Radiation Exposure

The first commonly used unit for measuring the biological effects of x-ray exposure was the roentgen. It was named after the German physicist Wilhelm Roentgen, who discovered x rays in 1895. A roentgen is the amount of radiation that produces a set number of charged ions in a certain amount of air under standard conditions. This unit is not, however, particularly useful for describing the potential effects of radiation on human or animal tissues. The rad unit is slightly better in this regard. It is a measure of the radiation dose absorbed by one gram of something. A rad is equal to a defined amount of energy (100 ergs) absorbed per gram.

The problem with rads as a unit of measurement for human radiation exposure is that a dose of one rad of radiation from plutonium produces a different effect on living tissue than one rad of a less harmful type of radiation. Consequently, scientists introduced the rem, which stands for "roentgen equivalent man." A rem is the dose of any radiation that produces the same biological effect, or dose equivalent, in humans as one rad of x rays.

Scientists continue to use these units, which were introduced earlier in the century, even as they become used to newer units for certain applications. The roentgen will still be the unit used to measure exposure to ionizing radiation, but the rad is being replaced with the "gray" as a measure of absorbed dose. One gray equals 100 rads. The sievert is replacing the rem as a measure of dose equivalent. One sievert equals 100 rems.