Effective radiation dose
Effective dose is a dose quantity in the International Commission on Radiological Protection system of radiological protection.
It is the tissue-weighted sum of the equivalent doses in all specified tissues and organs of the human body and represents the health risk to the whole body, which is the probability of cancer induction and genetic effects, of low levels of ionising radiation. It takes into account the type of radiation and the nature of each organ or tissue being irradiated, and enables summation of organ doses due to varying levels and types of radiation, both internal and external, to produce an overall calculated effective dose.
The SI unit for effective dose is the sievert (Sv) which represents a 5.5% chance of developing cancer. The effective dose is not intended as a measure of deterministic health effects, which is the severity of acute tissue damage that is certain to happen, that is measured by the quantity absorbed dose.
The concept of effective dose was developed by Wolfgang Jacobi and published in 1975, and was so convincing that the ICRP incorporated it into their 1977 general recommendations (publication 26) as "effective dose equivalent". The name "effective dose" replaced the name "effective dose equivalent" in 1991. Since 1977 it has been the central quantity for dose limitation in the ICRP international system of radiological protection.
According to the ICRP, the main uses of effective dose are the prospective dose assessment for planning and optimisation in radiological protection, and demonstration of compliance with dose limits for regulatory purposes. The effective dose is thus a central dose quantity for regulatory purposes.
The ICRP also says that effective dose has made a significant contribution to radiological protection as it has enabled doses to be summed from whole and partial body exposure from external radiation of various types and from intakes of radionuclides.
The calculation of effective dose is required for partial or non-uniform irradiation of the human body because equivalent dose does not consider the tissue irradiated, but only the radiation type. Various body tissues react to ionising radiation in different ways, so the ICRP has assigned sensitivity factors to specified tissues and organs so that the effect of partial irradiation can be calculated if the irradiated regions are known. A radiation field irradiating only a portion of the body will carry lower risk than if the same field irradiated the whole body. To take this into account, the effective doses to the component parts of the body which have been irradiated are calculated and summed. This becomes the effective dose for the whole body, dose quantity E. It is a "protection" dose quantity which can be calculated, but cannot be measured in practice.
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