Ionizing Radiation Roy E Albert MD

16 Radiation Protection

Historically, occupational standards were developed separately for radon-222, radium-226, and for whole body exposure.

16.1.1 Radon-222

Setting a standard for radon-222 to prevent lung cancer is complicated by the fact that radon-222 rarely exists in equilibrium with its daughters. The difficulty was circumvented by using, as a unit of radon activity, the Working Level (WL), defined as any combination of radon daughters in one liter of air that releases 1.3 x 105 MeV of alpha energy. This is equivalent to 100 pCi per liter of radon-222 in equilibrium with its daughters. A pCi is 10-12 Ci. A Ci is 3.7 x 1010 disintegrations per second (d/s). Therefore 100 pCi is 3.7 d/s for each of the isotopes in the decay chain. Most of the dose to the tracheobronchial mucosa comes from the alpha-emitting isotopes, polonium-218 (Ra A) and polonium-214 (Ra C'). When the disintegration rate equivalent to one WL continues for a working month, which is 170 hours, the cumulative exposure is called a Working Level Month (WLM). So far as the tracheobronchial mucosa is concerned, a WLM is a total dose from about 1.5 x 10' disintegrations of Ra A and Ra C'. This number of disintegrations can occur during any period of time, for example, a year. The dose to the bronchial epithelium is 0.2 mrem/year per pCi/L using a radiation weighting factor of 20. The occupational standard for radon-222 is 4 WLM per year. This is equivalent to a bronchial dose of 80 rem/year (65).

Radon in homes in the United States averages from 0.5-1.6 pCi/L and 2.7 pCi/L in Scandinavia. The U.S.EPA has set an "action level at 4 pCi/L suggesting that remedial action should be taken at levels higher than this. This affects 1 in 12 homes in the United States, a total of about 6 million. The action levels in Europe are 2 to 5 times higher. The EPA action level of 4 pCi/L is 100 times lower than the occupational standard. This action level translates into an effective dose of 0.8 rem/year and a cancer risk of 4 x 10-4/year. Despite the complexity and uncertainty of the lung dosimetry, these crude dose estimates for the occupational standard are far higher than those permitted for external exposure. The bronchial dose associated with the EPA action level for background radon exposure is far higher than the background exposure from other sources.

16.1.2 Radium-226

A standard for radium-226 was based on studies of radium dial workers. The standard is a maximum body burden of 0.1 mCi. This value was picked because at that body burden, no cases of bone cancer were observed. Subsequently, the standards for other bone-seeking isotopes were derived mainly from dog studies that compared the potency of radium-226 with strontium-90 and plutonium-239. The dosimetry of radioisotopes in bone is so complex that the standards were based on skeletal burdens rather than on dose estimates.

16.1.3 External Radiation Standards

External radiation standards were originally focused on preventing acute responses, first skin erythema in the 1920s. Later, with higher energy X-ray machines, the concern shifted to deeper tissues, particularly the bone marrow, as manifested by depressed white blood counts. Subsequently there was a shift to preventing more sensitive responses, namely, cancer and mutations. Today, standards are based largely on preventing cancer. All of these changes were accompanied by progressively lower standards.

As indicated earlier, the absorbed dose is absorbed energy per gram of tissue (100 ergs/gram). The absorbed dose is expressed in rads or Grays (Gy = 100 rads). The radiation weighting factor (Wr) is based on the RBE combined with judgment factors about the relative effectiveness of different kinds of radiation at low doses. The radiation weighting factor converts the absorbed dose to equivalent dose measured in rems or sieverts (Sv = 100 rems). The radiation weighting factor for gamma rays and electrons = 1; protons = 5; alpha particles, fission fragments and heavy nuclei = 20; neutrons = 5-20, depending on energy (66).

The concept of effective dose is used for stochastic effects (cancer and hereditary effects) with uniform whole-body radiation. It is a measure of the total harm that can be ascribed to the sum of the deleterious effects on individual organs. Therefore, effective dose is the sum of the equivalent dose multiplied by the tissue weighting factor (WT) for each organ. WT = 0.20 for the gonads; 0.12, for the colon, lung and stomach; 0.05 for the bladder, breast, liver, esophagus, and thyroid; 0.01 for the skin and bone surfaces; and 0.05 for the remainder (66).

The concepts of committed equivalent dose and committed effective dose are applied to internally deposited radionuclides where the total dose is obtained by integrating over a period of 50 years.

The concept of collective equivalent dose and collective effective dose is used to express the radiative dose to an exposed population. It is the average individual dose multiplied by the number of people exposed. When combined with the cancer risk per unit dose, for example, the collective dose gives the number of people who will get cancer. The collective equivalent dose is expressed in person-rads or person-Gy and the collective effective dose is expressed as person-rem or person-sievert.

The basic NCRP occupational whole body radiation standard is 1 rem/year (0.01 Sv/year). Occupational radiation exposure is not permitted under the age of 18, except for training purposes when the limit is that for the general population described later. The maximum occupational exposure in any one year, as an effective dose, is 5 rem (0.05 Sv). Extra exposure is allowed for limited areas of the skin (50 rem/year) and 15 rem/year for the lens of the eye. Emergency occupational limits up to 50 rem/year are allowed but with subsequent restrictions on exposure. Occupational exposure of the fetus after pregnancy is declared should be no more than 50 millirem/month (0.0005 Sv/month).

The limit for general population exposure is 0.1 rem/year (0.001 Sv) or 0.5 rem/year if such exposure is very infrequent. Individuals under the age of 18, if in occupational training, are allowed the general population exposure limit.

A uniform whole-body exposure to a population is estimated to produce a total detriment of 5.6 x 10-4/rem. This is made up of the sum of 4 x 10-4/rem for fatal cancer and equal contributions from nonfatal cancer and hereditary effects of 0.8 x 10-4/rem each. The comparable figures for the general population are somewhat higher because of the higher sensitivity of the young, namely, 7.3 x 10-4/rem, which is made up of 5 x 10-4/rem for fatal cancer, 1 x 10-4/rem for nonfatal cancer, and 1.3 x 10-4/rem for hereditary effects (66).

A negligible individual dose is one millirem (0.01 mSv). This is the dose below which further expenditure to improve radiative protection are unwarranted. It carries a risk of between 10 6 and

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