Radiation exposures to individuals are measured by the amount of energy deposited in their bodies; exposures to populations are measured by adding up the individual doses in that population. ¹ The unit of radiation dose is the gray. It is a measure of the amount of ionization caused by the radiation and is a strictly physical unit. Other factors such as the type of radiation involved (alpha, beta, etc.) and the parts of the body exposed affect the biological effect of the radiation. When corrections are made for these factors, the unit used is the sievert. One sievert of radiation should have about the same biological effect, whether it be one sievert of gamma radiation to the whole body or one sievert of alpha radiation to the lung. Where the total dose to groups or populations is being considered, units such as person-sieverts are used. Population doses are measured in person-grays and person-sieverts (person-Sv), depending on whether energy deposition or biological damage is being measured.

Two further units apply to uranium mines. The working level (WL) is the quantity of radon decay products (also called radon daughters or radon progeny) in one liter of air that will result in the emission of 130,000 million electron volts of alpha-particle energy. If the radon progeny are in equilibrium with radon in the air (that is, if the radon has remained in the air for some time), then about 100 picocuries (3.7 becquerels) of radon per liter of air equals one working level. The working level month (WLM) measures the total radiation dose a miner would receive by breathing air containing a concentration of 1 working level for one working month (170 hours).

Radiation doses may be due to sources outside the body or to substances that have entered the body in the course of eating, drinking, or breathing, or through a wound. It is relatively straightforward to estimate radiation doses due to gamma rays and beta particles from outside the body, provided a person wears appropriate measuring equipment, such as a film badge. However, it is generally much harder to estimate doses from substances inside the body. The size of the dose will depend on the chemical form of the material, its pathways and distribution in the body, and the rate of its elimination from the body, among other factors. The elimination of a radionuclide from the body is generally quite a complex phenomenon; it can be very approximately described by the concept of "biological half-life"-the time it takes for half the material to be eliminated from the body.

When estimating doses from environmental radioactivity, direct measurements are almost never available for the amounts of particular radionuclides in the body. Complex computer models have to be used, often with large numbers of parameters and associated uncertainties. This is especially true of dose estimation for off-site populations where there are no direct measurements for dose or for body-burdens of radioactive materials. However, radionuclides in food, water, and air can be measured. If done carefully, such measurements can provide a basis for estimating doses. If internal burdens are large, techniques such as whole-body counting (called in vivo measurements) and urine sampling can also be used.