Ionizing radiation comes in two basic forms, particles and rays.

Alpha particles are made up of two protons and two neutrons, elementary particles found in the nucleus of atoms. Alpha particles are relatively slow moving and do not have enough energy to penetrate, for example, paper or skin-although they may nevertheless harm the skin. They are most worrisome when a substance that emits them is eaten or inhaled, since alpha particles can then damage cells within the body.

Beta particles are single electrons, the elementary particles normally surrounding an atomic nucleus. Beta particles move at the speed of light and can penetrate paper and slightly into the skin. They cannot pass through, however, even a light metal such as aluminum.

Gamma raysare short bursts of high energy given off by an un- stable atomic nucleus. They easily pass right through the human body and...