Alpha radiation consists of alpha particles, which are helium nuclei made up of two protons and two neutrons. They are relatively large and heavy particles compared to other types of radiation, such as beta or gamma rays. As a result, alpha radiation has a low penetrating power and can be stopped by most materials.

Alpha particles can be stopped by a few centimeters of air, a sheet of paper, or a few millimeters of plastic or metal. They cannot penetrate the outer layer of human skin, and therefore, they are not considered a significant external radiation hazard.

However, alpha radiation can be dangerous if it is ingested or inhaled, as it can damage the cells in the body from the inside. Alpha-emitting isotopes, such as plutonium or uranium, are found in some radioactive sources and nuclear waste, and they require special handling and storage procedures to prevent exposure.

In summary, alpha radiation can penetrate only a very short distance in most materials and is effectively stopped by air, paper, plastic, or metal. It poses a greater risk when radioactive materials containing alpha-emitting isotopes are ingested or inhaled.