An alpha particle is the nucleus of a helium atom (with two neutrons and two protons each). Many heavy radionuclides, such as uranium-238 and plutonium-239, decay mainly by emitting alpha particles.

For example the decay of plutonium-239 results in uranium-235 with the emission of an alpha particle:

94Pu-239 ---- 92U-235 + 2He-4

The alpha particles emitted carry a lot of energy, about 5 million electron volts. A helium atom at room temperature has an energy of .025 electron volts. It is this large amount of energy associated with radioactive particles being deposited in cells that causes biological damage through ionization.

Alpha particles, being heavy, transfer their energy to other atoms and molecules within a shorter distance than the far lighter electrons which are the primary means of radiation damage for both gamma and beta radiation. Outside of the body, alpha particles do not pose a health hazard, since they do not penetrate the outer, dead layer of skin. However, once inside the body through ingestion, inhalation or through cuts and abrasions, alpha particles are very damaging, because they travel only a short distance within living tissue, repeatedly bombarding the cells and tissue nearby.

A beta particle is an electron or a positron (a positively charged particle otherwise identical to an electron). Beta particles are much lighter than alpha particles, and travel much further. If they are sufficiently energetic, they can penetrate the skin. Some beta-emitters therefore can pose a health hazard, especially to the lymphatic system, even when outside of the body. Most beta radiation can be stopped by light shielding, such as a piece of wood or several pieces of paper, though some, such as that from sodium-24 requires heavier shielding.

Radioactive decay is often also accompanied by emission of gamma radiation, which is very high frequency electromagnetic radiation, like X-rays. It takes heavy shielding of lead or steel to stop gamma rays. Gamma rays consist of photons, which are "packets" or quanta of electromagnetic energy. Emission of photons from a nucleus does not result in a transmutation. gama ray photons produce ionization (and hence biological damage). The incident photon collides with an electron in an atom (or molecule) and knocks it out, imparting some energy to it. A less energetic photon (the "scattered" photon) is also emitted in this process which is called the Compton effect.

These electrons, the electrons generated by further collisions, as well as electrons produced by the new photons, are responsible for the damage caused by gamma radiation.

Alpha, beta, and gamma radiation have very different properties in some respects, but are all ionizing radiation--that is, each is energetic enough to break chemical bonds, and thus possess the ability to damage or destroy living cells. Visible light, like gamma rays, is also electromagnetic energy, but of lower frequency. Visible light photons are not energetic enough to cause ionization. Radio waves are of even lower frequency than visible light.