Can anybody advise on the equilibrium factor to use for outdoor radon? Radon equilibrium factors for indoor and outdoor air are often quoted as 0.4 and 0.8 respectively. ICRP65 use 0.4 for indoor air, but I do not have a good reference for outdoor air. Is the equilibrium factor seasonal? How variable is it? Andries Leuschner andries.leuschner@pixie.co.za