What creates the complexity is that our instruments measure physical effects, while what is of interest to some are biological effects. A further complication is that units, as with words in any language, may fade from use and be replaced by new units. Radiation is not a series of distinct events, like radioactive decays, which can be counted individually. It is no longer used, but appears in many of the documents examined by the Advisory Committee. Neutrons , when expelled from atomic nuclei and traveling as a form of radiation, can also be a significant health concern.
Alpha particles are clusters of two neutrons and two protons each.
They are identical to the nuclei of atoms of helium, the second lightest and second most common element in the universe, after hydrogen. As they travel along, these large and heavy particles frequently interact with the electrons of atoms, rapidly losing their energy. They cannot even penetrate a piece of paper or the layer of dead cells at the surface of our skin. But if released within the body from a radioactive atom inside or near a cell, alpha particles can do great damage as they ionize atoms, disrupting living cells. Radium and plutonium are two examples of alpha emitters.