MSHA - Code of Federal Regulations - 30 CFR 57.5045
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MSHA - Title 30 CFR
30 CFR § 57.5045
Posting of inactive workings.

Inactive workings in which radon daughter concentrations are above 1.0 WL, shall be posted against unauthorized entry and designated by signs indicating them as areas in which approved respirators shall be worn.

For further information: See MSHA's Program Policy Manual