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3. Radiation dose limit

3.1 RATIONALE AND LEGAL BASIS OF DOSE-EQUIVALENT LIMITS

The purpose of a system of dose limits* is to ensure that the radiation dose received by any person (other than an accidental exposure, or a deliberate exposure as in medical diagnosis) is such that:

  • the dose is below the threshold for any biological effect (non-stochastic or deterministic) which requires a minimum dose for expression; and
  • the probability of any effect of the all-or-nothing (stochastic) type is small enough to be acceptable to the individual and to society.

The system of radiation dose limits in use in Canada (and in most other countries) is based on the recommendations of the International Commission on Radiological Protection (ICRP). Recommendations of the ICRP are voluntary but a legal basis for the limits is provided by the NSC Act Radiation Protection Regulations and Regulations O.C. 554-79 (1979) of the Public Health Protection Act of Quebec.

* In Canada, both the "effective dose limit" and the "equivalent dose limit" are regulated. See the Table in Section 3.8 for the effective or whole body dose limits. See Appendix B for equivalent dose limits to organs. See Glossary of Terms (Appendix K). For the sake of brevity, the effective dose and equivalent dose limit will be referred to in this Manual as the "dose limit"..

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3.2 DOSE LIMITS FOR OCCUPATIONALLY EXPOSED PERSONS

Any person who is exposed to ionizing radiation as a direct and necessary condition of his occupation, business or employment is said to be "occupationally exposed" and is subject to the dose limits for this group set out in Section 3.8. The category of occupationally exposed persons includes both "Radiation Users" as defined in Section 2.6.2 of this Manual and "Nuclear Energy Workers" as defined by the CNSC (see Section 2.6.3).

The effective dose limit for Nuclear Energy Workers, as prescribed by the CNSC, is 100 mSv (10,000 mrem) for 5 years (with a maximum of 50 mSv in any given year or an average of 20 mSv/yr for 5 years) for whole-body exposure. For Radiation Users and "members of the public" the corresponding limit is 1 mSv (100 mrem) per year.

At McGill University, the category "Radiation User" is employed to designate personnel who, from the strictly legal point of view, could be regarded as non-radiation workers or as members of the public. The dose limit for Radiation Users is, the same as that for members of the public. While it is legal for a Radiation User to receive more than 1 mSv per annum and up to the 50 mSv limit, any Radiation User who does so, or is likely to do so, must be re-classified as a Nuclear Energy Worker.

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3.3 THE ALARA PRINCIPLE

The effective and equivalent doses received by Radiation Users or other personnel, are dose limits. They are in no sense "dose allotments" which can and should be used up. On the contrary, the guiding principle of all radiation work is: the dose should be As Low As Reasonably Achievable, economic and social factors being taken into account. This is called the "ALARA Principle" and is central to all radiation safety. In fact, most Radiation Users are able to maintain an annual exposure not only well below the legal limit but well below even the lower limit operating in McGill University for this category of radiation workers. Similarly, the annual exposure of Nuclear Energy Workers, while exceeding the lower limit of 1 mSv per year, is normally well below the CNSC prescribed average effective dose limit of 20 mSv per year (based on a 100 mSv dose in 5 years).

Any Radiation User whose annual or quarterly dose, as measured by external monitoring or calculated from the results of bioassay procedures, greatly exceeds the normal value either for that individual or for persons carrying out similar work, is subject to investigation by the Permit Holder, in co-operation with the Environmental Safety Office. Persistently high levels may indicate that the individual concerned should be re-classified as a Nuclear Energy Worker.

The principle set out above also applies to Nuclear Energy Workers (NEWs) although the "normal value" for such persons is expected to be higher than for Radiation Users. In addition, there is an overall investigatory level of 10 mSv (1000 mrem) per 6 months, i.e. one-tenth of the statutory limit for NEWs, or 5 mSv (500 mrem) per quarter. Any recorded dose in excess of this level should be regarded as an indication of a need to improve radiation safety, even though no legal transgression has occurred.

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3.4 DOSE LIMITS FOR INTERNAL SOURCES

Both external and internal radiation may contribute to the dose received by an individual. External radiation arises from radiation sources, including X-ray machines, particle accelerators and radioisotope sources, which are external to the body. Internal exposure occurs when radioactive material is ingested, swallowed, inhaled or otherwise enters the body and is deposited in body tissues. Internal contamination is particularly hazardous for radioactive materials which have long half-lives and accumulate in bone or lung (e.g. radium, strontium-90), but all internal radiation carries some risk and should be avoided. For this reason stringent precautions are needed in laboratories where unsealed (solid, liquid and gaseous) radioisotopes are handled.

Appendix C of the Manual lists the Annual Limit on Intake (ALI) for commonly used radioactive materials. The ALI for a given radionuclide represents the activity, in Becquerels, that will deliver an effective dose of 20 mSv during the 50 years after it is taken into the body. For a given radionuclide, the ALI depends on the physical characteristics of the nuclide and on its chemical form, which in turn determines its metabolism in the body (see Section 5.5, for ALI in terms of monitoring radioisotope intake).

If a long-lived radionuclide is deposited in the body, such that both the physical decay and the biological excretion are small, the body tissues will continue to be irradiated for a long period, in some cases for the rest of the person's life. The committed dose* is the total whole-body dose arising from a single intake. The annual committed dose is approximately 1/50th of the committed dose, although in practice the annual committed dose is highest in the first year and decreases thereafter, at a rate that depends on the rates of decay and excretion. The annual committed dose, if any, has to be taken into account in assessing the total annual dose received by a radiation worker.

* Strictly speaking, the term is committed dose-equivalent: see note in paragraph 3.1. The CNSC utilizes the term, "committed effective dose-equivalent" to take account of the fact that an ingested radionuclide irradiates body tissues having different radiosensitivities, the whole-body dose-equivalent being the weighted sum of the individual tissue contributions.

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3.5 PREGNANT RADIATION USERS AND NUCLEAR ENERGY WORKERS

The dose limits set out in Section 3.8 include special provisions for female radiation workers of reproductive age:

  • the annual maximum permissible dose must be evenly distributed throughout the year;
  • if pregnancy is diagnosed, the worker's duties must be such that the dose to the abdomen after the licensee has been informed of the pregnancy shall not exceed the balance of pregnancy limit of 4 mSv (400 mrem) as prescribed by the CNSC.

These special limits apply at McGill University to all female radiation workers designated as Nuclear Energy Workers. Radiation Users are, by definition, working under conditions such that their annual dose will not exceed 1 mSv, which is well below the limits stipulated above. Hence, no additional restriction need be placed on pregnant Radiation Users if the working conditions are such that the 1 mSv annual limit can be adhered to without difficulty. A pregnant Nuclear Energy Worker, on the other hand, may need to be assigned to other duties if there is a risk that the 4 mSv balance of pregnancy limit may be approached or exceeded.

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3.6 NON-RADIATION WORKERS AND THE GENERAL PUBLIC

The legal dose limit for non-radiation workers and members of public (i.e. everyone except NEWs) is 1 mSv (100 mrem) per annum, i.e. one-hundredth of the maximum limit for Nuclear Energy Workers. This is the case under both the CNSC and Quebec Act Respecting Occupational Health and Safety (see Section 3.8). This limit covers radiation exposure of all types, except those arising from background radiation and medical procedures, whether received inside or outside university premises. Since McGill has no control over radiation sources outside the University, it should never be assumed that the radiation exposure at a given point within the University is the only source of exposure of the staff concerned. Dose levels in university premises should be interpreted with this in mind.

The ALARA principle applies to non-radiation workers as well as Nuclear Energy Workers. Every effort must therefore be made to reduce the doses received by "other personnel" and members of the public to a minimum level. This applies to any situation in which such persons are not directly involved in the work but may nevertheless be exposed to radiation to some extent. Such exposure may occur, for example, to clerical and other non-academic staff within a department using radiation sources, to members of adjoining departments and to members of the public in areas adjacent to buildings housing major radiation-emitting equipment. The public may also incur exposure when radioactive waste is disposed of via the sewers or into the atmosphere.

The difference between a "Radiation User" and a non-radiation worker or member of the public lies in the circumstances in which each is exposed to radiation. The latter is exposed incidentally or randomly, because he/she happens to come into the vicinity of radiation sources, of which he/she has no direct knowledge, interest or control. In contrast, the Radiation User is systematically exposed as a result either of his/her own work or of work carried out by colleagues in the same laboratory or department.

A corollary of the definition of a Radiation User is that no person outside the department or laboratory in which sources are stored or used, for example a member of a neighbouring department, should be subject to a level of exposure which would require him/her to be classified as a Radiation User. Shielding should therefore be sufficient to reduce the radiation levels in adjacent areas, which are outside the control of the Permit Holder concerned, to less than 1 mSv per annum (excluding background), occupancy being taken into account. In most cases this is not only feasible but corresponds to present practice. Radiation levels in adjacent areas higher than 1 mSv per year, up to 5 mSv per year, are legal but not recommended. Where it is difficult or impossible to meet this recommendation, the matter should be referred to the Radiation Safety Officer.

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3.7 ACTION LEVELS

The use of action levels plays an important part of an overall radiation management program. Action levels are designed to alert radiation safety personnel, before regulatory limits are reached.

Action levels are defined as "a specific dose of radiation or other parameter that, if reached, may indicate a loss of control of part of a Permit Holder's radiation protection program, and triggers a requirement for specific action to be taken". The primary goal of the action to be taken is to prevent a re-occurrence of the events. The table below lists the action level for the corresponding activity.

Table of Action Levels is shown below.
Action levels have been identified as required for the following activities:

Activity Initial respon- sibility of: Action level Action
Log occurrence & action taken (attempt to prevent future occurrences)
Personnel Dosimetry for:
(whole body/ extremity)
► NEW1 RSO2 > 10 mSv3 / 2 consecutive periods (3 mo. each) Investigate cause of exposure in all cases
► Pregnant NEW RSO > 0.1 mSv/month
► Radiation user RSO > 0.5 mSv/ 2 consecutive periods (3 mo. each)
► Public RSO Not applicable
Thyroid Bioassay RSO > 1000 Bq4 (investigation Level)
> 10,000 Bq (Reporting Level)
Investigate and report to CNSC5
Leak Testing for Sealed Radioisotopes RSO >200 Bq Investigate and dispose of sealed radioisotope
Radioactive Surface Contamination (i.e. wipe tests) Permit Holder & RSO >0.5 Bq/cm2 Investigate cause and decontaminate
Decommissioning RSO >0.5 Bq/cm2 for all emitters except alphas
>0.05 Bq/cm2 for alpha emitters
Investigate cause of contamination and decontaminate in both cases
Package Receipt Receiving Staff or Laboratory Personnel Damaged Package or radiation level > than package designation Reinforce CNSC guidelines:
► CNSC INFO 0426 "Receiving Radioactive Packages"
► CNSC "Packaging and Transport of Nuclear Substances Regulations"

1Nuclear Energy Worker
2Radiation Safety Officer
3millisievert
4Becquerel
5Canadian Nuclear Safety Commission

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3.8 REQUIREMENTS FOR RADIATION AND NON-RADIATION WORKERS

For each occupational and non-occupational classification, there are specific effective or whole body dose limits and requirements that need to be fulfilled. The requirements are summarized and presented in the table below.

Classification Effective Dose mSv (mrem)1 Requirements
Nuclear Energy Worker 2 100 (10000) / 5 yrs3
Note: A maximum of 50 (5000)6 in any one year or an average of 20 (2000)/yr
  • one year dosimetry period
  • doses likely to surpass 1 mSv/yr
  • mandatory personnel monitoring
  • medical surveillance
Pregnant Nuclear Energy Worker 4(400)
  • balance of pregnancy
  • mandatory personnel monitoring
  • medical surveillance
Radiation User 4 1 (100)/yr5
  • doses not to surpass 1 mSv/yr
  • recommended personnel monitoring
Public 1 (100)/yr
  • one calendar year

1mSv (mrem) = millisievert (millirem)
2CNSC classifications with legal dose limits. Includes pregnant Nuclear Energy Worker until employer informed of pregnancy.
3Yrs = years
4McGill classification with on legal dose limits, just a guideline. In fact, the maximum permissible dose for Radiation Users is the same as for the public, i.e. 1 mSv/yr.
5Yr = year