators (PRs) now recommended for use in health- care facilities, NIOSH has, on the basis of the well-documented mode of airborne transmission of tuberculosis, scientific and technical logic, and broad experience with personal respiratory protection programs in a variety of occupational settings, attempted a "best judgement." This is consistent both with NIOSH's mandates and prudent practice in the workplace.