A Perspective on Natural Versus Man-Made Radiation
The International Commission on Radiological Protection (ICRP) recommends an effective dose of 10 mSv as the annual dose reference level for humans [11]. The United Nations Scientifi Committee on the Effects of Atomic Radiation (UNSCEAR) estimates that the global population receives an average annual effective dose of ~3.1 mSv (~2.4 mSv and ~0.7 mSv from natural background and anthropogenic radiation, respectively) [12]. Terrestrial radiation sources include primordial radionuclides such as uranium (238U) and thorium (232Th) [2]. Indwelling radon comprises about half of the overall average annual dose. Medical procedures (i.e., X-rays, CT scans) account for the majority of anthropogenic sources of radiation [13]. For occupational workers, the recommended annual dose limit to the whole body and extremities is 20 and 500 mSv, respectively [12]. While the dose rate from natural radionuclides in the body is independent of geographical location, the level of exposure to cosmic and terrestrial radiation can vary signifi depending on altitude [1]. For example, at 3,000 m above sea level, people receive fi e times more radiation dose than people at sea level [2].
Distinguishing External from Internal Exposure
Exposure to radiation may be classified into three categories: (i) body exposure due to the proximity of a radiation source, (ii) external contamination, and (iii) internal contamination.
All types of ionizing radiation may result in total or partial body exposure, with the severity of irradiation dependent upon the type and energy of the radiation. In contrast, external or internal contamination occurs when radionuclides or fission products settle on or penetrate human bodies via three primary routes [4, 14]:
• inhalation of airborne radionuclide particles
• ingestion of contaminated water and foodstuffs
• direct exposure via open skin from contaminated surface deposition
Numerous factors influence the potential health effects after contamination with radionuclides [3, 4, 10]:
• chemical nature of the radionuclide or radiation source
• the physicochemical characteristics of the radionuclide (radiological and biological half-life, particle size, chemical composition, solubility, etc.)
• the behavior of radionuclides after radionuclide intake into the body
• radionuclide dose and dose rate
• type of radiation
• radiation dose-response relationships for individual tissue following radiation uptake
• sensitivity of different tissues and organs
• age and health of the contaminated individual
The chemical nature of the ingested radionuclide strongly dictates the extent of absorption in the GI tract. For example, iodine and cesium are almost completely absorbed, whereas less than 0.1 % of plutonium and americium are absorbed. The distribution of incorporated radionuclides in the body also depends on the solubility of the particles. In general, absorption is greater after ingestion of soluble inorganic forms than after ingestion of inorganic forms of an element. For example, ingestion of 239Pu as nitrate or citrate results in at least one order of magnitude greater absorption than as oxide particles. Similarly, intake of soluble radioactive materials via inhalation or open wounds results in greater absorption and deposition in other tissues. In addition, the pattern of radioactivity distribution (i.e., uptake and retention) throughout irradiated tissues may infl the degree of damage. This is particularly true for alpha emitters because of localized deposition of energy and their greater RBE compared with that of beta or gamma emitters. For example, α-emitting 239Pu localizes in tissues and causes fi ulceration, loss of tissue function, and even death [10]. Ingestion of insoluble forms of radionuclides with α or β emission may be largely confi to the gastrointestinal tract, whereas radionuclides with γ emission may irradiate neighboring tissues. After 7 half-lives, less than 1 % of the original activity remains and after 10 half-lives, less than 0.1 % of the original activity remains [4].
