Before we discuss the biological effects of ionizing radiation, it is important to remember the many sources of radiation that surround us. Individuals are exposed to minute amounts of radiation from the environment every day.

Large radioactive sources such as nuclear power plants and nuclear devices have received much attention recently. This has led to a greater understanding of the hazards from these sources of radiation and the need for emergency plans to respond to situations involving an accident or threat associated with them.

However, radiation and radioactivity is present all the time, and they were present in the universe long before the earth existed. Even humans are slightly radioactive. Living things such as humans also have traces of radioactive elements (potassium 40) in their bodies.

Select this link for a chart showing the Average Annual Effective Radiation Dose Equivalents in the United States (i.e., the sources and average effective radiation dose individuals in the United States receive each year). This chart shows that both natural and man-made sources each contribute 50 percent of the total dose equivalents and that of the 50 percent man-made sources, two percent is from consumer sources and 48 percent is from medical sources.