30 CFR § 57.5046
Protection against radon gas.
Where radon daughter concentrations exceed 10 WL, respirator protection
against radon gas shall be provided in addition to protection against radon
daughters. Protection against radon gas shall be provided by supplied air
devices or by face masks containing absorbent material capable of removing
both the radon and its daughters.
For further information: See MSHA's Program Policy Manual