A particle emitted by certain kinds of radioactive materials. An alpha particle is identical to the nucleus of a helium atom, consisting of two protons and two neutrons. Some common alpha-particle emitters are uranium-235, uranium-238, radium-226, and radon-222. Alpha particles have relatively low penetrating power. They can be stopped by a thin sheet of paper or by human skin. They constitute a health problem, therefore, only when they are taken into the body. The inhalation of alpha-emitting radon gas escaping from bedrock into houses in some areas is thought to constitute a health hazard.