Which type of radiation penetrates human skin the least effectively?

Alpha particles have the lowest penetration power when it comes to interacting with human skin. These particles are composed of two protons and two neutrons, making them positively charged and relatively large compared to other types of radiation. Due to their size and charge, alpha particles can be easily stopped by simple barriers, such as a sheet of paper or the outer dead layer of human skin.

While they can be highly damaging if ingested or inhaled, their inability to penetrate the skin effectively means they pose less of a direct external hazard. In contrast, beta particles, gamma rays, and X-rays have greater penetrating abilities, with beta particles being able to penetrate further than alpha particles, and gamma rays and X-rays being able to pass through soft tissue and materials much more effectively. This makes alpha particles distinct in their interaction with human skin, as they do not have the capacity to penetrate it beyond the outermost layer.