Rad (unit)

The rad is a deprecated unit of absorbed radiation dose, defined as 1 rad = 0.01 Gy = 0.01 J/kg. It was originally defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by one gram of matter. It has been replaced by the gray in SI but is still used in the United States, though "strongly discouraged" in the chapter 5.2 of style guide for U.S. National Institute of Standards and Technology authors. A related unit, the roentgen, is used to quantify the radiation exposure. The F-factor can be used to convert between rads and roentgens.

The material absorbing the radiation can be human tissue or silicon microchips or any other medium (for example, air, water, lead shielding, etc.).

A dose of under 100 rad will typically produce no immediate symptoms other than blood changes. 100 to 200 rad delivered to the entire body in less than a day may cause acute radiation syndrome, (ARS) but is usually not fatal. Doses of 200 to 1,000 rad delivered in a few hours will cause serious illness with poor outlook at the upper end of the range. Whole body doses of more than 1,000 rad are almost invariably fatal. Therapeutic doses of radiation therapy are often given and well tolerated even at higher doses to treat discrete and well defined anatomical structures. The same dose given over a longer period of time is less likely to cause ARS. Dose thresholds are about 50% higher for dose rates of 20 rad/h, and even higher for lower dose rates.

Radiation increases the risk of cancer and other effects at any dose. The International Commission on Radiological Protection maintains a model of these risks as a function of absorbed dose and other factors. That model calculates an effective radiation dose, measured units of rem, which is more representative of the risk than the absorbed dose in rad. In most power plant scenarios, where the radiation environment is dominated by gamma or x rays applied uniformly to the whole body, 1 rad of absorbed dose gives 1 rem of effective dose. In other situations, the effective dose in rem might be thirty times higher or thousands of time lower than the absorbed dose in rad.

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