The type of radiation referred to in this course is ionizing radiation-invisible particles or waves of energy emitted from radioactive atoms or radiation-producing machines. Non-ionizing radiation (e.g., laser light and microwave radiation) presents very different hazards and are controlled through the Non-ionizing Radiation programs. The common types of ionizing radiation are alpha, beta, neutron, x-ray, and gamma radiation. Some radioactive atoms (e.g., uranium-238 and thorium-232) are natural; others (e.g., plutonium-239 and iodine-131) are man-made.

If the energy from the radiation is deposited in a person, he or she receives a radiation dose. Radiation doses are measured in millirems (mrem) or rems. One thousand millirems equal one rem (1000 mrem = 1 rem).

	Figure 1. Annual Radiation Doses in the United States from Natural and Manufactured Radiation Sources. 1

Background radiation is radiation from our natural environment. Everyone is exposed to some amount of background radiation. This exposure primarily comes from cosmic rays, radioactive material in the earth (such as uranium-238), ingestion of naturally occurring radionuclides in food (such as potassium-40), and inhalation of radon gas. In the United States, the average background radiation dose is 300 mrem/yr.

Manufactured sources contribute an additional background radiation dose of approximately 60 mrem /yr. Of this amount, approximately 54 mrem are from medical procedures (e.g., x-rays and certain diagnostic tests). Consumer products such as lantern mantles, smoke detectors, and uranium-glazed pottery contribute roughly 5 mrem /yr. Fallout radiation that is present in our environment contributes less than 1 mrem /yr. Figure 1 shows typical annual radiation doses in the United States.