The primary aim of the ICRP’s Recommendations is to contribute to 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. In protecting individuals, it is the control (in the sense of restriction) of radiation doses that is important, no matter what the source. In view of what is known about the effects of radiation, the human health objectives are relatively straightforward: to manage and control exposures to ionising radiation so that deterministic effects are prevented and the risks of stochastic effects are reduced to the extent reasonably achievable. Before examining how these objectives are achieved, however, it is first useful to consider the situations that would result in radia­tion exposure in the first place.

The ICRP currently recognises three types of exposure situations: (i)

occur unintentionally to those where there is a clear intention to perform a malevolent act. Specific guidance has been given in relation to radiological attacks.15

(ii) Emergency exposure situations, which are unexpected situations such as those that may occur during the operation of a planned situation, requiring urgent attention. They are, inevitably, unpredictable in detail, and often require particular attention being paid to deterministic health effects.

(iii) Existing exposure situations, which are exposure situations that already exist when a decision on control has to be taken, such as those caused by natural background radiation.

Individuals may be exposed to radiation from more than one source. Provided that doses are below the threshold for deterministic effects (harmful tissue reactions), the presumed proportional relationship between the additional dose attributable to each situation and the corresponding increase in the probability of stochastic effects makes it possible to deal separately with each one.

The term ‘‘practice’’ has become widely used in radiological protection and denotes an activity that causes an increase in exposure to radiation or in the risk of exposure to radiation. A practice can be a business, trade, industry or similar activity. It can also be a government undertaking or a charity. Regardless of its purpose, however, it is implicit in the concept of a practice that the radiation sources that it introduces or maintains can be controlled directly by action on that source.

The exposure of people to ionizing radiation can also be categorized in different ways:

(i) Medical exposure of patients, which includes radiation exposure resulting from diagnostic, interventional and therapeutic procedures.

(ii) Occupational exposure, in which radiation exposure is incurred as a result of work.

(iii) Public exposure, which includes all exposures of the public other than occupational or medical exposure of patients, and includes exposures of the embryo and foetus of pregnant workers.

Of course any particular individual could belong simultaneously to all three categories.

‘‘Patients’’ are defined as individuals who receive an exposure to radiation associated with a diagnostic, interventional or therapeutic procedure. Dose limits and dose constraints do not apply to individual patients because they may reduce the effectiveness of the patient’s diagnosis or treatment, thereby doing more harm than good. Emphasis is therefore placed on the justification of the medical procedures and on the optimisation of protection and, for diagnostic procedures, the use of diagnostic reference levels.

‘‘Workers’’ are defined as any person who is employed, and who has recognised rights and duties in relation to occupational radiological protection.

Workers in medical professions involving radiation are occupationally exposed, and air crew may also be considered to lie in this category — but not ‘‘frequent fliers’’. (Exceptional cases of cosmic radiation exposures, such as exposure in space travel, where doses may be significant and some type of control war­ranted, are dealt with separately, taking into account the special type of situations that can give rise to this type of exposure).

An important function of an ‘‘employer’’ is that of maintaining control over the sources of exposure, and over the protection of workers who are occupa­tionally exposed. In this respect, the classification of areas of work is preferable to the classification of workers. There are usually two types of designation — ‘‘controlled areas’’ and ‘‘supervised areas’’. A ‘‘controlled area’’ is a defined area in which specific protection measures and safety provisions are, or could be, required for controlling normal exposures or preventing the spread of contamination during normal working conditions, and preventing or limiting the extent of potential exposures. A ‘‘supervised area’’ is one in which the working conditions are kept under review but for which special procedures are not normally needed. A controlled area is often within a supervised area, but need not be. Workers in controlled areas of workplaces are of necessity well informed and specially trained, and form a readily identifiable group. Such workers are monitored for radiation exposures incurred in the workplace, and occasionally may receive special medical surveillance.

Particular attention is paid to pregnant workers and breast feeding mothers. If a female worker has declared that she is pregnant, additional controls have to be considered to protect the embryo or foetus, to a level that is equivalent to that provided for members of the public. The working conditions of a pregnant worker should therefore be such as to ensure that the additional dose to the embryo or foetus would not exceed about 1 mSv during the remainder of the pregnancy. The principal implication is that the employer should carefully review the exposure conditions of pregnant women and, if required, alter their working conditions so that the probability of accidental doses and radionuclide intakes is extremely low.16,17

Finally, a ‘‘member of the public’’ is defined as any individual who receives an exposure that is neither occupational nor medical. A large range of different natural and man-made sources contribute to the exposure of members of the public but, in general, each source will result in a distribution of doses over many individuals. For the purposes of protection of the public, the term ‘‘critical group’’ has long been used to characterise an individual receiving a dose that is representative of the more highly exposed individuals in the population. Dose restrictions were then applied to the mean dose in the appropriate critical group. A considerable body of experience has now been gained in the application of the critical group concept, particularly in the UK. There have also been developments in the techniques used to assess doses to members of the public, including the increasing use of probabilistic techniques. The adjective ‘‘critical’’ has also had the connotation of a ‘‘crisis’’, which was never intended by ICRP. Furthermore, the word ‘‘group’’ can be confusing in the context where the assessed dose is to an individual.

So the ICRP now recommends the use of the “Representative Person’’ for the purpose of radiological protection of the public instead of the earlier critical group concept.18 This Representative Person may be real or hypothetical, but it is important that the habits (e. g., consumption of foodstuffs, breathing rate, location, usage of local resources etc.) used to characterise the Representative Person are typical habits of a small number of individuals representative of those most highly exposed, and not the extreme habits of a single member of the population. Thus although consideration may be given to some extreme or unusual habits, they should not dictate the characteristics of the Representative Persons considered. Dose coefficients are available for the calculation of pro­spective doses to different age categories, but for practical reasons it is now recommended that three age categories be used: 0-5 years (infant); 6-15 years (child); and 16-70 years (adult), the dose coefficients and habit data for a 1 year old, a 10 year old, and an adult being used respectively.

All of these concepts and definitions need marshalling together in order to provide advice that is both consistent and logical across all exposure situations, and across all categories of exposure. In order to do so, it is necessary to construct some form of principled framework. Such a framework obviously needs to be based on the scientific information that exists, and the LNT model, whilst also allowing for the incorporation and interpretation of new informa­tion as it arises. However, it also needs to be able to accommodate other factors relating to sociological, financial and other relevant considerations if it is to be of value as a decision making tool. The ICRP has attempted to rise to this challenge by basing its advice on the following three key principles:

(i) The Principle of Justification: any decision that alters the radiation exposure situation should do more good than harm.

(ii) The Principle of Optimisation of Protection: the likelihood of incurring exposure, the number of people exposed and the magnitude of their individual doses should all be kept as low as reasonably achievable, taking into account economic and societal factors.

(iii) The Principle of Application of Dose Limits: the total dose to any individual from regulated sources in planned exposure situations, other than medical exposure of patients, should not exceed the appropriate limits specified by the ICRP.

The principles of justification and optimisation apply in all three exposure situations, whereas the principle of application of dose limits applies only to doses expected to be incurred with certainty as a result of planned exposure situations.