Radiation Protection – Dose Limits

90204

DOSE LIMITS (Abstract)

A dose limit is defined in the International Basic Safety Standards (BSS) as “The value of the effective dose or the equivalent dose to individuals from controlled practices that shall not be exceeded.  The limits on effective dose for occupational exposure apply to the sum of effective doses from external sources and committed effective doses from intakes in the same period.

The exposure of the public or any worker shall not exceeded:

Public:
a. an effective dose of 1 mSv per year to a member of the public

Worker:
b. an effective dose of 20 mSv per year averaged over five consecutive years;<
c. an effective dose of 50 mSv in any single year;
d. an equivalent dose to the lens of the eye of 150 mSv in a year; and
e. an equivalent dose to the extremities (hands and feet) or the skin of 500 mSv in a year.

The equivalent dose limits for the skin apply to the average dose over 1 cm2 of the most highly irradiated area of the skin. Skin dose also contributes to the effective dose, this contribution being the average dose to the entire skin multiplied by the tissue weighting factor for the skin.

There are three classifications of dose in radiological protection.

1. Effective dose: calculated for the whole body, and is sometimes called whole-body dose.
2. Equivalent dose: 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.
3. Absorbed dose: is the amount of energy deposited by radiation in a mass (the person) and is a measurable, physical quantity,

The total effective dose Et received or committed during any time period t can be estimated from the following expression:

where Hp (10) is the personal dose equivalent at a depth of 10 mm in soft tissue during time period t, e(g) j,ing and e(g) j,inh are the dose coefficients for, respectively, ingestion and inhalation of radion

Derivation for the dose limit:

The worker dose limit was derived from several sources of radiological data including reference anatomical and physiological models of humans, studies at the molecular and cellular level, experimental animal studies, and human epidemiological studies including Japanese atomic bomb survivor studies. Early in the studies it was clear that no observable effects were found below 50 mSv. From that observation it was surmised that 10 times lower was surely safe. A worker limit of 50 mSv annually was established. It was considered extra safe because the dose would be delivered over time rather than instantaneous as with the bomb dose.

A risk-benefit argument was developed for the worker dose, assuming there were indeed a risk (the 0.5 Sv safe level might be in error) that risk would be small. A worker derived benefit (income) from exposure to the risk, but the public did not. A factor of 10 was introduced for the public and the limit set at 5 mSv. This level was reduced to 1 mSv since it was obvious that most public exposures were less than 1 mSv.

The linear, no-threshold model was introduced for a conservative and easily calculated risk model. The LNT assumption lead to no-safe-dose of radiation. No-safe-dose lead to As Low As Reasonably Achievable as a radiation protection policy. The limit of 1 mSv/y is no longer defensible if one can claim that a lower dose is reasonable.

In no case has the original observation of no effects below 50 mSv been demonstrated to be incorrect.


Reference
1. IAEA Safety Standards – Occupational Radiation Protection
2. IAEA Safety Standards – Radiation Protection of the Public and the Environment