Patty's Industrial Hygiene, Physical and Biological Agents. Группа авторов

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СКАЧАТЬ radiations that originated in the ore. Ernest Rutherford named the first two radiations alpha and beta rays. P.V. Villard later named a third type of radiation, the gamma ray (later investigations showed that X‐rays and gamma rays were the same type of radiation). Alpha and beta particles are still occasionally referred to as alpha rays and beta rays as a result of this discovery.

      The discovery of these radiations opened new fields of scientific investigation and uses for these radiations. In parallel with the studies of the physics and chemistry of radioactive elements and the associated radiation, researchers were also examining the biomedical effects. With this expanded use of radiation, there were further reports of harmful effects, such as skin burns and hair loss when X‐rays were used in medical diagnosis. In 1899, the first case of a cancer, a basal cell carcinoma on a woman's face, was cured by X‐rays. In 1906, two French physiologists, Bergonie and Tribondeau, published their classical paper on the relative radiosensitivity of different cells and tissues. They found that the less differentiated a cell was, and the more frequently it divided, the more radiosensitive it was. Although to date we have found nothing to contradict their observations, much is understood now about the molecular biology basis for their findings. Since the time of their publication, an enormous amount of information on the nature of the interaction of radiation with living tissue and on the dose–effect and dose–response relationships has been amassed. Sources of information, such as the experiences of the radium dial painters, early radiologists, and uranium miners, showed that occupational overexposures led to harmful effects. Other major sources of information include populations that had been medically exposed to diagnostic and therapeutic radiation, survivors of the atomic bombings in Japan, data from radiation accidents, and epidemiological studies of populations exposed to low‐level radiation from nuclear facilities and from natural background. This body of knowledge forms the scientific basis for the radiation safety standards currently in use. After radiation safety guidelines were developed, occupational exposures that were within the radiation safety guidelines did not lead to harmful biomedical effects.

2 SOURCES OF IONIZING RADIATION

One is exposed to radiation from both naturally occurring and anthropogenic sources. Natural background radiation includes cosmic radiation that originates in the sun and in the stars, and terrestrial radiation that is emitted from radioactive minerals on or within the earth. These naturally occurring background radiations must be accounted for when making radiation measurements and decisions, and in certain instances may pose a potential health threat.

      2.1 Natural Radiation

      Cosmic radiation, when it enters the earth's atmosphere, consists mainly (90%) of high kinetic energy protons, with the remaining 10% being alpha particles, neutrons, and electrons. The interaction of these very high‐energy particles with the atmosphere leads to the production of certain radioactive isotopes (notably tritium (³H) and radiocarbon (¹⁴C)), muons (extremely high‐energy, heavy electrons that become ordinary electrons when they lose their energy), and gamma rays. Cosmic ray intensity varies according to altitude and latitude. The atmosphere acts as a shield whose total thickness is equivalent to about 30 ft (approximately 9 m) of water. Therefore, cosmic ray intensity is lowest at sea level, and increases with increasing altitude. In the United States, the average dose from cosmic rays at sea level is about 30 millirems (mrem) (0.3 mSv) per year. The mrem is a unit of radiation dose equivalent that is used for safety and regulatory purposes, while the sievert is the SI unit for radiation dose equivalent. One Sievert is equivalent to 100 rems. In Denver, CO, at an altitude of one mile (1.6 km), the cosmic ray dose rate is about twice that at sea level. Thus, jet aircraft crew members receive significant radiation doses in the course of their work (although very much less than the regulatory dose limit for occupational exposure).

      The latitude effect is due to the earth's magnetic field. Charged cosmic ray particles entering the earth's atmosphere in the equatorial region travel in a direction that is nearly perpendicular to the earth's magnetic field, whereas charged particles that enter the atmosphere at higher latitudes cross the magnetic lines of force at a glancing angle. The equatorial particles thus experience a much greater magnetic force, which turns them away from their initial path toward the earth, than the particles that enter at the high latitudes. The cosmic ray intensity is therefore lowest at the equator, and increases continuously as the latitude, both north and south, increases.

      Terrestrial radiation comes mainly from three different groups of naturally occurring radioisotopes. Primordial radioisotopes that were present when the earth was formed are

       238U, T1/2 = 4.5 × 109 years

       235U, T1/2 = 7.1 × 108 years

       232Th, T1/2 = 1.4 × 1010 years

       40K, T1/2 = 1.3 × 109 years.

      These radioactive elements are ubiquitous; average uranium and thorium concentrations in the soil are of the order of several parts per million. Natural potassium, which is abundant in the earth's crust, contains approximately 0.012% of ⁴⁰K.

      2.1.1 Uranium and Thorium Progeny

      The primordial U and Th isotopes are the progenitors of three chains of sequentially decaying radioisotopes that have several common characteristics. Each series contains a radioisotope of the gaseous element radon, and each series terminates with a stable isotope of lead. Radon gas diffuses out of the ground, and the radon daughters, which are solids under ordinary circumstances, attach themselves to atmospheric dust particles. This naturally occurring airborne radioactivity must be accounted for when using the measured radioactivity in a dust sample to compute the concentration of an airborne radiocontaminant. Although the mean concentration of these naturally occurring radioisotopes is relatively low, the actual radioactivity concentration from this source varies according to the concentrations of uranium and thorium in the ground. Areas that have uranium concentrations much higher than the average thus may be expected to have relatively high atmospheric radon concentrations.

      2.2 Anthropogenic Sources

       X‐rays. Any electronic devices in which electrons fall through potentials of 10 kV or more produce X‐rays, and should be considered a potential source of exposure. Examples include medical diagnosis and therapy, industrial radiography for inspection, and X‐ray fluorescence and X‐ray diffraction analysis in research laboratories.

       Accelerators. Intense beams of all radiation types are produced for medical radiation therapy, radioisotope production, neutron production for neutron activation analysis, and research.

       Radioisotopes. This category includes sealed sources (for industrial uses such as gauging, inspection, food and drug sterilization, and medical radiation therapy), sealed neutron sources (for inspection, neutron activation analysis, and oil‐well logging), and unsealed radioisotopes (used in medical research, diagnosis, therapy, and generally in scientific and industrial research).

       Nuclear reactors. Properly operating nuclear reactors produce negligible environmental radioactivity and exposure to the public. Nuclear reactors produce large quantities of neutrons in the fuel, resulting in the activation of many components in close proximity to the fuel. The fuel will contain both alpha‐ and beta‐emitting nuclides.

       Technologically enhanced natural radioactivity. The concentration of radioactivity may be technologically increased to produce the so‐called TENORM (technologically enhanced naturally occurring radioactive material). Since this natural radioactivity is found in water and oil, in addition to rocks, coal, and soil, any scale that builds up in pipelines used for oil and gas production or for carrying water and brine in industrial facilities concentrates СКАЧАТЬ