All exposures to ionizing radiation carry a risk of biological damage, although this risk decreases as the exposure decreases. For example, the risk associated with natural background radiation is very small. For workers in certain industries, such as miners and medical radiographers, the risk is slightly higher. Exposure levels as a result of a nuclear or radiological emergency can vary widely, depending on the nature of the incident, the type of radiation involved, and even weather patterns.
There are a number of factors that must be taken into consideration in calculating the quantity, or dose, of radiation a person has received, including
This calculation is referred to as a dose assessment.
The terminology used for specifying radiation dose and its effects on the human body has been developed to account for all the factors that are important in determining its biological impact. The most frequently used dose measurements are absorbed dose, equivalent dose, effective dose, and collective dose, which are all briefly described below.
As radiation passes through matter, some of its energy is left behind; that is, it is "absorbed" by the matter. This is referred to as the absorbed dose, and the measurement of this dose is given in grays (Gy).
Different types of radiation have different effects on tissue. In order to account for these differences, the absorbed dose is multiplied by a radiation weighting factor. This factor is dependent upon the type and amount of radiation involved. The result is referred to as the equivalent dose, and it is expressed in sieverts (Sv).
Different tissues and organs are affected differently by radiation. For example, lung tissue is more likely to be affected by radiation than is the skin. In order to account for the differing sensitivities, the equivalent dose is multiplied by a tissue weighting factor: the resulting unit is referred to as the effective dose. The effective dose is also given in sieverts.
The collective dose refers to the amount of radiation received by a group of people. It is calculated by multiplying the average effective dose received by the number of persons exposed. The collective dose is expressed in person-sieverts (person-Sv).
The Canadian Nuclear Safety Commission (CNSC) is the federal regulator of nuclear facilities and materials in Canada. It is responsible for setting radiation dose limits in order to protect workers, such as nuclear power workers and medical personnel who work with sources of ionizing radiation, from overexposure to those sources. The CNSC also sets dose limits to protect the general public from overexposure to radiation from licensed nuclear facilities or substances in Canada.
This table shows the current Canadian dose limits for exposure to licensed sources of radiation:
|Population Group||Dose Limits: Over Five Years||Dose Limits: Annual|
|Worker||100 mSv||50 mSv|
Health Canada's Radiation Protection Bureau supports and maintains the National Dosimetry Services and the National Dose Registry, which monitor, track, and record doses received by those who work with sources of ionizing radiation. Health Canada is also responsible for setting dose limits for emergency responders in the event of an uncontrolled release of radioactive materials.