DIdn't see measurements given in the article. Curious if "exposure to radon in the visitor center, main caverns and Spider Cave were reportedly below the Occupational Safety and Health Administration’s permissible limits." means, as OSHA interpreted at https://www.osha.gov/laws-regs/standardinterpretations/2002-12-23 :
"The OSHA radon exposure limit is an average concentration for 40 hours in any workweek of 7 consecutive days. The still applicable 1971 radon-222 exposure limit for adult employees is 1 x 10-7 microcuries per milliliter (µCi/ml) [100 picocuries/liter (pCi/L)] averaged over a 40-hour workweek. However, OSHA would consider it a de minimis violation if an employer complied with the current NRC radon-222 (with daughters present) exposure limit for adult employees of 3x10-8 µCi/ml [30 pCi/L] averaged over a year (DAC-derived air concentrations)." [emphasis mine]

And since the statement "Radon levels in the visitor center were also below the General Services Administration’s guidelines for federal buildings" mentions only the visitor center, does that mean that the main caverns and Spider Cave are over that level? See 25 pCi/L at https://www.gsa.gov/real-estate/environmental-programs/hazardous-materials/radon-management Not a big issue for the occasional visitor, but certainly wondering about cumulative exposure for workers in the caverns. And I'll admit to curiosity if bats live long enough to get lung cancer.