A summary and review of information on the biological effects of ionizing radiation are included in ICRP Publication 103 and, on the basis of the review, quantitative estimates are made of the consequences of radiation exposure, as discussed in Chapter 4. Estimates are made both for stochastic effects and for harmful tissue reactions. To reiterate
:
Stochastic effects are those for which the probability of an effect occurring, rather than its severity, is regarded as a function of dose, without threshold. The most important somatic stochastic effect is the induction of cancers, for which the risk must be regarded as increasing progressively with increasing dose received, without threshold. Similarly, at the dose levels involved in radiation protection, genetic effects are regarded as being stochastic.
Harmful tissue reactions are those for which the severity of the effect varies with the dose, and for which a threshold may exist. Examples of tissue reactions are the acute radiation effects and the late effects such as cataract of the lens of the eye, damage to blood vessels and impairment of fertility. The severity of these effects varies with the size of the radiation dose received, but they are not detectable at all unless a quite high threshold dose is exceeded.
The aim of radiation protection, as stated by the ICRP, is to prevent detrimental tissue reactions and to limit the probability of stochastic effects to levels deemed to be acceptable without unduly limiting the desirable benefits associated with the exposure. This aim is achieved by:
1. setting dose limits at levels that are sufficiently low to ensure that no threshold dose is reached, even following exposure for the whole of an individual’s lifetime – prevention of harmful tissue reactions;
2. keeping all justifiable exposures as low as is reasonably achievable, economic and social factors being taken into account, subject always to the boundary condition that the appropriate dose limits shall not be exceeded – limitation of stochastic effects.
The ICRP considered three levels related to the degree of tolerability of an exposure or risk. These are unacceptable, tolerable and acceptable. A limit represents a selected boundary in the region between ‘unacceptable’ and ‘tolerable’ levels. Tolerable implies that the exposure (or risk) is not welcomed, but can reasonably be tolerated, and acceptable means that the level of protection has been optimized and can be accepted without further improvement. The dose limits therefore represent the level at which continued exposure would begin to be unacceptable.
6.3.2 Recommended dose limits for workers
To limit stochastic effects, the ICRP recommends an annual effective dose limit for uniform irradiation of the whole body of 20 mSv, averaged over a period of 5 years. It is permissible to exceed 20 mSv in any one year, but the dose should never exceed 50 mSv in a year. For non-uniform irradiation of the body, weighting factors have been assigned to the various individual organs, relative to the whole body as 1.0, reflecting the harm attributable to irradiation of each organ. The sum of the weighted organ doses is known as the effective dose, E. Thus:
E =冱wTHT
T
where wT is the weighting factor for tissue T, and HT is the equivalent dose in tissue T. The annual limit on effective dose is 20 mSv, and so in any one year
冱wTHT £ 20 mSv
T
The weighting factors are given in Table 6.2.
The use of an annual effective dose limit of 20 mSv implies that, if the conditions of exposure were such that only a single tissue T were exposed, the limiting annual equivalent dose for that tissue would be:
Dose limitT = 20/wT mSv
For example, in the case of the lung, the weighting factor has a value of 0.12, and this implies an annual limit on equivalent dose to the lung of about 170 mSv. Similarly, for the thyroid, the weighting factor has a value of 0.04 and so the annual equivalent dose limit for the thyroid is 500 mSv. For most of the organs and tissues of the body, the ‘stochastic’ equivalent dose limits are lower than the threshold doses at which tissue reactions might start to occur (generally about 500 mSv, although a few tissues show higher radiosensitivities). Thus the restrictions on effective dose are sufficient to ensure the avoidance of tissue reactions in almost all tissues and organs. The exceptions are the bone surfaces, brain, salivary glands, skin, and the hands and the feet, for which an equivalent dose limit of 500 mSv in 1 year is recommended, and the lens of the eye, for which a limit of 150 mSv/year is currently recommended. However, in a subsequent statement in April 2011, the ICRP recommended a much reduced equivalent dose limit for the lens of the eye of 20 mSv in a year, averaged over defined periods of 5 years with no single year exceeding 50 mSv.
Recommended dose limits 51
Based on current ICRP risk factors, a 20 mSv effective (or whole body) dose leads to a fatal cancer risk of approximately 1 in 1000, which is, in an occupational situation, just tolerable.
Example 6.1
Calculate the allowable equivalent dose to the thyroid of a worker for a year in which he is exposed to non-uniform irradiation involving the whole body and the lung, as well as the thyroid. During the year he receives equivalent doses of 10 mSv to the whole body and 50 mSv to the lungs.
Using the weighting factor formula:
冱wTHT £ 20 mSv
T
wT (whole body) ¥ HT (whole body) + wT (lung) ¥ HT (lung) + wT (thyroid) ¥ HT (thyroid) £ 20 mSv
Thus,
1.0 ¥ 10 mSv + 0.12 ¥ 50 mSv + 0.04 ¥ HT (thyroid) = 20 mSv, in the limit that is,
10 mSv + 6 mSv + 0.04 HT (thyroid) = 20 mSv HT (thyroid) = 20 – 16 = 100 mSv
0.04
Thus, the worker is permitted to receive up to 100 mSv equivalent dose to the thyroid during the year in question.
Table 6.2 Tissue weighting factors
Tissue or organ Tissue weighting factor, wT
Gonads 0.08
Bone marrow (red) 0.12
Colon 0.12
Lung 0.12
Stomach 0.12
Breast 0.12
Bladder 0.04
Liver 0.04
Oesophagus 0.04
Thyroid 0.04
Skin 0.01
Bone surface 0.01
Brain 0.01
Salivary glands 0.01
Remainder 0.12
The values are averages across a population of all ages and both sexes. They may be applied to workers and to members of the public.
Example 6.2
Using the weighting factors in Table 6.2, calculate the implied limits for the gonads and the thyroid, assuming that each organ is irradiated completely in isolation.
For gonads, wT = 0.08, and so
implied annual limit = 20 = 250 mSv 0.08
For thyroid, wT = 0.04, and so
implied annual limit = 20 = 500 mSv 0.04
6.3.3 Notes on the dose limits for workers
The following points should be stressed in applying this system of dose limitation:
1. All unnecessary exposures should be avoided.
2. While it is permissible to average a worker’s dose over 5 years, the effective dose should not exceed 50 mSv in any single year.
3. The Commission lays considerable emphasis on the fact that only a few workers would be expected to receive annual doses close to the recommended limit. Experience shows that in many industries the distribution of doses has often been such that the average worker has received an annual whole-body equivalent dose of about 2 mSv. Using the risk factors quoted in Publication 103, this implies that the average risk of death in such occupations involving radiation exposure is comparable to the average risk in other industries which are normally considered ‘safe’.
4. The basis for control of the occupational exposure of women is the same as for men except that, when a pregnancy is declared, the level of protection to the fetus should be broadly similar to that for a member of the public.
5. Workplaces should be subject to classification:
Controlled area – in which normal working conditions require workers to follow well-established procedures.
Supervised area – where no special procedures are normally needed but exposure conditions are kept under review.
6.3.4 Recommended dose limits for individual members of the public
In Publication 103, the ICRP recommends an annual effective dose limit of 1 mSv for individual members of the public. However, it also recommends that, in special circumstances, a higher value of effective dose could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv/year. To prevent harmful tissue reactions, the ICRP recommends equivalent dose limits of 15 mSv/year for the lens of the eye and 50 mSv/year for the skin (Table 6.3). In their 2011 statement on the dose limit for the lens of the eye, the ICRP makes no reference to the dose limit for members of the public but the implication is that it would remain at 15 mSv/y.
Planned exposure situations 53
Table 6.3 Recommended dose limits*
Dose limit Occupational Public
Effective dose 20 mSv per year, averaged over defined period of 5 years†
*The limits apply to the sum of the relevant doses from external exposure in the specified period and the 50-year committed dose (to age 70 years for children) from intakes in the same period. †The effective dose should not exceed 50 mSv in any single year. ‡In special circumstances, a higher value of effective dose could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv/year. §For other organs, stochastic effects are limiting and hence the dose to these other organs is controlled by the limit on effective dose. **Based on the April 2011 Statement of the ICRP on dose limits for the lens of the eye.