eli5 How come if highly radioactive radium was on watches that the wearers didn’t succumb to radiation poisoning at the wrist?

Question

eli5 How come if highly radioactive radium was on watches that the wearers didn’t succumb to radiation poisoning at the wrist?

In: 186

Answer 1

The entire radium decay chain* emits only alpha and beta particles (and no gamma radiators)

Alpha particles are blocked by a piece of paper, so these are mainly dangerous if you get the material inside your body (if you for example lick a brush with radium paint on it) and beta radiation is slightly better at penetrating materials but is generally stopped by a very thin sheet of metal (like the metal body of a watch).

There is also very little radium in a watch (very thin layer of paint that contains radium and a material that absorbs radiation and then emits that energy as a photon, ie visible light).

*Radium collapses into radon->Polonium 218->Lead 214->Bismuth 214 etc etc. Each time it emits either a helium-4 particle (alpha radiation) or an electron/positron (beta radiation).