How can different types of radiation exposures be compared?
In the diagram at left, a man is exposed to a collimated x-ray beam. In the
diagram at right, he is exposed to a non-collimated beam. For this example, we
shall assume the exposure in the beam is 50 mR in both cases.
Although the radiation output of the x-ray machine is the same in both cases
(50 mR), it is evident that the biological risks from these two exposures is NOT
EQUAL, since a greater portion of the body is exposed to the x-rays in the
To be meaningful, a measure of radiation dose should be adjusted for these
types of factors. This "adjusted" dose is known as the Effective Dose
Effective Dose Equivalent [measured in rem]
This is a risk-based measure of radiation dose. It is adjusted for:
- the type of radiation, and
- the relative radio-sensitivity of the exposed portion of the body.
Effective Dose Equivalent = rad x Quality Factor x Weighting Factor
Quality Factor is a factor that expresses the relative biological
effectiveness for a specific type and energy of radiation. For x-rays and gamma
rays the Quality Factor (QF) is 1.
Weighting Factor is a number that adjusts for the relative radio-sensitivity
of the exposed organ or part of the body.
For whole body exposures (WBE) the weighting factors for organs/body parts
area added together and give a Weighting FactorWBE = 1