Which type of ionizing radiation is the least penetrating to human tissue?

Alpha radiation is the least penetrating type of ionizing radiation when it comes to human tissue. This is primarily due to its massive particles, which consist of two protons and two neutrons, making alpha particles relatively heavy compared to other forms of radiation. Because of their size and charge, alpha particles have a very limited range in materials. They can be stopped by just a few centimeters of air or even by the outer layer of human skin.

This property makes alpha radiation particularly hazardous when alpha-emitting materials are ingested or inhaled, as they can then cause significant damage to internal tissues. However, outside the body, alpha particles pose a minimal risk due to their inability to penetrate the skin.

In contrast, beta radiation consists of lighter particles (electrons) that can penetrate further into human tissue, while gamma radiation and X-rays are forms of electromagnetic radiation that are highly penetrating, capable of passing through the body and requiring dense materials like lead or several centimeters of concrete to shield them effectively.