Types of ionizing radiation differ in the way in which they interact with biological materials, so that equal absorbed doses (meaning equal amounts of energy deposited) do not necessarily have equal biological effects.
For instance, 1 Gy to tissue from alpha radiation is more harmful than 1 Gy from beta radiation because an alpha particle, being slower and more heavily charged, loses its energy much more densely along its path. In order to put all the different types of ionizing radiation on an equal basis with respect to their potential for causing harm, we need another quantity. This is the equivalent dose. It is expressed in a unit called the Sievert, symbol Sv. Submultiples of the Sievert are commonly used, such as the millisievert, mSv, which is one-thousandth of a Sievert. The Sievert is named after the Swedish physicist Rolf Sievert .
Equivalent dose is equal to the absorbed dose multiplied by a factor that takes into account the way in which a particular type of radiation distributes energy in tissue so that we can allow for its relative effectiveness to cause biological harm. For gamma rays, X rays, and beta particles, this radiation-weighting factor is set at 1, so the absorbed dose and equivalent dose are numerically equal. For alpha particles, the factor is set at 20, so that the equivalent dose will be 20 times the absorbed dose. Values of the radiation weighting factor for neutrons of various energies range from 5 to 20.
Defined in this way, the equivalent dose provides an index of the likelihood of harm to a particular tissue or organ from exposure to various types of radiation regardless of their type or energy. So 1 Sv of alpha radiation to the lung, for example, would create the same risk of inducing fatal lung cancer as 1 Sv of beta radiation. The risk to the various parts of the human body varies from organ to organ. For example, the risk of fatal malignancy per unit equivalent dose is lower for the thyroid than for the lung.