radon radiation is primarily alpha. That is never good in any dose.
>In this multicenter trial, researchers enrolled 114 patients with moderate-to-mild knee osteoarthritis across three academic centers in Korea. Participants were randomly assigned to receive one of two radiation regimens — a very low dose (0.3 Gy) or a low dose (3 Gy)
This is gamma. Those doses are "low" because delivery here is localized. If given full-body the 3Gy has something like 25-50% mortality.
The cancer patient are delivered like 20-80Gy into the tumor and surrounding tissues which just kills cells outright.
The point is that "radiation" is multiparametric.
It's not really the radiation from radon itself which is the problem, but rather radon's daughters. Radon is a noble gas, so it's not going to accumulate in you and doesn't get much chance to do damage to people even if they inhale it; they'll just exhale it moments later before barely any of it can decay. But radon's daughters, the chain of atoms which are produced from the decay of radon and each other, aren't gases, so if stagnant air with radon sticks around it's going to 'rain' an atomic dust of radioactive isotopes which can accumulate and, if disturbed, can be inhaled and stick with people.