|NEWS AND INFORMATION
|Year : 2012 | Volume
| Issue : 1 | Page : 55-56
Basis for the ICRP dose limits
Editor, Radiation Protection and Environment C/o Radiation Safety Systems Division, Bhabha Atomic Research Centre; Mumbai 400 085, India
|Date of Web Publication||6-May-2013|
Editor, Radiation Protection and Environment C/o Radiation Safety Systems Division, Bhabha Atomic Research Centre; Mumbai 400 085
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Pushparaja. Basis for the ICRP dose limits. Radiat Prot Environ 2012;35:55-6
The International Commission on Radiological Protection (ICRP) has developed, maintained, and elaborated the International System of Radiological Protection which is used world-wide as the common basis for radiological protection standards, legislation, guidelines, programs, and practice.
The System has been developed based on the current understanding of the science of radiation exposures and effects and value judgments taking into account societal expectations, ethics, and experience gained in application of the system through-out the world. The various scientific and technical committees of the ICRP examines all the available sources of radiological data including reference anatomical and physiological models of humans, biological studies at the molecular and cellular level, experimental animal studies and human epidemiological studies, including those conducted on the Japanese A-bombing survivors and medical exposure cases.
The ICRP Publication 103 (2007) states: "the primary aim of the Commission's Recommendations is to contribute an appropriate level of protection for people and the environment against the detrimental effects of radiation exposure without unduly limiting the desirable human actions that may be associated with such exposure."
One of the important operating parts of the System of Radiological Protection is the dose limits for occupational and public exposures. The ICRP dose limits are intended to prevent deterministic health effects, and limit the probability of stochastic effects. Deterministic effects are often of an acute nature, are mostly the result of damage/death of somatic cells following radiation exposure, and only appear if the radiation dose exceeds a threshold value.
The stochastic effects are cancer or heritable (genetic) effects and may occur in either mature somatic cells or through the mutation of germ (reproductive) cells. It is, however, conservatively assumed for the purpose of radiation protection that there is no safe level of exposure, i.e., a linear non-threshold (LNT) approach is assumed. This implies that all the radiation exposures should be kept "as low as reasonably achievable," below the dose limits. The LNT assumption is scientifically plausible, but has been challenged and further experimental work is necessary to determine the actual dose-response relationship.
The total harm to health experienced by the exposed group of people and their descendants is termed as detriment. The detriment is based on a consideration of a collection of attributes which measure harm caused by exposure to radiation. These attributes included lifetime attributable probability of death, the time lost if the attributable death occurs, the reduction of life expectancy, the annual distribution of the attributable probability of death, and the increase in the age-specific mortality rate.
It is recognized that the time of risk received (age at the time of exposure) is important, and the additional dose/risk received later in life is less important than the risks added in earlier years in life. This is due to latency period before probable onset of cancer after exposure to radiation and for majority of cancers the latency period can be as long as 15-30 years. For the occupational dose limit, the ICRP sought to select a dose that correlated to a most tolerable consequence based on the quantifiable attributes of health detriment.
The ICRP examined several possible values of doses that an individual could receive over a full working lifetime (age 18 years to 65 years). For the annual effective doses of 10, 20, 30 and 50 mSv, the corresponding levels of the attributable fatal cancer risk are compared with the most tolerable risk and the dose corresponding to this risk level is selected as the dose limit. Thus, the dose limit represents the peak level of fatal cancer risk at which continued exposure year after year would begin to cross an unacceptable level of risk. Broadly, the ICRP concluded that a risk of death, one in 1000/year is ordinarily accepted and it serves as a dividing line between what is just tolerable and what is intolerable or unacceptable.
ICRP determined that for the occupational dose limit, an approximate lifetime dose of 1 Sv (100 Rem) is tolerable. This works out to an annual average dose of 20 mSv (2 Rem). The dose limits cover exposures from both external sources and internal sources.
To allow for operational flexibility, the ICRP also recommended that the dose can exceed 20 mSv in any single year, but not to exceed 50 mSv (5 Rem) in any year, over a defined period of five working years.
Annual limits (for occupational exposure in planned exposure situations) on equivalent dose were also recommended for some tissues such as lens of the eye and skin to prevent deterministic effects (tissue reactions). Subsequent to the ICRP publication 103, the equivalent dose limit for the lens of the eye was reduced by the ICRP to 20 mSv a year from the earlier limit of 150 mSv a year. There is also provision for averaging the dose over a defined period of 5 years with a single year maximum of 50 mSv. This change was necessitated on re-examination of the evidence for radiation associated cataract risk and observed lower threshold dose for effects on lens of the eye.
Using the same approach for the public dose limit as for occupational dose limit, the ICRP examined the relation between annual effective dose and the attributes of fatal cancer, due to exposure for individuals aged 0-75 years. Based on the data, the ICRP estimates that the combined risk from the doses received from all the radiation sources, including exposure to natural sources modified by human activities, above 1 mSv (100 mRem)/year may be unacceptable, and will justify the introduction of protection actions for members of the public. The ICRP thus recommended the dose limit of 1 mSv in a year, with a higher value allowed in special circumstances ensuring that the average of 5 years not exceeding 1 mSv/year.
Exposure from normal background radiation sources is not included in the dose limit. However, the variations in the natural background radiation levels in one of the considerations in recommendation of the dose limit.
The dose to the members of the public is not obtained by individual monitoring as in the case of occupational exposure. It is determined by environmental measurements, diet habits and mathematical modeling. The dose also includes external exposures from the radionuclides present in air, soil or water.
Thus, regulatory controls on radionuclide releases from various routes for each source, including radioactive waste disposal sites, is necessary to ensure the public dose limit of 1 mSv (100 mRem) in a year in not exceeded and the corresponding risk level lies within the acceptability level for the public. Considering the negligible level of risk of cancer or malformation at a dose level of 1 mSv to the fetus, the ICRP recommends this dose as the limit for the pregnant women for the protection of the fetus (Extracted from ICRP publications).
In India, the national regulatory body, Atomic Energy Regulatory Board, stipulated maximum dose limit of 30 mSv (3 Rem) in any 1 year over a sliding 5 year period. Occupational exposures in Indian nuclear fuel cycle facilities are reported to be maintained at about one-tenth of the limit stipulated, and are at the levels of dose received from natural background radiation.