Why is alpha radiation most dangerous with internal human exposure?

This is due to the fact that the stream of alpha rays is a stream of alpha particles – helium nuclei, which has 2 protons and 2 neutrons in its composition.

They, penetrating into the body, are able to cross the DNA strand and destroy it and, as a result, shorten and divide into separate parts, breaking the continuity. This, in turn, causes a mutation of the body cells at the genetic level. Which leads to radiation sickness.