This document presents the recommendations of the National Institute for Occupational Safety and Health (NIOSH) regarding occupational exposure to radon progeny in underground mines. Ventilation is discussed as an important control technology approach in the management of excessive concentrations of radon daughters. Data is presented pertaining to the linkage of radon exposure with the risk of human lung cancer. The NIOSH recommended exposure limit (REL) takes into account studies and documentation of the Mine Safety and Health Administration (MSHA) and the United States Environmental Protection Agency (EPA), as well as its own risk assessment modeling, accumulated measurement data, and technical feasibility assessments. The REL allows for 0.2 pCi/l (picocuries per liter of air) at 1 working level month (WLM - 170 hours per month). This should be considered as an upper limit and it is recommended that mine operators limit exposure to the lowest levels possible. In addition, it is emphasized that this recommended standard contains many important provisions in addition to the annual exposure limit. These include recommendations for limited work shift concentrations of radon progeny, sampling and analytical methods, recordkeeping, medical surveillance, posting of hazardous information, respiratory protection, worker education and notification, and sanitation.