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The magnitude of radiation effect is directly related to the concentration of absorbed energy in tissue (or in any other medium that is being irradiated). Other factors that must be considered in assessing the biological effect of radiation include the type of radiation (alpha, beta, or gamma) and the radiosensitivity of the irradiated tissue.

The first dosimetric quantity used in radiation safety, the roentgen (R) as originally defined, is actually a measure of exposure to X‐rays. The amount of energy absorbed from a given roentgen exposure depends on the nature of the absorber. Soft tissue absorbs about 96 ergs g⁻¹ for 1 R of X‐ray exposure. Because of the dependence of absorbed dose on the nature of the absorber, the traditional dosimetric unit, the rad, was introduced. The rad is defined as that dose in which 100 ergs of energy are absorbed per gram of absorber.

Radiation safety technicians often make the assumption that 1 R of X‐rays is approximately equal to 1 rad. Regulators typically allow this assumption as doses will be slightly overestimated.

For purposes of radiation safety control, it is customary to use the subunit millirad (mrad), which represents 0.001 rad. The rad is applied to all types of radiation dosimetry – to external radiation as well as to internally deposited radionuclides and to doses from alpha, beta, and gamma radiations. The traditional radiation unit is being replaced by the SI system unit, called the gray, symbolized by Gy. One gray is defined as that dose in which 1 J of energy is absorbed in 1 kg of the absorbing medium

Since 1 J = 10⁷ ergs and 1 kg = 1000 g,

For radiation safety purposes and for regulatory purposes, a dose unit is used that is normalized for the type of radiation and for the relative radiosensitivity of the irradiated tissue. The relative radiosensitivity is important when dealing with nonuniformly irradiated tissues and organs. The normalizing factor that accounts for the type of radiation is called by the US Nuclear Regulatory Commission (USNRC) the quality factor, Q, and by the International Commission on Radiological Protection (ICRP) the radiation weighting factor, w_R. Note that the recommendations of ICRP 26 and ICRP 30 are currently the basis for the USNRC regulations, and other weighting factors are included in this chapter for reference. Relative radiosensitivity of irradiated tissue is considered by the tissue weighting factor, w_T, which ranges from 1 for uniform whole body radiation to 0.03 for the thyroid. The traditional dose equivalent unit is called the rem, H. For whole body irradiation, the rem is defined as the product of the quality factor and the dose:

In the SI system, the unit for the dose equivalent is called the sievert, Sv, and is defined by

Values used for the normalizing factor for several radiations are listed in Table 1.

In the belief that the probability of a stochastic effect should be the same whether the whole body is uniformly irradiated or whether the radiation is nonuniformly distributed, ICRP 30 introduced the concept of the effective dose equivalent. The safety standards assume that the probability of harm to any tissue or organ is proportional to the dose to that tissue. However, because of the differences in radiosensitivity among the various tissues, the value for the proportionality factors differs among the tissues. Generally, the higher the mitotic rate of a cell line, and the less differentiated a cell line is, the more radiosensitive is that cell line. (This observation was first enunciated by two French physiologists, Bergonie and Tribondeau, in 1906.) The differences in radiosensitivity are accounted for by tissue weighting factors, w_T, Table 2. The effective dose equivalent is given by

(8)

TABLE 1 Values for the quality factor, Q, and the radiation weighting factor, wR.

Radiation	Q or wR
X‐rays	1
Gamma rays	1
Beta particles	1
Alpha particles	20

Note that these specific recommendations are unchanged for ICRP 26, 60, and 103.

TABLE 2 Values for tissue weighting factors, wT.

Tissue (or organ), wT	ICRP 26	ICRP 60	ICRP 103
Gonads	0.25	0.20	0.08
Breast	0.15	0.05	0.12
Red bone marrow	0.12	0.12	0.12
Lung	0.12	0.12	0.12
Thyroid	0.03	0.05	0.04
Bone surface	0.03	0.01	0.01
Colon	Not given	0.12	0.12
Stomach	Not given	0.12	0.12
Bladder	Not given	0.05	0.04
Liver	Not given	0.05	0.04
Esophagus	Not given	0.05	0.04
Skin	Not given	0.01	0.01
Salivary glands	Not given	Not given	0.01
Brain	Not given	Not given	0.01
Remainder	0.30	0.05	0.12
Total	1.00	1.00	1.00

The values are based on a reference population of equal numbers of both sexes and a wide range of ages. In the definition of effective dose, they apply to workers, to the entire population, and to both sexes.

Example. A laboratory technician had accidentally inhaled ¹³¹I. Using data from whole body counts and bioassay measurements, the health physicist calculated a thyroid dose equivalent of 6000 mrem (60 mSv) and a whole body dose equivalent of 13 mrem (0.13 mSv). What was the technician's effective dose according to the USNRC criterion?

Substituting into the equation for effective dose equivalent,