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What is Radioactive Dose Equivalent?

Dose equivalent is defined as the product of the dose D and a quality factor Q, which is based on the type of radiation (e.g. alpha, beta, gamma), so that H = D.Q. The dose equivalent is intended to estimate the biological effect of different types of radiation.

For equivalent dose, the unit corresponding to rads is the rem (roentgen equivalent man). If the absorbed dose is in grays then the unit for dose equivalent is sievert (Sv). Thus, 1 Sv = 100 rem. Roughly 1 rem is the average dose received in three years of exposure to natural radiation. One sievert is at the lower end of a range of doses that are likely to cause radiation sickness.

The chief sources of radiation exposure are classified into three categories: natural sources, medical sources and other sources. Natural sources are cosmic rays, indoor radon, radionuclide and terrestrial radiation. Medical sources are diagnostic X-rays and radionuclide used in diagnosis or radiotherapy. Other sources are nuclear fuel cycle, drinking water, building materials, radiation through reactor accidents, weapons, and weapons tests.

Members of the public or workers in the nuclear industry may receive exposure to what are commonly referred to as "low" doses of radiation. This is not precisely defined but it would include doses below 10 mSv per year. At doses below 1 Sv (100 rem), there is little likelihood of radiation sickness. However, much lower doses of 0.2 Sv (20 rem) or above increase the risk of cancer.

High radioactive doses are above 3 Gy (300 rad), which are very dangerous. Doses above 6 Gy (600 rad) are almost always fatal, leading to death within months. Very high doses are expressed in terms of grays. Radiation above 1 Gy causes a complex of symptoms such as nausea and blood changes, known as radiation sickness.

A further weighting factor can be applied depending on the part of the body or type of tissue that is exposed to the radiation.