Which type of radioactive particles presents the greatest risk once inside the body?

Alpha particles present the greatest risk once inside the body due to their high mass and charge, which allows them to cause significant damage to biological tissues at a very close range. When alpha-emitting materials are ingested or inhaled, these particles can interact with cellular structures and DNA, leading to severe biological damage and increasing the risk of cancer.

Although alpha particles are generally less penetrative and can be stopped by a sheet of paper or the outer layer of skin, their danger arises from their potential to cause harm once inside the delicate environment of the body. This makes them particularly hazardous in cases of internal contamination.

In contrast, gamma rays have a much higher penetration power and can pass through the body without causing direct harm unless in high doses from external exposure. Beta particles can also penetrate tissues but are less damaging when compared to alpha particles because of their smaller mass and charge, which results in less energy being deposited in the body. Delta particles are not a recognized type of radioactive particle, further underscoring the focus on alpha particles as the most hazardous when inside the body.