MSHA - Code of Federal Regulations - 30 CFR 57.5039
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MSHA - Title 30 CFR
30 CFR § 57.5039
Maximum permissible concentration.

Except as provided by standard §57.5005, persons shall not be exposed to air containing concentrations of radon daughters exceeding 1.0 WL in active workings.

For further information: See MSHA's Program Policy Manual