- Alpha particles pose the greatest internal hazard because, despite their low penetration ability externally, they release a large amount of energy over a very short range inside the body.
- When inhaled or ingested, alpha emitters directly damage nearby cells and DNA, significantly increasing the risk of severe biological effects such as cancer.
- In contrast, gamma rays and X-rays penetrate deeply but typically have less localized damage internally, and beta particles have intermediate penetration and energy.

Thus, internal exposure to alpha particles is far more dangerous than to other radiation types.

Reference: Radiobiology for the Radiologist, 7th Edition, Chapter 2, p. 30