The statute provides that OSHA can regulate on the basis of the “best available evidence” [(29 USC 655(f), 6(b)(5)]. The U.S. Supreme Court has said that “scientific certainty” is not required; rather, actions need only be supported by a “body of reputable scientific thought” (448 U.S. 607 at 655). The court also said that OSHA could also use assumptions that risked error on the side of overprotection. The court explicitly acknowledged the relevance of epidemiologic evidence.

The agency has traditionally focused on materials used in industrial processes to which exposure was relatively predictable and measurable. A cotton dust standard in the cotton-textile industry, for example, could assume that workers in a cotton-textile mill would be exposed to cotton dust.

The 1997 proposed rule on occupational tuberculosis was only the second that OSHA has developed to deal with an infectious disease hazard. The other led to the 1992 standard on bloodborne pathogens. Regulation of the occupational risk of communicable disease introduces at least three additional complications for regulators that must be kept in mind in assessing the proposed rule on occupational tuberculosis.

First, exposure to Mycobacterium tuberculosis is not readily predictable and cannot reliably be measured, so exposure must be inferred from epidemiologic and other data. Because exposure depends upon numerous factors that vary considerably from workplace to workplace, it cannot be assumed that health care and other workers will actually be exposed to M. tuberculosis.

Second, the risk of exposure and negative health effects has the potential to change rapidly because of events outside the workplace, requiring unusual flexibility and coordination with other actors involved in preventing the transmission of tuberculosis. If community prevalence drops substantially or infection control measures change significantly, OSHA’s risk assessment or regulatory response may cease to be relevant.

Third, the risk of communicable disease may not originate in the workplace. No one brings formaldehyde or other regulated toxins into the workplace, but workers, patients and others do bring communicable diseases into workplaces. Thus, OSHA must deal with the question of whether infections in a hospital or other covered workplace are to be attributed to a worker’s occupational risk or community risk.

For the most part, OSHA’s direct regulatory focus is on private employers. Separate provisions of the statute describe its application to federal workers and to state and local employees.