ICRP = International Commission on Radiological Protection; An advisory body providing recommendations and guidance on radiation protection; Functioning since 1928.
While the use of ionising radiation for beneficial purposes can entail significant risks if not appropriately controlled, it needs to be treated with care rather than fear and its risks should be kept in perspective, both with the benefits of uses and with other risks. The procedures available to restrict the exposures from ionising radiation are sufficient, if used properly, to ensure that the associated risks remain a minor component of the spectrum of risks to which people are exposed.
TR, R T.
T R TR R Unit: Sievert
Radiation type Radiation weighting Alpha particles, fission fragments, heavy nuclei Electrons, muons & ionising photons factor w R 20 1 Protons 2 Neutrons Continuous function
1000 100 µ 10 1 0.1 0.01 1 ectron 1 MeV electr 2 1 MeV proton 3 article 1 MeV α-partic
Unit: Sievert
T Tissue/Organ Tissue weighting factor Bone marrow, Breast, Colon, Lung, 0.12 Stomach Bladder, Oesophagus, Gonads, 0.05 Liver, Thyroid Bone surface, Brain, Kidneys, 0.01 Salivary glands, Skin Aggregate value for 14 remainder 0.10 tissues
Adipose tissue; Adrenals; Connective tissue; Extra-thoracic airways; Gall bladder; Heart wall; Lymphatic nodes; Muscle; Pancreas; Prostate; SI Wall; Spleen; Thymus; Uterus/cervix.
The ICRP s tissue weighting factors quantify cancer-risk detriment to tissues, averaged over all ages and both genders.
Some radiations are more damaging than ionising photons, and stochastic effects are more likely in some tissues than in others. In order to improve the correlation between dose quantities applied in radiation protection, and the effects considered, two types of weighting factors have been introduced, a radiation weighting factor and a tissue weighting factor. These weighting factors are needed for the calculation of the effective dose.
The definition for the effective dose E, weighs the physics quantity absorbed dose D by weighting factors derived from radiobiology & radioepidemiology. The effective dose E is a protection quantity a convolution of physics, radiobiology and radio-epidemiology. The radiation weighting factor w R quantifies the effect of the micro-deposition of radiation energy in tissue, on its carcinogenic potential, and is intimately related to the structure of DNA, and the fact that carcinogenesis is related to radiation insult to DNA.
The tissue weighting factor quantifies the empirically determined radio-sensitivities of tissues, based on clinical observation and results from radiobiology & radioepidemiology.
Bone marrow; Colon Female breast Stomach Lungs are organs most susceptible to the development of radiationrelated cancer, whereas the Bone surface Brain Kidneys Salivary glands Skin are relatively insensitive.
Studies of exposed populations have confirmed the substantial breast cancer risk from ionising radiation. Risk is higher for exposure at younger ages, especially the age when the breasts develop. The ICRP's 2007 estimate of the risk of radiation- related breast cancer is 3 times higher than their 1990 estimate. Breast cancers contributed about 11% of the total radiation-related (excess) cancers, averaged over males and females.
Effective dose is a protection quantity, a unit of detriment. It is not a physics quantity (even though the ICRP has been stating this ever since 1990!)
600 500 400 300 200 100 0 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 Year
Since 1934, dose limits for occupational exposure have been sharply reduced as new knowledge regarding the potential negative health effects of ionising radiation came to light. Before 1934, there was no proper, formal regulation of dose limits. A limit of 2mGy per day, which amounts to approximately 600 msv per annum, was introduced in 1934. In 1950 five years after the detonation of two nuclear bombs in Japan the dose limit was lowered to 3 mgy per week, i.e. roughly 150 mgy per year.
Just over half a decade later, in 1956, the annual dose limit was further reduced to 50 msv. In 1990, the International Commission on Radiological Protection (ICRP), recommended a new set of dose limits for radiation workers as well as the general public the limit on effective dose was lowered to an average of 20 msv per radiation worker per year. The dose limit of 20 msv/yr was again recommended in the 2006 recommendations of the ICRP.
16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1950 1960 1970 1980 1990 2000 2010 2020 Year
This steady lowering of the dose limit was the direct result of improved risk quantification for radiation-related ill health, which led to a sharp increase in, especially, the estimated risk of radiation-related cancer, between 1960 and 1990.
5 ICRP est timate of radiatio on-induc ced cancer mortality risk (%..Sv -1 ) 4 3 2 1 0 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 Year
In the same period that the risk estimate for radiation-related cancer was repeatedly adjusted upward, the risk estimate for radiation-related heritable ill health was repeatedly lowered.
2.5 radiationl health ri -induced isk ICRP est timate of he eritable il (%.S Sv -1 ) 2.0 1.5 1.0 0.5 0.0 1970 1980 1990 2000 2010 2020 Year
Tissue injury and its various organspecific manifestations are commonly called tissue or organ reactions. The induction of tissue reactions is generally characterised by a dose- threshold.
The reason for the presence of this dose-threshold is that radiation damage (serious malfunction or death) of a critical population of cells in a given tissue needs to be sustained before injury is expressed in a clinically relevant form. Above the dose-threshold the severity of the injury, including impairment of the capacity for tissue recovery, increases with dose.
Early reactions Example * Inflammatory type Erythematous skin reaction * Cell loss type Mucositis, epidermal desquamation Late reactions * Generic type Vascular occlusion leading to tissue necrosis * Consequential Mucosal ulceration leading type to intestinal stricture
The present estimate of the ICRP is that the risk for radiation-related heritable ill health is 0.2% per Gy, i.e. 1 case in 500 live births per Gy. This value relates to continuous low dose rate exposures over 2 generations, i.e. doses to the grandparental and parental generations.
In the pre-implantation period of embryonic development, ionising radiation has an all-or-nothing effect on the embryo. It is found that the embryo either dies, or is not harmed at all. At doses of a few tens of mgy, lethal effects are very infrequent.
Malformations Gestation-age dependent patterns of in- utero radiosensitivity, with maximum radiosensitivity during the period of major organo-genesis. Threshold of ~100 mgy for the induction of malformations. The risk of malformations at doses in the range up to a few tens of mgy, is discounted by the ICRP.
Mental retardation A-bomb data supports the existence of a threshold of ~300 mgy for the induction of severe mental retardation in the most sensitive pre-natal period, namely 8 15 weeks post-conception. Risk of radiation-related severe mental retardation is essentially absent at doses in the range up to a few tens of mgy. Radiation-related loss in IQ is judged to be 25 IQ points per Gy during the period 8 15 weeks post-conception. The absence of a threshold dose for IQ reduction is possible. Because it is impossible to reliably measure IQ shifts below about 3 IQ points, doses below a few tens of mgy will be practically insignificant as far as IQ reduction is concerned.
In-utero irradiation and cancer risk The largest studies of in utero medical irradiation provided evidence of increased childhood cancer. Life-time cancer risk following in utero exposure will be similar to that following irradiation in early childhood. The ICRP (2005) judges that the risk posed by in-utero exposures to doses below the limit of 1 msv over the duration of the pregnancy, is negligible.
Maximum effective dose per Situation to which it applies year 100 msv Extraordinary, severe emergency situations; radiation workers. 20 msv Radiation workers trained, monitored. Occupational exposure. 1 msv Exposure without direct individual benefit; no training or monitoring. 0.01 msv Minimum value of any 0.01 msv Minimum value of any constraint.
Organisations are encouraged to develop RP Standards with dose constraints lower that the recommended limits, but not lower than a factor of ~10. Example: Set constraint on effective dose at 10 msv/yr, and lower it, in steps, to an eventual target value of 5 msv per year.
Maximum dose per year Situation to which it applies 20 msv Effective dose to radiation workers. 1 msv Total effective dose to member of public. 1 msv Radiation weighted dose to unborn baby over entire known period of pregnancy. 20 msv Equivalent dose to lens of eyes. 500 msv Equivalent dose to skin, hands & feet.
In all situations the constraints are complemented by the requirement to optimise the level of protection achieved. This is because there is presumed to be some probability of health effects even at small increments of exposure to radiation above the natural background.
Nuclides Exclusion activity concentration (Bq/g) Artificial α-emitters 0.01 Artificial β-emitters 0.1 Head-of-chain activity level, 238 U, 232 Th 40 K 10 1