LST 2-4

Part 2 Continued


2-4. The radiations we are exposed to. Part I-8 presented a very simplified explanation of the pattern of radiation exposures to which the public is subjected. No details of the kinds of radiation or their actual sources were given. Part II will provide such details for those readers who wish more specific information to enable them to better shape their judgment or opinions on radiation-related questions.

So far, we have examined the ways in which man not only creates or releases sources of radiation but also tries to control it, to the point where it should be acceptable, or a matter of limited concern. Actually, it is only within the past decade that we have suddenly come to the realization that our main sources of radiation exposure are derived from radiation in the environment and that, to a considerable extent, these exposures are beyond our control

As far as a cell in our body is concerned, it " does not know or care" anything about the source of whatever radiation may strike it. A single x-ray in a chest x-ray examination may or may not damage a cell in exactly the same way as a single gamma ray from radioactive potassium originating from natural sources in some other part of the body. Actually, it now appears that of all the radiation exposure that the average person in this country is subjected to, some 82% is from sources that occur naturally in the environment. No place is exempt from this radiation. Radioactive material and radioactive gases in individual areas may vary over fairly wide limits, but there is literally no place in the world that is entirely free from ionizing radiation.

To examine a few of these radiation sources, it will be useful to refer back to the chart in Figure 4. When the world was formed, it was made up of a mixture of most of the chemical elements that we know today. Of the natural radioactive elements, the oldest is uranium, which has an isotope with a half life of 4.5 billion years. To give some idea of the amounts of uranium we are thinking about, let us imagine skimming off one square mile of earth's surface to a depth of one foot. If we then extracted all of the radioactive material from this material, we would find, on the average, some: 3 tons of uranium, 6 tons of thorium, and 1 gram (1/35 ounce) of radium. From these, radon and thoron, radioactive gases, are constantly being produced by the decay of radium and thorium. Most of these gases are trapped in the rocks and soil. However, some of the gaseous radioactive elements are continually escaping into the air and are responsible for a major portion of our atmospheric radioactivity.

As already noted, over three-fourths of the radiation to which man is exposed is from natural sources in the environment, and there is little we can do to reduce it by any substantial degree. Eleven percent of our overall radiation exposure is derived from the use of x rays, which are, indeed, produced by humans and are within our complete control. X rays, used for a variety of purposes, are generated and utilized only when needed, but all are expected to contribute directly to the better health and welfare of humanity. Four percent of our overall exposure is from nuclear medicine - again in the service of humanity - employing radiations which are derived from radioactive materials , some natural in origin, but now mostly man-made. As in the case of x rays, these applications of radiation are also within our control.

Well informed people do not fear a medical x-ray examination just because it involves the use of ionizing radiation. Similarly, well informed people do not fear living near a nuclear power plant because the plant releases nuclear materials and radiation. Actually, radiations released in the year-round operation of a nuclear power plant are less than 1/10 of 1% (one thousandth) of those from the natural sources which surround everyone. To address the fears of the ill-informed, we will provide a brief and non-technical explanation of these natural and man-made radiations in hopes of demystifying them. During these explanations, try to keep in mind that over the past thousands of centuries, the human race has developed to its present stature in a climate of radiation which, if anything, was even stronger in the past than it is today.

The fear of living in the vicinity of a nuclear plant of any kind is more reasonably based on the possibility, however remote, of some kind of an accident. The chance of such an occurrence is very small. It is reasonable to accept these small risks, as we accept the risks of the other accident conditions that threaten us -- such as airplane crashes, dam failures, refinery explosions, and the disruptions caused by hurricanes, tornadoes, and major earthquakes.

2-4.1 X-rays X-rays cover a wide range of energies, some of which overlap those produced by gamma rays. They are essentially the same as gamma rays, but can be turned on and off at will. Because the applications of x rays are so familiar to everyone, some of their properties will be discussed in more detail. Today, x rays are produced in vacuum tubes by applying very high voltages to the terminals of the tube. These voltages may vary from a few tens of thousands to a few million volts. (A light bulb in the home requires 110 volts). The ability of x rays to penetrate or pass through matter increases as the voltage used to produce them increases; the energy or penetrability is expressed in kilovolts (1 kilovolt =1000 volts)

Dental x-ray examinations, with which most of us are quite familiar, are made with 50 to 75 kilovolt x-rays. Under most dental office conditions, shielding by 1/32 inch of lead is usually adequate shielding to protect both the dental technicians and persons in adjacent rooms. Two or three inches of concrete, four to eight inches of cinder-block, or 3/4 inch of barium plaster will provide the same protection. A distance of 5 or 6 feet behind the x-ray unit - opposite to the direction of the x-ray beam - is usually adequate for the protection of the dentist and x-ray technologist. Well known radiation protection design principles apply to each individual situation.

To give a very rough idea of radiation protection requirements for medical x-ray installations, the following examples will serve. Diagnostic x-rays up to 125 kilovolts may need shielding ranging from 1/32 to 3/32 inch of lead or its equivalent in other materials. Therapeutic x-ray installations operating at 200 kilovolts will need shielding up to 1/4 inch of lead, which, because of its heavy weight, is usually structurally impractical; 12 to 16 inches of concrete would be equivalent. For "super-voltage" x-rays, ranging up to 10 to 20 million volts, concrete walls and floors may need to be from four to six feet thick.

2-4.2 Radioactivity All matter, physical or biological, is made up of atoms of many kinds and varieties. Each kind of material or element, for example, iron, copper, oxygen, and so on, has its own kind of atoms;. Even the tiniest speck of one of these materials contains countless atoms -- numbers so large that they lose all meaning to the average person. These atoms are referred to as stable atoms; they do not change with time; a piece of copper remains copper forever.

In addition to the stable atoms that make up most of the world, as we know it, there are unstable atoms which undergo continuous change - actually changing from one kind of atom to some other kind. Usually the new atom is also unstable and at some time will, in turn, change into yet another kind of atom. Unstable atoms are called radioactive atoms -- "radio" because, in the process of changing or disintegrating, they give off some kind of radiation -- "active", because they are indeed active; they are changing all the time. Each atom has its own assortment of radioisotopes and the rate of disintegration differs from one radioisotope to another. Some examples are given in Figure3. This process of atoms changing is frequently referred to as " radioactive decay". With each such change, an atom gives out a small part of its vast supply of stored-up energy. For some atoms the decay process ends in a stable atom, entirely different from the original unstable atom. An example of this will be given below. (See para. 2-4.3)

As a part of the process of disintegration, with each change, a small part of the energy in the atom is given up in the form of ionizing radiation. One kind of radiation is alpha radiation, which can penetrate only two or three inches of air, or be stopped completely (absorbed) by a single sheet of paper or an equal thickness of human tissue. The outer non-living layer of skin on the human body is thick enough to protect people from external alpha radiation. Alpha rays are positively charged particles and are emitted by naturally occurring elements such as radium or uranium and some of their decay products like radon, a radioactive gas. The special hazard of radioactive materials that emit alpha rays lies in the possibility of their being breathed into the lungs, as with radon, or being taken into the body along with food and water. Within the body cellular components are not shielded from alpha radiation. Alpha rays are also emitted by some man-made radioactive materials, such as plutonium, but exposure of the public to hazardous conditions involving plutonium is very rare.

There are also beta rays given off in some decay processes; they can penetrate 1/2 to 3/4 inch of water or human flesh - the thickness of a human hand. An eighth of an inch of aluminum will stop, or absorb, most of them. Beta rays are negatively charged particles and are emitted by some naturally-occurring radioactive materials and by some of the waste products of nuclear reactors. Beta rays are useful in the treatment of diseases involving the skin. They are of lesser concern than the more penetrating x- and gamma radiation, as a source of exposure of the general public.

Gamma rays are produced in the radioactive decay process and occur with a wide range of energies, depending upon the particular kind of radioactive material from which they come. They can penetrate the human body, which will absorb only a small part of them. Three to four feet of concrete or several inches of lead are needed to completely stop or absorb them. Concrete and lead are the materials most commonly used in the shields required to protect people from exposure to gamma rays.

Neutron radiation is generally of no concern to the public, since it exists primarily inside nuclear reactors, in which the neutrons can be adequately contained. Neutron therapy is useful for some disease conditions. Some neutrons are produced naturally at very high altitudes but do not reach the earth.

2-4.3 The origin of natural radioactivity. At the beginning of this planet, some 4-1/2 billion years ago, the earth was a tremendous ball of stuff, the greatest portion of which was the stable atoms that we are familiar with every day of our lives. Included in this mass were several kinds of radioactive atoms which, because they were there from the very beginning, are called primordial atoms. Important among these, are a few radioactive materials to which we can trace the great bulk of our natural radioactivity on the earth. Three such primordial radioactive elements will be discussed below because it is from those that we derive the greater part of the "sea of ionizing radiation" within which life has evolved and the human race has developed.

The primordial radioisotopes began to disintegrate the moment they were created, and they have been doing so ever since. Each radioisotope changes into a new kind, and it, in turn, disintegrates and changes into another kind, and so on and on. For certain of the primordial radioactive elements there is a long series of such changes before the last one. The final remaining atom is lead -- stable lead -- the kind of lead we use for shielding ourselves from some radiation sources, or for making sinkers for fishing.

Figure 5 shows the series of radioisotopes that result from the disintegration of Uranium-238 which contributes an important part of our radiation background. Given at each stage is the name of the radioisotope and its half life. (As explained earlier, the half-life is the time for any given number of the same kind of radioactive atoms to decrease to half). Because the precise description of each of these steps is complicated beyond the need or purpose of these discussions, they are being listed by their general names and numbers.

It is not the purpose of Figure 5 to provide a mass of information which everyone need study or know. It is primarily to show, in the process of nature and natural radioactivity, where some of the radiation and radioactive elements of concern come from. Important members of the uranium series, for example, are radium-226 and radon-222. Radium occurs in appreciable amounts in all soil throughout the world. Radon is a radioactive gas that results from the disintegration of radium. It has a very short half life, but supplies about 4/5 of the radon shown in the pie chart (Figure 4). The remaining 1/5 of the radon included in Figure 4 is Radon-220, a decay product of naturally occurring Thorium-232. Thorium is twice as plentiful as uranium in the soil.

The distribution of uranium and thorium in soil is quite variable. In other words, there are some square miles of the earth's surface which show very little radioactivity, but there are other areas that show amounts many times higher than the average. Such a region with large uranium deposits with consequent radon seepage, for example, occurs in south-east Pennsylvania, cutting across into upper New Jersey and New York state. It is referred to as, "The Reading Prong" because it is shaped like a spearhead, and passes near the city of Reading.

In this and several other countries, uranium deposits have accumulated under ground, due to local geological conditions, to the point where it can be mined and refined at reasonable costs. For example, many such mines exist in the Colorado-New Mexico area, and lesser deposits are scattered around the country.

The presence of substantial quantities of uranium in the ground can be detected by measuring the radon which seeps up through the soil and is trapped at the surface. One of the unintended ways that radon is trapped is in a dwelling. Houses are built on the ground, with or without basements, and the average home covers about 1500 square feet of the earth. Depending upon a wide variety of circumstances, the radon that is creeping up through the ground under the 1500 square foot foundation may find ways into the house. Factors such as the air-tightness of the house, its heating and air-conditioning system, and its general ventilation may cause the radon level in homes to build up to undesirable levels. There are, however, no known cases of specific injury to resident caused by this radioactive gas. It is nevertheless a potential hazard that needs, at least, to be considered. (It is of interest to know that in the average home, with normal family traffic in and out, there will be two to three complete air exchanges per hour).

Sometimes the radium deposits in soil are substantially larger than average, although not large enough to warrant extracting the radium for commercial purposes. This can result in unusually high levels of the decay product, radon gas, in homes built over such deposits. If the presence of such deposits is known, or suspected, at the time of building, relatively inexpensive steps can be taken to avoid the potential seepage of radon into the home. Thus, there are adequate means of constructing new homes so as to avoid any radioactivity problems inside. It is unfortunate that it has only been within the last few years -- about a decade -- that there has been a public awareness of the possible local seriousness of radon in homes. Prevention of seepage into an already constructed house can sometimes be quite difficult. However, in most situations, keeping a window open an inch or two, together with the normal house-air leakage, will provide adequate venting of the radon without an unacceptable loss of heat.

Other examples of unusually high radiation levels at the surface of the earth, exist very locally in parts of India and Brazil. In these locations the ground is covered with a black substance known as monazite sand, a relatively radioactive ore that is derived from thorium deposits. While described as local areas, some of these are large enough to contain small villages with substantial populations. The radiation level a foot or two above the surface of the sands may be as much as 20 times the average background level elsewhere, exposing people who live on this soil to annual doses of 5,000 to 10,000 millirems (50 to 100 millisievert). Studies of these populations, including people whose families have lived on the sands for several generations, have not disclosed any unusual trends in cancer or any other radiogenic disease.

An interesting little sidelight on this is the fact that the radioactivity of these sands, and lesser levels in some European spas or springs, has been, and is probably still being exploited as having a curative value. People pay to lie down on this relatively highly radioactive soil or soak in the radioactive waters for days at a time. There are no records of anyone having been injured -- or cured of anything -- as a result of these practices.

2-5 How we are all exposed to ionizing radiation. The largest single source of radiation exposure of the U.S. general population, a little more than half -- about 55% -- comes from radon. Out of a total of 82%, this leaves 27 % of man's exposure due to other natural or environmental sources. These, in turn, may be divided into three categories: 1) Cosmic radiation, i.e. radiation originating outside of the earth's atmosphere; 2) Terrestrial sources, due to radiation from radioactive materials that are in the rock and soils everywhere (both sources are external to the body); and, 3) Internal sources which are derived from radioactive material that is in the body, partly in the form of material taken in as food and part that is in the body tissues. The radiation dose to the body from each of these three sources is very roughly the same, as shown in Figure 4.

2-5.1 Cosmic radiation. The average annual dose to a person from cosmic radiation is about 26 millirem in a year at sea level. Some of the cosmic rays are absorbed in the air above the earth's surface; thus, as we go to higher altitude, there is less air between us and the source of the cosmic rays and our dose from this source increases. The cosmic ray intensity doubles for each 1-1/4 mile of altitude. Thus, in Denver, which has an altitude of one mile, the intensity of cosmic radiation is about 50 millirem a year, as compared to about 26 mrem a year at sea level. It is also worth noting, that air travel at an altitude of 39,000 feet, a common altitude for commercial flight, gives an enhanced cosmic ray dose of about one-half mrem per hour of flight for each individual passenger. This would add about one mrem in a year to the dose of an average member of the US population.

2-5.2 Terrestrial gamma radiation. As already noted, the radiation from terrestrial sources comes from the radioactivity that is widely distributed in soil and rock. However, because most building materials -- such as wood, stone brick, pipes, and nails -- are derived from the soil, each will contain a small amount of radioactivity, however infinitesimal, and will contribute some dose to the residents from the radioactive materials built into homes, schools , and other structures. While one might think that by going into his home he could avoid that extra radiation from the soil, the fact is that the structure itself would only shield him from about 20% of his outdoor exposure, the other 80% deriving from the housing materials. The average dose to a person in the US population from these sources is about 29 millirems a year.

2-5.3 Radioactivity in the body. The principal internal contributors to irradiation of the body tissues are potassium-40, polonium-210, and rubidium-87. Potassium-40 is a primordial radioactive atom; that is, it has been present since the start of the world. Potassium is an important part of our body and food system and enters the body by way of food, milk, water, and the air we breathe. The body incorporates the radioactive potassium-40 into any tissues containing the element potassium. It starts as radioactive potassium, and then decays directly to stable calcium or argon. Its average contribution to man's dose is about 40 millirems in a year. The contribution by the rubidium-87 is only about 1 millirem and is included in the internal body dose listed for potassium-40. The remaining internal whole-body dose of radiation comes from the uranium/thorium/ radium/polonium series discussed above. All of these radioactive decay products together, contribute about 95 millirems a year to the average person in this country.

2-5.4 Inhaled radioactivity. Most of the exposure from inhaled radioactive material comes from radon, discussed in detail above. Radon contributes approximately 200 millirems to an average member of the US population. Radioactive carbon-14 is minor source of inhaled radioactivity. The dose from this isotope is substantially uniform over the world because it is formed primarily by interaction between cosmic rays and the carbon dioxide which is in the atmosphere. Carbon-14 contributes about one millirem a year to the dose of an average person.

2-5.5 Summary of annual natural radiation doses

The dose to an average person in a year from natural radiation sources is summarized in the following table:

Drawn from the data in sections 2.5.1 to 2.5.4, above, it is evident that there is little flexibility in reducing most of our irradiation from the natural sources of radioactivity. Probably the only significant opportunity for dose reduction would be in better controlling the entry of radon into homes, and on this there is a reasonable question as to actual need and practicability in many cases.