The IAEA has developed a system of categorization of radioactive sources [11] to provide a simple, logical means for ranking them based on their potential to cause harm to human health. In addition, it provides a means for grouping the applications in which these sources are used into discrete categories.
In recognition of the fact that human health is of paramount importance, the categorization system is based primarily on the potential for radioactive sources to cause deterministic health effects. The categorization system is therefore based on the concept of ‘dangerous sources’ which are quantified in terms of D-values. The D-value is the radionuclide-specific activity of a source which, if not under control, could cause severe deterministic effects for a range of scenarios that include both external exposure from an unshielded source and inadvertent internal exposure following dispersal (such as fire or explosion) of the source material.
The activity of a radioactive material (A) in a source varies over many orders of magnitude; D-values are therefore used to normalize the range of activities in order to provide a reference in comparing risks – this is done by taking the activity A of the source (in TBq) and dividing it by the D-value for the relevant radionuclide.
In some situations it may be appropriate to catagorize a source on the bases of A/D alone, for example, when the practice for which the source may be used is unknown or not confirmed, as may happen at the time of import or export of the source. However, when the circumstances of use of the source is known, the regulatory body may make a judgement to modify this initial categorization using other information about the source or its use. In some circumstances it may be convenient to assign a category on the basis of the practice in which the source is used (see Table 1).
Within this categorization system, sources in Category 1 are considered to be potentially the most ‘dangerous’ because they can pose a very high risk to human health if not managed safely and securely. An exposure of only a few mintes to an unshielded Category 1 may be fatal. At the lower end of the categorization system, sources in Category 5 are potentially the least dangerous; however, even these sources could give rise to doses in excess of the dose limits if not properly controlled, and therefore need to be kept under appropriate regulatory control.
A detailed and comprehensive description of the categories for sources may be found in Ref. [11].
Throughout Sections 5 and 6, sources and devices are categorized according to their potential to cause harm, as described in this outline.
| Category | Sourcea and practice | Activity ratiob (A/D) |
| 1 | Radioisotope thermoelectric generators (RTGs) Irradiators Teletherapy sources Fixed, multi-beam teletherapy (gamma knife) sources | A/D ≥ 1000 |
| 2 | Industrial gamma radiography sources High/medium dose rate brachytherapy sources | 1000 > A/D ≥ 10 |
| 3 | Fixed industrial gauges that incorporate high activity sources Well logging gauges | 10 > A/D ≥ 1 |
| 4 | Low dose rate brachytherapy sources (except eye plaques and permanent implants) Industrial gauges that do not incorporate high activity sources Bone densitometers Static eliminators | 1 > A/D ≥ 0.01 |
| 5 | Low dose rate brachytherapy eye plaques and permanent implant sources X ray fluorescence (XRF) devices Electron capture devices Mossbauer spectrometry sources Positron emission tomography (PET) check sources | 0.01 > A/D and A > exempt |
a Factors other than A/D alone have been taken into consideration in assigning the sources to a category.
b This column can be used to determine the category of a source purely on the basis of A/D. This may be appropriate, for example, if the practice is not known or is not listed, if sources have a short half-life and/or are unsealed, or if sources are aggregated.
| Category of source | Risk in being close to an individual source | Risk in the event that the radioactive material in the source is dispersed by fire or explosion |
| 1 | Extremely dangerous to the person: This source, if not safely managed or securely protected, would be likely to cause permanent injury to a person who handled it or who was otherwise in contact with it for more than a few minutes. It would probably be fatal to be close to this amount of unshielded radioactive material for a period in the range of a few minutes to an hour. | This amount of radioactive material, if dispersed, could possibly — although it would be unlikely — permanently injure or be life threatening to persons in the immediate vicinity. There would be little or no risk of immediate health effects to persons beyond a few hundred metres away, but contaminated areas would need to be cleaned up in accordance with international standards. For large sources the area to be cleaned up could be a square kilometre or more.a |
| 2 | Very dangerous to the person: This source, if not safely managed or securely protected, could cause permanent injury to a person who handled it or who was otherwise in contact with it for a short time (minutes to hours). It could possibly be fatal to be close to this amount of unshielded radioactive material for a period of hours to days. | This amount of radioactive material, if dispersed, could possibly – although it would be very unlikely — permanently injure or be life threatening to persons in the immediate vicinity. There would be little or no risk of immediate health effects to persons beyond a hundred metres or so away, but contaminated areas would need to be cleaned up in accordance with international standards. The area to be cleaned up would probably not exceed a square kilometre.a |
| 3 | Dangerous to the person: This source, if not safely managed or securely protected, could cause permanent injury to a person who handled it or who was otherwise in contact with it for some hours. It could possibly — although it would be unlikely — be fatal to be close to this amount of unshielded radioactive material for a period of days to weeks. | This amount of radioactive material, if dispersed, could possibly – although it would be extremely unlikely — permanently injure or be life threatening to persons in the immediate vicinity. There would be little or no risk of immediate health effects to persons beyond a few metres away, but contaminated areas would need to be cleaned up in accordance with international standards. The area to be cleaned up would probably not exceed a small fraction of a square kilometre.a |
| 4 | Unlikely to be dangerous to the person: It is very unlikely that anyone would be permanently injured by this source. However, this amount of unshielded radioactive material, if not safely managed or securely protected, could possibly — although it would be unlikely — temporarily injure someone who handled it or who was otherwise in contact with it for many hours, or who was close to it for a period of many weeks. | This amount of radioactive material, if dispersed, could not permanently injure persons.b |
| 5 | Most unlikely to be dangerous to the person: No one could be permanently injured by this source.b | This amount of radioactive material, if dispersed, could not permanently injure anyone.b |
a The size of the area to be cleaned up would depend on many factors (including the activity, the radionuclide, how it was dispersed and the weather).
b Possible delayed health effects are not taken into account in this statement.