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Contents Intoroduction Preliminaries Chapter 1 Chapter 2
Chapter 3 Chapter 4 Appendix Index


Natural radiation and dose-equivalent limits


Man has always been exposed to radiation from his natural environment, the basic sources of natural radiation exposure being cosmic rays, radioactivity in rocks and soil, and radioactive nuclides incorporated into his tissues. The does of natural radiation that a person receives depends on a number of factors such as the height above sea-level at which he lives, the amount and type of radioactive nuclides in the soil in his neighbourhood, and the amount that he takes into his body in air, water and food. The total absorbed dose rate in most human tissues from natural radiation is about one-thousandth of a gray per year, but absorbed dose rates up to one-hundredth of a gray per year, or more, have been reported from certain limited areas of the would.


Man-made modifications of the environment and man's activities can increase the "normal" exposure to natural radiation. Examples of this include mining, flight at high altitudes, and the use of building materials containing naturally-occurring radioactive nuclides. Even living within a house is often sufficient to increase radiation exposure because restricted ventilation tends to lead to an accumulation of radioactive gases and their decay products.

Dose-equivalent limits for individual members of the public


Radiation risks are a very minor fraction of the total number of environmental hazards to which members of the public are exposed. It seems reasonable therefore to consider the magnitude of radiation risks to the general public in the light of the public acceptance of other risks of everyday life. This acceptance (when related to risks that could not be reduced or avoided entirely) is motivated by the benefits that would not otherwise be received, by an assessment of the social cost of achieving a possible reduction of risk, or by an implicit judgment that the risk is negligible.


The acceptable level of risk for stochastic phenomena for members of the general public may be inferred from consideration of risks that an individual can modify to only a small degree and which, like radiation safety, may be regulated by national ordinance. An example of such risks is that of using public transport. From a review of available information related to risks regularly accepted in everyday life, it can be concluded that the level of acceptability for fatal risks to the general public is an order of magnitude lower than for occupational risks. On this basis, a risk in the range of 10-6 to 10-5 per year would be likely to be acceptable to any individual member of the public.


The assumption of a total risk of the order of 10-2 Sv-1 (see paragraph 60) would imply the restriction of the lifetime dose to the individual member of the public to a value that would correspond to 1 mSv per year of life-long whole body exposure. For the reasons given in the following paragraphs, the Commission's recommended whole body dose-equivalent limit of 5 mSv (0.5 rem) in a year, as applied to critical groups, has been found to provide this degree of safety and the Commission recommends its continued use under the conditions specified in paragraphs 120-128.

Statement from the 1985
Paris Meeting of the International
Commission on Radiological Protection

Dose Limits for Members of the Public

In the recommendation on effective dose-equivalent limits* for members of the public, made in its 1977 Recommendations (IRCP Publication 26), two values were mentioned. The use of the limit of 5 mSv in a year was endorsed, but only under the conditions described in paragraphs 120 to 128 of ICRP Publication 26. For other circumstances the Commission recommended that it would be prudent to limit exposures on the basis of a lifetime average annual dose of 1 mSv.

The Commission's present view is that the principal limit is 1 mSv in a year. However, it is permissible to use a subsidiary dose limit of 5 mSv in a year for some years, provided that the average annual effective dose equivalent over a lifetime does not exceed the principal limit of 1 mSv in a year.

With this limitation on the effective dose equivalent, the non-stochastic organ limit of 50 mSv in a year becomes unnecessary for most organs. However, since the dose equivalents in the skin and the lens of the eye are not included in the computation of effective dose equivalent for the individual, organ dose limits are still needed for these two tissues. The recommended dose-equivalent limit for both the skin and the lens is still 50 mSv in a year for members of the public.

* The Commission's dose-equivalent limits apply to the sum of the effective dose equivalent resulting from external exposure during 1 year and the committed effective dose equivalent incurred from that year's intake of radionuclides.

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