System of radiation protection module 2
Exposure Categories and Situations
Categories of exposure and exposure situations are used to consider how best to approach radiological protection in different circumstances.
Exposure Categories are: occupational, public, and medical.
Exposure Situations are: planned, existing, and emergency.
Categories and Situations are considered together to help guide the best approach to radiological protection in a particular circumstance.
Exposure of workers incurred as a result of their work |
Exposure of members of the public other than occupational and medical exposures, and not including the normal local natural background radiation |
Exposure of patients as part of their diagnosis or treatment, volunteers helping in the support and comfort of patients, and volunteers in biomedical research | |
|---|---|---|---|
Situations where radiological protection can be planned in advance, and exposures can be reasonably predicted |
e.g. working in a hospital, uranium mine, or nuclear power plant | e.g. visiting a hospital, living near a nuclear power plant | e.g. getting an x-ray, CT scan, or radiation treatment |
Situations that already exist when a decision on control has to be taken |
e.g. aircrew and astronauts exposed to cosmic radiation | e.g. radon gas in the home | n/a |
Unexpected situations that may require urgent protective actions |
e.g. in the immediate response to an accident | e.g. during a major accident | n/a |
Radiation Dose
Radiation dose is a measure of the amount of exposure to radiation. There are three kinds of dose in radiological protection. Absorbed dose is a measureable, physical quantity, while equivalent dose and effective dose are specifically for radiological protection purposes.
Absorbed Dose
Absorbed dose is the amount of energy deposited by radiation in a mass. The mass can be anything: water, rock, air, people, etc. Absorbed dose is expressed in milligrays (mGy).
Absorbed dose is a measurable, physical quantity. It is expressed in grays (Gy), or, more frequently milligrays (mGy), which are 1/1000th of a gray. 1 gray = 1 joule of energy deposited in 1 kilogram of material i.e. 1 Gy = 1 J/kg
Equivalent Dose
Equivalent dose is calculated for individual organs. It is based on the absorbed dose to an organ, adjusted to account for the effectiveness of the type of radiation. Equivalent dose is expressed in millisieverts (mSv) to an organ.
Equivalent dose = absorbed Dose multiplied the appropriate radiation weighting factor. The radiation weighting factors are needed because different types of radiation (like alpha, beta, gamma, and neutrons) can have different effects even if the absorbed dose is the same. Equivalent dose is expressed in sieverts (Sv), or, more frequently, millisieverts (mSv) which are 1/1000th of a sievert, and the organ should always be specified (for example "25 mSv to the skin"). In the simplest cases, for gamma (photon) and beta (electron) radiation, the radiation weighting factor is 1, and therefore, for example, an absorbed dose of 1 mGy in an organ equals an equivalent dose of 1 mSv to that organ.
Effective Dose
Effective dose is calculated for the whole body. It is sometimes called whole-body dose. It is the addition of equivalent doses to all organs, each adjusted to account for the sensitivity of the organ to radiation. Effective dose is expressed in millisieverts (mSv).
Effective dose = sum for all organs of (equivalent dose to the organ times the appropriate tissue weighting factor)
The tissue weighting factors are needed because different organs have different levels of sensitivity to radiation, even if the equivalent dose is the same. Effective dose is expressed in sieverts (Sv), or, more frequently, millisieverts (mSv) which are 1/1000th of a sievert. This is the most frequently used dose in radiological protection. Unless you see mention of a specific organ, a "dose" in Sv or mSv is the effective dose. In the simplest cases, for uniform whole-body exposure to gamma (photon) or beta (electron) radiation, the radiation weighting factor is 1, and the tissue weighting factors add up to 1, and therefore, for example, an absorbed dose of 1 mGy equals an effective dose of 1 mSv.
Effective dose in particular is a central feature of radiological protection. It sums up any number of different exposures into a single number that reflects, in a general way, the overall risk. The concept may be complex, but it makes radiological protection practical to implement.
Dose Limits
Dose limits help ensure that no person is exposed to an excessive amount of radiation in normal, planned situations.
They are the strongest form of restriction on dose to an individual. Exceeding a dose limit is contrary to regulations in most countries.
Dose Limits Recommended by ICRP
| Type of Dose Limit | Limit on Dose from Occupational Exposure | Limit on Dose from Public Exposure |
|---|---|---|
| Effective Dose | 20 mSv per year, averaged over defined periods of 5 years, with no single year exceeding 50 mSv After a worker declares a pregnancy, the dose to the embryo/fetus should not exceed about 1 mSv during the remainder of the pregnancy |
1 mSv in a year In special circumstances, a higher value could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv per year |
| Equivalent Dose to the Lens of the Eye | 20 mSv per year, averaged over defined periods of 5 years, with no single year exceeding 50 mSv |
15 mSv in a year |
| Equivalent Dose to the Skin Averaged over 1 cm2 of skin regardless of the area exposed |
500 mSv in a year | 50 mSv in a year |
| Equivalent Dose to the Hands and Feet | 500 mSv in a year | - |
| Dose limits are primarily from ICRP Publication 103 Table 6. The recommendation for pregnant workers is from ICRP Publication 103 Paragraph 186. The occupational limit for the lens of the eye is from Paragraph 3 of the ICRP Statement on Tissue Reactions in ICRP Publication 118.
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Dose limits alone are not enough to ensure adequate protection. They function in combination with the fundamental principles of justification and optimisation.
These limits apply only to doses received above the normal local natural background radiation.
Limits on effective dose, combined with optimisation of protection, are designed to avoid a risk of stochastic effects that would be considered intolerable in a planned exposure situation.
Limits on equivalent dose to an organ, combined with optimisation of protection, are designed to prevent the occurance of deterministic effects.
Dose limits apply only in planned exposure situations. In other situations, restrictions on individual dose are called reference levels. They provide the additional flexibility needed in emergency and existing exposure situations to make sure protection is optimised.
Dose limits do not apply to medical exposures. If they did, the effectiveness of diagnosis or treatment might be reduced, doing more harm than good for the patient. The emphasis is on justification of medical procedures and optimisation of protection.
Take me back to the ICRP's Guide to the System of Radiological Protection!