NIOSH Evaluates Radon Exposures in Park Caverns, Buildings

A report recently published by NIOSH details the agency’s evaluation of employee exposures to radon gas at a national park. The U.S. National Park Service requested NIOSH’s assistance to examine potentially elevated radon concentrations within an underground tourist cavern and its connected buildings. Staff from NIOSH’s Health Hazard Evaluation Program visited the park four times during different seasons to assess exposures to radon and radon decay products within the park’s main cavern, attached visitor’s center, a cave that had periodic guided tours, and other administrative buildings. Investigators also evaluated ventilation within the visitor’s center. NIOSH personnel found that radon gas was entering the visitor’s center by way of the elevator shaft connected to the main cavern and noted that radon concentrations in the center were greater than concentrations found outdoors and in other park buildings nearby. NIOSH ultimately recommended that the park use engineering and administrative controls to decrease workers’ exposures to radon.

Radon gas concentrations measured inside the visitor’s center, main cavern, and the other cave were all below OSHA’s permissible exposure limit of 100 picocuries/liter (pCi/L), averaged over 40 hours in any workweek of 7 consecutive days. Since radon is a radioactive material, OSHA’s whole-body ionizing radiation dose limit also applies. NIOSH’s report warns that employees working in the cavern have the potential to exceed this dose limit—1.25 roentgen equivalent man (rem), or 12.5 millisieverts, per quarter—depending on how much time they spend in the cavern. NIOSH recommended administrative controls to limit the number of hours that workers spend in the cavern to help prevent long-term adverse health effects. The agency also noted seasonal differences in radon concentrations and urged the park to schedule non-time-sensitive cavern work during wintertime, when concentrations are lower.

“In the higher radon concentrations months (i.e., summer months), employees should be limited to no more than 225 hours per quarter [in the main cavern],” NIOSH’s report states. “During the months with lower radon concentrations (i.e., winter months), employees could spend up to 665 hours per quarter in the cavern before exposures would exceed the OSHA quarterly dose limits.”

Estimating approximately 500 work hours per quarter, park employees would be able to spend their full work time in the cavern during winter months with lower radon concentrations.

At the time of NIOSH’s visit, employees led tours in the other cave for three hours per week. NIOSH cautions that it is possible that the PEL for radon could also be exceeded in the other cave if the duration or frequency of tours increases. In the visitor’s center, NIOSH recommended HVAC modifications to prevent the entry and mixing of cavern air with air inside the building. The agency expects that these modifications should reduce radon levels in the visitor’s center.

Further details are available in NIOSH’s report (PDF).