Alpha particles are a type of subatomic particle that consist of two protons and two neutrons, making them identical to the nucleus of a helium atom. They are positively charged and are emitted during the radioactive decay of certain heavy elements, such as uranium and radium. Due to their relatively large mass and charge, alpha particles have low penetration power and can be stopped by a sheet of paper or even human skin. However, if ingested or inhaled, they can cause significant damage to biological tissues, making them a concern in radiation safety.