Alpha (α) particles
α particles, identical to helium nuclei, are positively charged particles consisting of two protons and two neutrons. As they are positively charged, their motion will be affected by an electromagnetic field. In general, heavy atoms (having atomic numbers greater than 82) would tend to emit α particles. Uranium and radium are examples of α emitting nuclei. As α particles are relatively large and positively charged, they ionize matter easily and lose their energy very quickly. The penetrating power of α particles is the lowest among all types of ionizing radiation. α particles can be stopped easily by the outer layer of skin or a sheet of paper.
However, if α particles are taken into our body by inhalation or ingestion, such as inhaling part of a radioactive plume, α particles can directly affect tissues in the body. Although their penetrating power is weak, their ionization power is strong. The severity of biological effects inflicted will not be lower than other radiations.