Think of a radioactive object as a person with a gun who randomly shoots bullets in different directions.
In our case, the radioactive object is the radium. It fires 3 types of bullets: alpha particles, beta particles, and gamma radiation.
Alpha particles are huge, so they tend to get stopped by almost anything they hit, from your skin or the back of the watch or the glass of the watch.
Beta particles are smaller and pass through more things: they can go a half inch through skin but something like a thin sheet of plastic can stop it.
Gamma rays can pass through the human body, ionizing (or changing the atomic makeup of atoms) as it goes.
With all that in mind, go back to the person with a gun shooting 3 types of bullets from the radium watch. Half the bullets are fired away from the body and won’t hit you. The other half are fired towards the back of the watch and that stops alpha and most beta particles.
So, you are left with gamma rays, which might kill a handful of blood cells or marrow cells as they pass. Not great, but not the end of the world.
Now, imagine you are painting the watches and, as part of your practice, you lick the radium coated brushes. Now, the little person shooting radiation bullets is inside your body! 1) Every direction it shoots, it hits your body. There’s no shielding and no firing off in a direction that won’t hit you. 2) It’s not hitting skin, it’s hitting organs and things inside of you. 3) Unlike a watch, you can’t take it off. It’s inside your stomach and passes out your bowels or gets integrated into your cells. 4) The dose of radioactive material gets higher over time as you consume more radium, much more than the tiny watch arm.
Even if you are ingesting radium, the dose only becomes really deadly over time: killing smallish numbers of cells per day, until the dose accumulates and starts killing more and more cells at a higher rate and possibly mutating DNA, leading to cancers.
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