Sunday, December 27, 2009

Radiation protection : Application of individual dose limits

3- Application of individual dose limits

A limit should be applied to the dose received by any individual as the result of all the practices (other than medical diagnosis or treatment) to which he or she is exposed.

Proper justification is also required for the use of radiation in diagnostic medicine. Few of us would question the practice: the benefits are undoubted even though individual doses for some examinations, and collective doses generally, are high. Nevertheless each procedure needs to be judged on its own merits. A mass X ray screening pro­gram for cancer that might cause more cancers than it was likely to reveal would clearly be unacceptable. For this reason, there is unlikely to be clinical justification for the routine screening of employees except in special circumstances, such as the prevention of tuberculosis. Medical irradiation during pregnancy in particular requires clear justification and careful techniques. Radiological examinations for legal or insurance purposes. are usually unwarranted since they do not benefit the health of the exposed person.

Practices are proposed from time to time that fail to satisfy the test of justification: these include the production of toys and jewelers containing radioactive material and other devices such as security tags for which there are perfectly adequate non-radioactive alternatives.

The third requirement for practices is an obligation not to expose individuals and their descendants to an unacceptable degree of risk. This is fulfilled by imposing strict dose limits and applying the principle of optimization of protection. The International Basic Safety Standards (BSS) specify dose limits for workers of 20 mSv per year (averaged over a five-year period, with no more than 50 mSv in any year) and for members of the public of 1 mSv in a year.




Effective dose

Prime limit






Equivalent dose

Lens of eye



Area of skind






Table (2) : International dose limit and constraints ( mSv/y).

a = For students and apprentices. three-tenths of these values.

b = There are no agreed international values; constraints should be set according to the particular circumstance (e.g. type of industry or operation).

C = Prospective value for a single new source of exposure.

d = Averaged over any 1 cm2 of skin regardless of area exposed.

e = Forearms and ankles as well as hands and feet.

These prime limits, expressed in terms of effective dose, are intended to control the incidence of serious effects such as cancer and hereditary harm that involve an element of probability. Another set of limits, expressed in terms of equivalent dose, is to protect the eyes, skin and extremities against other forms of damage.
In some cases, as for example after an accident that releases radioactive material to the environment or when high indoor levels of radon occur, it may be necessary to intervene to reduce the exposure of people. Under such circumstances, ICRP recommends a system of radiological protection for intervention based on two further principles that mainly differ from the first set in that they omit dose limits for individuals. Specifying limits, however, might require measures out of all proportion to the likely benefit and would, therefore, be in conflict with the first principle. The application of this system again requires the exercise of judgement.
Both systems of radiological protection are endorsed in the International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources or BSS, which are sponsored by the IAEA and five other international organizations.
There are two common misconceptions about dose limits. The first is that they mark an abrupt change in biological risk, a line of demarcation between safe and unsafe. It should. be clear from the discussion on dose and risk that this is not so. It should also be apparent from the fact that there are different dose limits for workers and members of the public. These limits differ because higher risks are seemed- more acceptable for workers, who receive a benefit from their employment, than for members of the public, whose risk is involuntary.
The second misconception is that keeping doses below the limits is the only important requirement in radiological protection. On the contrary, the overriding require­ment is to keep doses as low as reasonably achievable. This is reflected in the increasing emphasis on investigations levels, which are, of course, set below dose limits.

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