The National Institute for Occupational Safety and Health (NIOSH) received a Health Hazard Evaluation (HHE) request for technical assistance from the Department of the Interior (DOI) on January 13, 2004. The request described concerns about potential exposures to national park employees from naturally occurring radioactive mineral deposits in piping removed from renovated bathhouses at Hot Springs National Park (HSNP), Hot Springs, Arkansas. On January 26, 2004, NIOSH and DOI investigators made on-site radiation measurements and identified radium-226 (226Ra) in mineral deposits in water pipes. NIOSH investigators made recommendations regarding safe handling of the water pipes and also recommended conducting radon concentration measurements throughout HSNP. Gamma-radiation levels measured by NIOSH, the DOI, and the State of Arkansas in six bathhouses were low and similar to previously reported values, except for a few localized areas in the Hale Bathhouse basement. These areas of higher gamma-radiation were associated with rock in the bathhouse foundation which was rich in naturally occurring radioactive material (NORM), particularly 226Ra. On July 1, 2004, additional radon-progeny measurements and water samples were collected to assess radioactive constituents in the air and spring water. Radon-progeny measurements determine the amount of alpha activity in the air that could be inhaled. The alpha activity measurement is called a "Working Level" (WL) and determines if a respirator is recommended based on NIOSH criteria. These WL measurements are used with radon concentration measurements to calculate the equilibrium factor (EF). The EF is an important site-specific parameter because it is used to convert radon concentrations (picoCuries [pCi/L]) to WL estimates. The site-specific EF results indicated that the radon concentration and WL relationship in HSNP buildings behaved more like thermal spa environments than the typical indoor residential environment. Following the NIOSH recommendation, indoor radon measurements were made by park officials in every HSNP bathhouse and the park's Administration Building between May 2004 and February 2005. Only the Hale Bathhouse and the Administration Building Tower Room had airborne radon concentrations above the U.S. Environmental Protection Agency (EPA) recommended remedial action level of 4 pCi/L. The Tower Room, not routinely occupied or accessible to building occupants, was posted with a warning sign. Water sampling showed that 226Ra concentrations in the Hale spring water were below the proposed EPA maximum concentration limit (MCL = 5 pCi/L), but above the proposed EPA MCL (300 pCi/L) for 222Rn (the Administration Building public water fountain source did not exceed either proposed limit). NIOSH investigators conclude that a potential health hazard exists for some workers at the HSNP due to the elevated radon concentrations in the Hale Bathhouse basement and the Administration Building Tower Room. Remedial actions and ongoing radiation monitoring by HSNP officials have reduced exposure potentials to levels as low as reasonably achievable. NIOSH investigators recommend continued routine radon monitoring and starting a water monitoring program for radium and radon in public drinking water.