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 It has been replaced by the gray in some countries. A related unit, the roentgen, is used to quantify the number of rad deposited into a target when it is exposed to radiation. The F-factor can be used to convert between rad 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.).
Health effects 
A dose of under 100 rad will typically produce no immediate symptoms other than blood changes. 100 to 200 rad delivered in less than a day will 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. Doses of more than 1,000 rad are almost invariably fatal. 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 stochastic 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 stochastic 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.
Material effects 
Silicon-based microelectronics break down under exposure to radiation. Radiation-hardened components designed for military or nuclear applications can survive up to 10 gigarad (100 MGy).
Metals creep, harden and become brittle under the effect of radiation.
Foods and medical equipment can be sterilized with radiation.
Dose examples 
|25||rad||-lowest dose to cause clinically observable blood changes|
|0.2||krad||-local dose for onset of erythema in humans|
|0.4||krad||-whole body LD50 for acute radiation syndrome in humans|
|1||krad||-typical radiation tolerance of ordinary microchips|
|4 to 8||krad||-typical radiotherapy dose, locally applied|
|10||krad||-fatal whole-body dose in Wood River Junction criticality accident|
|1||Mrad||-typical tolerance of radiation-hardened microchips|
In the 1930s the roentgen was the most commonly used unit of radiation exposure. This unit is obsolete and no longer clearly defined. One roentgen deposits 0.877 rad in dry air, 0.96 rad in soft tissue, or anywhere from 1 to more than 4 rad in bone depending on the beam energy. These conversions to absorbed energy all depend on the ionizing energy of a standard medium, which is ambiguous in the latest NIST definition. Even where the standard medium is fully defined, the ionizing energy is often not precisely known.
In 1940, Gray, who had been studying the effect of neutron damage on human tissue, together with Mayneord and Read published a paper in which a unit of measure, dubbed the "gram roentgen" (symbol: gr) defined as "that amount of neutron radiation which produces an increment in energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one roentgen of radiation" was proposed. This unit was found to be equivalent to 88 ergs in air. It marked a shift towards measurements based on energy rather than charge.
The Röntgen equivalent physical (rep), introduced by Herbert Parker in 1945, was the absorbed energetic dose to tissue before factoring in relative biological effectiveness. The rep has variously been defined as 83 or 93 ergs per gram of tissue (8.3/9.3 mGy) or per cc of tissue.
In 1953 the ICRU recommended the rad, equal to 100 erg/g as a new unit of absorbed radiation, but then promoted a switch to the gray in the 1970s.
The International Committee for Weights and Measures (CIPM) has not accepted the use of the rad. From 1977 to 1998, the US NIST's translations of the SI brochure stated that the CIPM temporarily accepted the use of the rad (and other radiology units) with SI units since 1969. However, the only related CIPM decisions shown in the appendix are with regards to the curie in 1964 and the radian (symbol: rad) in 1960. The NIST brochures redefined the rad as 0.01 Gy. The CIPM's current SI brochure excludes the rad from the tables of non-SI units accepted for use with the SI. The US NIST clarified in 1998 that it was providing its own interpretations of the SI system, whereby it accepted the rad for use in the US with the SI, while recognizing that the CIPM did not. NIST recommends defining the rad in relation to SI units in every document where this unit is used. Nevertheless, use of the rad remains widespread in the US, where it is still an industry standard. Although the United States Nuclear Regulatory Commission still permits the use of the units curie, rad and rem alongside SI units the European Union required that its use for "public health ... purposes" be phased out by 31 December 1985.
See also 
- International Bureau of Weights and Measures (2008). United States National Institute of Standards and Technology, ed. The International System of Units (SI). NIST Special Publication 330. Dept. of Commerce, National Institute of Standards and Technology. Retrieved 29 May 2012.
- The Effects of Nuclear Weapons, Revised ed., US DOD 1962, pp. 592–593
- "The 2007 Recommendations of the International Commission on Radiological Protection". Annals of the ICRP. ICRP publication 103 37 (2-4). 2007. ISBN 978-0-7020-3048-2. Retrieved 17 May 2012.
- "Converting rad to rem, Health Physics Society .".
- Goans, R E; and Wald, N (1 January 2005). "Radiation accidents with multi-organ failure in the United States". British Journal of Radiology: 41–46. doi:10.1259/bjr/27824773.
- "APPENDIX E: Roentgens, RADs, REMs, and other Units". Princeton University Radiation Safety Guide. Princeton University. Retrieved 10 May 2012.
- Sprawls, Perry. "Radiation Quantities and Units". The Physical Principles of Medical Imaging, 2nd Ed. Retrieved 10 May 2012.
- Gupta, S. V. (2009-11-19). "Louis Harold Gray". Units of Measurement: Past, Present and Future : International System of Units. Springer. p. 144. ISBN 978-3-642-00737-8. Retrieved 2012-05-14.
- Cantrill, S.T; H.M. Parker (1945-01-05). The Tolerance Dose. Argonne National Laboratory: US Atomic Energy Commission. Retrieved 14 May 2012.
- Dunning, John R.; et al (1957). A Glossary of Terms in Nuclear Science and Technology. American Society of Mechanical Engineers. Retrieved 14 May 2012.
- Bertram, V. A. Low-Beer (1950). The clinical use of radioactive isotopes. Thomas. Retrieved 14 May 2012.
- Guill, JH; Moteff, John (June 1960). "Dosimetry in Europe and the USSR". Third Pacific Area Meeting Papers - Materials in Nuclear Applications - American Society Technical Publication No 276. Symposium on Radiation Effects and Dosimetry - Third Pacific Area Meeting American Society for Testing Materials, October 1959, San Francisco, 12–16 October 1959. Baltimore: ASTM International. p. 64. LCCN 60-14734. Retrieved 15 May 2012.
- International Bureau of Weights and Measures (1977). United States National Bureau of Standards, ed. The international system of units (SI). NBS Special Publication 330. Dept. of Commerce, National Bureau of Standards. Retrieved 18 May 2012.
- International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), ISBN 92-822-2213-6
- Lyons, John W. (1990-12-20). "Metric System of Measurement: Interpretation of the International System of Units for the United States". Federal Register (US Office of the Federal Register) 55 (245): pp. 52242–52245.
- Hebner, Robert E. (1998-07-28). "Metric System of Measurement: Interpretation of the International System of Units for the United States". Federal Register (US Office of the Federal Register) 63 (144): p. 40339. Retrieved 9 May 2012.
- Handbook of Radiation Effects, 2nd edition, 2002, Andrew Holmes-Siedle and Len Adams
- 10 CFR 20.1004. US Nuclear Regulatory Commission. 2009.
- The Council of the European Communities (1979-12-21). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". Retrieved 19 May 2012.