Current Intelligence Bulletin 10: Radon Daughters


May 11, 1976
DHHS (NIOSH) Publication Number 78-127

Edward J. Baier Correspondence

It has recently come to the attention of the National Institute for Occupational Safety and Health (NIOSH) that measurements of the radioactive decay product “daughter” concentrations of radon gas in a number of National Park Service caves are near the occupational limits as set forth in Occupational Safety and Health Administration’s (OSHA) standards for uranium miners. National Park Service caves in which one or more samples greater than 0.30; working levels (WL) were found include Carlsbad Cavern National Park, New Mexico; Lehman Caves National Monument, Nevada; Mammoth Cave National Park, Kentucky; Oregon Caves National Monument, Oregon; and Round Spring Cave in Ozark National Scenic Riverways, Missouri. In addition to the radiation levels inside the caves, buildings above the ground at Mammoth Cave, cooled with cave air, had 0.6 WL alpha radiation. Studies of uranium miners have shown that the alpha radiation emitted by the radon “daughters” caused an increase in pulmonary malignancies which became evident ten or more, years after individuals first started mining.

The OSHA requirements for uranium mines and deemed by Environmental Protection Agency (EPA) applicable to natural caves are as follows:

  • Above 0.1 WL alpha radiation — all-underground smoking stopped
  • 0.1 to 0.2 WL — monitor workspace at least once yearly
  • 0.2 to 0.3 WL — monitor workspace quarterly
  • Above 0.3 WL — monitor workspace weekly and maintain exposure records on all exposed employees
  • 1.0 to 2.0 WL — immediate corrective action to lower the concentration below 1.0 WL
  • Above 2.0 WL — withdraw all workers not necessary to lower the concentrations below 1.0 WL
  • Cumulative individual exposure shall not exceed 4 working level months in any calendar year

EPA states that the individual exposure limit of 4 WL months per year recommended by OSHA cannot be characterized as safe since the risk of lung cancer would be expected to double after 10 to 20 years employment. Therefore, it might be advisable to rotate long-term employees working in high radiation areas.

NIOSH supports the above recommendations, and is taking this opportunity to advise State Radiological Health officials of the potential hazards for privately owned caves and “cave air” conditioned buildings. Because there are a number of state and privately owned caves throughout the United States, NIOSH would also recommend the radiation levels in these caves be assessed.

Sincerely yours,
Edward J. Baier
Deputy Director

Page last reviewed: June 6, 2014