Health and Medicine

Health Hazards and Risks

How is human exposure to radiation measured?

The radiation absorbed dose (rad) and the roentgen equivalent man (rem) were used for many years to measure the amount and effect of ionizing radiation absorbed by humans. While officially replaced by the gray and the sievert, both are still used in many reference sources. The rad equals the energy absorption of 100 ergs per gram of irradiated material (an erg is a unit of work or energy). The rem is the absorbed dose of ionizing radiation that produces the same biological effect as one rad of X rays or gamma rays (which are equal). The rem of X rays and gamma rays is therefore equal to the rad; for each type of radiation, the number of rads is multiplied by a specific factor to find the number of rems. The millirem, 0.001 rems, is also frequently used; the average radiation dose received by a person in the United States is about 360 millirems per year. Natural radiation accounts for about 82 percent of a person’s yearly exposure, and manufactured sources for 18 percent. Indoor radon has only recently been recognized as a significant source of natural radiation, with 55 percent of the natural radiation coming from this source.


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