Okay. Uranium is radioactive, which means that its nucleus is unstable and at some point, depending on how unstable it is, it will decay away to another nucleus that is smaller. It will lose some of the nucleus. Then if that's stable, fair enough. If that is radioactive, it will decay away to another material, and you go down through a chain. We call that a daughter chain. So you start with a parent radionuclide like uranium and you get a succession of radioactive decays all the way down—in the case of uranium—through radium. Then radium decays to radon. Then there are more alpha particles and more radiations, until you come down to lead. When it gets to lead, it's now stable and there's no further radioactivity.
Radon is a gas, and whereas the uranium stays in the rocky material, the radon diffuses out. In the early days when the mines were poorly ventilated the concentrations of radon in the mines in Canada and elsewhere in the world were really very high, so the lung doses from the inhaled radon were very high. The amounts of uranium these people inhaled were very small. The bigger problem was actually inhalation of silica.
So yes, in that case, if you work it out and do the dose symmetry, asking how much of the radiation is coming from the uranium and how much of it is coming from radon and the daughters of radon, you find the dose from uranium is minuscule compared with the dose from both normal radon, which comes from uranium, and another radon isotope that we call thoron, that comes from thorium, which is present in the uranium deposits. Those two, and their daughters, contribute 99.9% of the dose when you work it out.