Our hand-applied American-made radium-packed specimen "RaSWISSRa"
Measuring radioactivity from old watches is rather telling, considering even tritium was once a little radioactive. Tritium, nowadays, is similar to background radiation (completely safe to wear).
Radium on the other hand, is about as radioactive as receiving a CT scan on your wrist daily.
Time To Sell Your Old Watches?
The answer we've all been waiting for...
Should you be worried, knowing this?
No, not really.
You see, the radiation emitted by radium watches (especially the isotope used in them) is primarily alpha rays with some gamma rays.
Alpha rays barely penetrate any solid, and are virtually blocked by steel. This means that a Geiger counter on the crystal will give alarming reads, but one on the caseback will be virtually safe (yet still more radiation compared to tritium). While the radiation is still ionizing (cancer causing), it isn't the most serious as compared to touching your skin to the compound.
Now if you're very, very diligent with your health, perhaps you'd like to stay away from radium. But it's still possible to relume/restore the paint (**vintage collectors all cringing at once**).
That brings me to the point of opening the watch up. Radium is essentially safe to wear, but the moment you open the caseback, or pop out the dial, it becomes extremely unsafe.
Radium particles can get in the air, and if any are consumed, you'll have ionizing radiation-producing compounds in your body essentially forever (unless you live to be older than 1600).
The body treats these compounds similar to calcium, so it will be deposited in your bones - yeah, that's some scary shit.
Additionally, radium decomposes to radon gas, a cancer causing dense gas that is easily inhaled into the lungs.
All in all, pretty safe to wear regularly, absolutely safe to wear on occasion, extremely dangerous to open or service.