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.
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.