> Alpha radiation is completely harmless and doesn’t even get through a sheet of paper.
It is indeed harmless outside of your body, but it is devastating inside of your body.
The reason is that outside of your body, it is blocked by your layer of dead skin, if it gets to it, your dead skin ends up pretty messed up as it absorbs all the energy of the alpha particle. But no big deal, it is dead, doesn't take part of your biology, can't turn cancerous. But now, if that alpha emitter ends up inside of your body, maybe in your lungs as you breathe in radioactive dust, it will end up dumping all of its energy inside of your live cells, damaging DNA and doing everything bad ionizing radiation can do.
Gamma radiation, the unstoppable one is actually less dangerous if it finds its way inside your body, that's because it will go right through it, it will mess up a few cells on its way out, but most of its energy will be dumped outside of you.
Apparently the author has never heard of Polonium-210, arguably the deadliest substance on Earth. what makes it so deadly is it has an extremely short half life (~138 days) and releases almost all of that energy as alpha particles.
How leathal? Roughly 6.8 trillionths of a gram [1].
Luckily Po-210 isn't a huge danger because of the short half life and it's really only produced by governments in very small quantities. But the point is that any alpha particle emitter ingested is potentially a massive health risk.
And what happens at an accident like Chernobyl? It scatters a ton of dust over a huge area that consists of many radioactive isotopes, some of which are just toxic by themselves (eg Caesium) but also some of them are alpha particle emitters. That dust gets into the food chain.
> And in one famous case, injected by a secret agent using an umbrella
I think you're mixing up the Litvinenko murder, which used polonium in a beverage, and Makenko, who got injected with poision with the tip of an umbrella.
It is indeed harmless outside of your body, but it is devastating inside of your body.
The reason is that outside of your body, it is blocked by your layer of dead skin, if it gets to it, your dead skin ends up pretty messed up as it absorbs all the energy of the alpha particle. But no big deal, it is dead, doesn't take part of your biology, can't turn cancerous. But now, if that alpha emitter ends up inside of your body, maybe in your lungs as you breathe in radioactive dust, it will end up dumping all of its energy inside of your live cells, damaging DNA and doing everything bad ionizing radiation can do.
Gamma radiation, the unstoppable one is actually less dangerous if it finds its way inside your body, that's because it will go right through it, it will mess up a few cells on its way out, but most of its energy will be dumped outside of you.