Because the biologic effects of radiation depend not only on dose, but also on the type of radiation, the dosimetric quantity relevant to radiation protection is the dose equivalent (H). It is defined as:

where D is the absorbed dose and Q is the quality factor for the radiation.¹

The SI unit for both dose and dose equivalent is joules per kilogram, but the special name for the SI unit of dose equivalent is sievert (Sv):

If dose is expressed in units of rad, the special unit for dose equivalent is called the rem:

Because Q is a factor and has no units:

The use of quality factor in radiation protection is analogous to the use of relative biologic effectiveness (RBE) in radiation biology. However, the quality factor is a somewhat arbitrarily chosen conservative value based on a range of RBEs related to the linear energy transfer (LET) of the radiation. Thus, the Q factor encompasses RBEs in a very broad sense, independent of the organ or tissue or of the biologic endpoint under consideration.

Although the dose equivalent for particular situations can be calculated (1), it is convenient for quick-and-rough calculations to have a table of practical quality factors available. Table 16.1 gives these approximate factors for a variety of radiations used in radiation therapy.

Whole body exposures are rarely uniform. For a given exposure received, internally or externally, dose equivalents for various tissues may differ markedly. Also, tissues vary in sensitivity to radiation-induced effects. To take into account these nonuniform irradiation situations, the concept of effective dose equivalent has been adopted by the ICRP and the NCRP. The effective dose equivalent
(H_E), is defined as “the sum of the weighted dose equivalents for irradiated tissues or organs” (2). Mathematically:

where W_T is the weighting factor of tissue T and H_T is the mean dose equivalent received by tissue T.

The weighting factors represent the proportionate risk (stochastic) of tissue when the body is irradiated uniformly. They are derived from risk coefficients (i.e., risk per unit dose equivalent). Table 16.2 gives the weighting factors and the corresponding risk coefficients for various types of tissues and organs.

Radiation is a part of the natural environment. This background radiation is contributed principally by three sources: terrestrial radiation, cosmic radiation, and radiation from radioactive elements in our bodies. Table 16.3 gives average values of background radiation to which various parts of the body are exposed annually. The total effective dose equivalent for a member of the population in the United States from various sources of natural background radiation is approximately 3.0 mSv/year (300 mrem/year).

The terrestrial radiation varies over the earth because of differences in the amount of naturally occurring elements in the earth’s surface. In addition, building materials may incorporate naturally occurring radioactive materials. Many buildings may have elevated levels of radon emitted by naturally occurring uranium-238 in the soil. It has been estimated (3) that the average annual dose equivalent to bronchial epithelium from radon decay products is approximately 24 mSv (2.4 rem).

Cosmic radiation levels change with elevation. For example, air travel exposes individuals to increased radiation exposure. It has been estimated that at 30,000 feet the dose equivalent is approximately 0.5 mrem/h (4).

The internal irradiation arises mainly from ⁴⁰K in our body, which emits β and γ rays and decays with a half-life of 1.3 × 10⁹ years.

In addition to the background radiation, the population is exposed to radiation from various medical procedures—the planned exposure of patients, as distinct from occupational exposures received by health personnel. It was estimated by the U.S. Public Health Service that the average annual genetically significant dose equivalent² in 1970 was approximately 20 mrem/year from radiologic procedures.

Under ordinary circumstances, exposures from natural background radiation and medical procedures are not included in the occupational exposure controls for the individual cases.

A vast literature exists on the biologic effects of radiation. Discussions pertinent to radiation protection can be found in reports of the United Nations Scientific Committee on the Effects of Atomic Radiation (6).

Whereas large doses of radiation produce identifiable effects within a relatively short period, the effects are difficult to ascertain at low doses (e.g., <10 cGy). The difficulty is due mainly to the extremely low frequency with which these effects might occur. The statistical problems are enormous in identifying small effects in the constant presence of spontaneously occurring effects. However, certain effects have been demonstrated in humans and other mammals at doses lower than those required to produce acute radiation syndrome but greatly in excess of dose limits recommended by the standards-setting bodies. Thus, exposures to low-level radiation may produce (a) genetic effects, such as radiation-induced gene mutations, chromosome breaks, and anomalies; (b) neoplastic diseases, such as increased incidence of leukemia, thyroid tumors, and skin lesions; (c) effect on growth and development, such as adverse effects on the fetus and young children; (d) effect on life span, such as diminishing of life span or premature aging; and (e) cataracts or opacification of the eye lens.

The harmful effects of radiation may be classified into two general categories: stochastic effects and nonstochastic effects. The NCRP (2) defines these effects as follows.

A stochastic effect is one in which “the probability of occurrence increases with increasing absorbed dose but the severity in affected individuals does not depend on the magnitude of the absorbed dose.” In other words, a stochastic effect is an all-or-none phenomenon, such as the development of a cancer or genetic effect. Although the probability of such effects occurring increases with dose, their severity does not.

A nonstochastic effect is one “which increases in severity with increasing absorbed dose in affected individuals, owing to damage to increasing number of cells and tissues.” Examples of nonstochastic effects are radiation-induced degenerative changes such as organ atrophy, fibrosis, lens opacification, blood changes, and decrease in sperm count.

Whereas no threshold dose can be predicted for stochastic effects, it is possible to set threshold limits on nonstochastic effects that are significant or seriously health impairing. However, for the purpose of radiation protection, a cautious assumption is made that “the dose-risk relationship is strictly proportional (linear) without threshold, throughout the range of dose equivalent and dose equivalent rates of importance in routine radiation protection.”

Many analysts believe that these two assumptions may overestimate the biologic effects at low dose levels. Some have proposed a linear quadratic dose-response curve that assigns relatively reduced effects to low doses. However, in the absence of more reliable data, it seems prudent to adopt a conservative model, the nonthreshold linear response, for predicting low-dose effects. For further discussion of dose-response models, the reader is referred to references 7 to 10.

NCRP (2) recommendations on exposure limits of radiation workers are based on the following criteria: (a) at low radiation levels the nonstochastic effects are essentially avoided; (b) the predicted risk for stochastic effects should not be greater than the average risk of accidental death among workers in “safe” industries; and (c) the ALARA principle should be followed, for which the risks are kept as low as reasonably achievable, taking into account social and economic factors.

It is important to compare radiation risks with the risks in other industries when setting radiation protection standards. Table 16.4 gives data on annual fatality rates from accidents in different occupations. On the basis of these data, “safe” industries are defined as “those having an associated annual fatality accident rate of 1 or less per 10,000 workers, that is, an average annual risk of 10^-4 (2). The available data for the radiation industries show an average fatal accident rate of less than 0.3 × 10^-4 (11). From this perspective, the radiation industries compare favorably with the “safe” industries. For radiation protection purposes, the total risk coefficient is assumed to be 1 × 10^-2 Sv^-1 (1 × 10^-4 rem^-1).

Radiation protection guidelines for the design of structural shielding for radiation installations are discussed in the NCRP Reports 49 and 51 (12,13). These reports contain the necessary technical information as well as recommendations for planning new facilities and remodeling existing facilities. The reader is referred to these reports for comprehensive details on this subject. This section will discuss only some of the basic factors that are considered in the calculation of barrier thicknesses.

Protective barriers are designed to ensure that the dose equivalent received by any individual does not exceed the applicable maximum permissible value. The areas surrounding the room are designated as controlled or noncontrolled, depending on whether or not the exposure of persons in the area is under the supervision of a radiation protection supervisor. For protection calculations, the dose-equivalent limit is assumed to be 0.1 rem/week for the controlled areas and 0.01 rem/week for the noncontrolled areas. These values approximately correspond to the annual limits of 5 rem/year and 0.5 rem/year, respectively.

Protection is required against three types of radiation: the primary radiation, the scattered radiation, and the leakage radiation through the source housing. A barrier sufficient to attenuate the useful beam to the required degree is called the primary barrier. The required barrier against stray radiation (leakage and scatter) is called the secondary barrier. The following factors enter into the calculation of barrier thicknesses.