The radiation in the earth's natural environment, including radiation originating outside the earth's atmosphere and radiation from the naturally occurring radioactive elements on earth. These elements may be found both in the environment and inside the bodies of men and animals.

A radioactive metallic element that is chemically similar to uranium and is obtained in nuclear reactors as a by-product in the production of plutonium from uranium.

The unit of force, which when applied to a one kilogram mass will give it an acceleration of one meter per second per second (1N = 1kg X 1m/s2).

A health effect, the severity of which increases with the size of the dose and for which a threshold exists. Radiation-induced cataract formation is an example of a nonstochastic effect. Also called a deterministic effect.