As mentioned in Section 13.4.1, the organs have different sensitivities to radiation. The sensitivity of an organ to radiation is determined by the proliferation and dif­ferentiation of its cells and tissues. This means that cells with faster proliferation are more sensitive to radiation. Moreover, living organisms, in which the biological functions of the cells and tissues are strongly differentiated, are also more sensitive to radiation. These facts have important consequences, both in medical applications and in radiation protection. For example, the cells of tumors can be damaged by irradiation because their proliferation is faster than that of healthy cells and tissues. At the same time, children and pregnant women have to be protected against radia­tion very carefully.

The radiation exposure can be acute or present for a long period of time (i. e., chronic). Acute radiation exposure refers to the delivery of (usually high) doses of radiation in a short period of time (within days). Acute radiation exposure of human beings can occur through accidents, wars, criminal activity, or medical impacts. Some radiation exposures can be present constantly in the environment, such as background radiation and the continuous elevated doses of radiation in radioactive workplaces.

Radioactive irradiation can cause somatic and genetic effects. The somatic effects manifest themselves in the individual, while the genetic effects (mutations) are observed in their descendants. The most serious genetic effect is when the indivi­dual’s reproductive capacity is affected, and there are consequently no descendants.

The low and high doses have different biological impacts, which are called “sto­chastic” and “deterministic” effects, respectively. The term “stochastic” means a random effect that is only the probability of damage (e. g., the induction of cancer and genetic defects) that can be caused by a certain radiation exposure. Stochastic effects are usually related to exposures to low levels of radiation exposure over a long period of time. Stochastic effects have no threshold level of radiation exposure below which we can say with certainty that cancer or genetic effects will not occur.

Deterministic effects are related to much higher levels of radiation exposure, usually over a much shorter period of time than is the case for stochastic effects. The deterministic effects have a threshold radiation dose, below which the deter­ministic effects are not observed. However, above the threshold dose, the severity of the deterministic effect is proportional to the radiation dose.

The effect of high doses is illustrated in Figure 13.7. Some curves of Figure 13.7 have been obtained in animal tests. The curve for humans has been constructed from the data collected after accidents. As seen, the tumor frequency and absorbed dose are in well-determined correlations. Moreover, there are thresh­old doses below which no excess cancer cases have been reported.

In Table 13.6, the biological impacts of high radiation doses are listed in the case of acute and cronic radiation exposures. The data are based on the radiation effects observed in human populations following the nuclear explosions in Hiroshima and Nagasaki in 1945.

As seen in Table 13.6, both acute and chronic radiation exposures can cause radiation sickness. This sickness has a variety of symptoms, including the following:

• General symptoms: fainting, fatigue, weakness, nausea and vomiting, diarrhea, dehydra­tion, and hair loss.

• Cutaneous symptoms: inflammation of exposed areas (redness, tenderness, swelling, bleed­ing), bruising, skin burns (redness, blistering), open sores on the skin, and sloughing of skin.

• Mucosal symptoms: mouth ulcers, ulcers in the esophagus, stomach, or intestines, bleed­ing from the nose, mouth, gums, and rectum, vomiting blood, and bloody stool.

20 40 60 80 100 120 (gray) Figure 13.7 Tumor frequency as a function of absorbed dose. (A) Sr-90 induced osteosarcomas in female CBA mice. (B) Bone tumors in men from incorporated Ra-226. (C) Kidney tumors in rats by X-rays. (D) Skin tumors in rats by electrons. Source: Reprinted from Choppin and Rydberg (1980), with permission from Elsevier.

Low radiation exposure has stochastic effects. This means that only the proba­bility of effects can be provided. The effect—dose functions may show linear, sub­linear (with threshold dose), and supralinear curves (with effects increasing with the increase in the radiation dose). In addition, there are views that very small doses stimulate the repair activity of DNA, so the radiation may have a desirable impact. This process is called “hormesis,” and there are many disputes whether this effect even exists at all. In conclusion, we do not have exact information on the radiation effects, which, in fact, is the most important in life for most of us.

As mentioned in Section 13.4.2, the impact of the radiation is due to radical for­mation. The effects of low doses can hardly be estimated because there are many other factors that also produce radicals in living organisms. This means that the effects of low-level radiation cannot be separated from the effects of other environ­mental risks such as stress, carcinogens (tobacco smoke, nonradioactive species, etc.), aging, and individual physical conditions. As mentioned previously, living organisms have different ways to protect against radicals by natural radical scaven­gers, and they have various repair mechanisms for restoring the damages to the bio­logical molecules. If, however, this repair mechanism fails, different diseases appear. Since any radiation exposure has some risk of producing radicals, it should be avoided if possible. For this reason, the limits of radiation are determined by the “linear-no-threshold” hypothesis. This hypothesis is questioned from time to time; however, it provides a pragmatic means of estimating radiation risks and is consis­tent with the (limited) data that are available.

The standards of radiation protection control the receipt, possession, use, trans­fer, and disposal of radioactive material in such a manner that the total dose to an individual (including doses resulting from licensed and unlicensed radioactive material and from radiation sources other than background radiation) does not exceed the standards for protection against radiation prescribed in the regulations. However, nothing shall be construed as limiting actions that may be necessary to protect health and safety. The standards have three basic aspects:

1. It must be proved that the application of radioactive material results in more improve­ments for the community than the risk to health.

2. The risk has to be decreased as low as reasonably achievable (ALARA).

Table 13.6 The Biological Impact of High Radiation Doses

Acute Radiation Exposure

1000 Sv Death immediately

100 Sv Damage of the central nervous system; death within hours

10 Sv Damage of blood-forming tissues; death within days

1 Sv Radiation thickness

Chronic Radiation Exposure

0.01 Sv/nap Weakness after 3—6 months; death after 3—6 years

0.001 Sv/nap Radiation sickness, symptoms are observed after several years

3. There are dose limits that are strictly prohibited under any conditions. There are dose lim­its for individual members of the public and occupational dose limits.

The standards for individual members of the public say that the total effective dose equivalent to individual members of the public from the licensed operation does not exceed 1 mSv/year, exclusive of the dose contributions from background radiation, medical investigations, and occupational doses. The annual limit of the effective occupational dose is 100 mSv/5 years, but the 50 mSv/year is permitted only in a 5-year period. Therefore, the mean occupation dose limit is 20 mSv/year. Other dose limits are defined for any tissues including occupational and public dose limits. For example, the occupational dose limit for the lens of the eye is 150 mSv/year; for skin, hands, and feet, it is 500 mSv/year. These limits for the members of the public are the tenth part of the occupational dose limits; namely, 15 mSv/year for the lens of the eye, and 50 mSv/year for skin, hands, and feet. The occupational dose limits have been determined such that they should be similar to the risk of other occupational limits (e. g., the risk of death of bus drivers).

Medical applications of radiation have no dose limits. The basic principles of protection for medical exposures can be summarized as follows: medical exposures should be justified by weighing the diagnostic or therapeutic benefits they procure against the radiation detriment they might cause, taking into account the benefits and risks of available alternative techniques that do not involve radiation exposure. The doses from medical exposure should be the minimum necessary to achieve the

required diagnostic objective or the minimum required to the normal tissue for the required therapeutic objective. This principle is in accordance with the ALARA principle. In Figure 13.8, the mean effective dose of the patients is shown in several nuclear medical diagnostic methods.

In conclusion, we can say that background radiation has always been present during the history of humanity. Protection from excess radiation exposure is legally controlled. The standards are very strict for protecting the members of the public. The occupational dose limits, or course, must be higher, so that people working with radioactive material and radiation face a higher level of risk. However, safety regulations are to be followed strictly in order to minimize this risk.

Further Reading

Feher, I. and Deme, S. (2010). Sugarve’delem (Radiation Protection). ELTE Eotvos Kiado, Somos Kornyzetvedelmi Kft, Budapest.

United States Nuclear Regulatory Comissions, 2007. < http://www. nrc. gov/reading-rm/doc- collections/cfr/part020/ > (accessed 28.03.12.)

Szabo, S. A. (1993). Radioecology and Environmental Protection. Akademiai Kiado/Ellis Horwood, Budapest/New York, NY.

Valentin, J. (2006). The Scope of the Radiological Protection Regulation. Elsevier, Amsterdam, http://www. icrp. org/docs/Scope_of_rad_prot_draft_02_258_05v06.pdf (accessed 28.03.12.)