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PART 1 continued
RADIATION AND THE WORLD WE LIVE IN
1-6. How were radiation safety standards developed? Even after the introduction and use of the Coolidge x-ray tube, the detailed effects of radiation on the body were poorly understood. Moreover, there were no uniform methods for measuring or controlling the amounts of radiation to which the physicians of that period were exposed. In fact, soon after World War-I ended in 1918, medical practitioners began to fear that they might have to give up the use of x rays altogether, because of the unacceptable hazards to themselves.
Largely as a result of this concern , the radiologists of the world organized The First International Congress of Radiology which met in London in 1925. The first important objective of that Congress was the immediate establishment of a committee to develop and reach international agreement on a standard method and unit in which to measure radiation - to be the "feet and inches" of radiation, as it were. Up until that time, trying to describe the amounts of radiation that scientists or physicians were using was like trying to measure a piece of property without any rulers or tape measures.
The other objective was to plan the organization of a second committee and program on protection against radiation at the time of the next Congress, which was to be held in Stockholm in 1928. It was hoped that by then there would be at least tentative agreement on the measurement problems. Until such agreement was in hand, little progress could be made on the issue of protection.
By the close of the 1928 Congress, the measurement committee was able to report agreement on a new quantity, and a unit to be named the Roentgen, for measuring x rays. That unit remained in use until 1953, at which time two, more fundamentally based units were added -- the rad and the rem. These terms have been used frequently in news accounts and will be discussed in more detail later.
The Congress also led to international acceptance of the initial recommendations on radiation protection for users of both x-rays and radium. It is important to realize that the possibility of radiation injury was a matter of concern for at least 45 years prior to the development of controlled nuclear energy, as we now know it. The central problem in developing standards was to reach agreement on a specific, numerical level of exposure to which humans might be subjected with reasonable assurance that they would not be harmed. In the 1920's that was not yet possible. However, agreement was reached on a series of recommendations for the shielding of radiation workers from x rays and the gamma rays from radium.
British and German radiologists had developed relatively simple x-ray protection recommendations as early as 1913. By 1920, the British, followed by radiologists in the United States and several other countries, developed more complete protection recommendations for both x-rays and radium. In Stockholm in 1928, two new international commissions held their first meetings: (1) the International Committee on X-Ray Units (ICRU), (later to be The International Commission on Radiation Units and Measurements), and (2) The International Commission on Radiological Protection (ICRP). Thus, by the end of the summer of 1928, international agreement had been reached on a system for radiation measurements and another on standards for radiation protection.
At the time, development of safety principles and practices was a matter primarily of concern to the medical profession, but it was recognized by the professional medical and radiological societies in the United States that there was no single organization equipped to speak for all of them and obtain an agreement on radiation protection standards designed specifically to meet the medical needs of this country. As a result, an Advisory Committee on X-ray and Radium Protection was organized in 1929, to represent the various medical organizations and x-ray equipment manufacturers. That organization continues to function, now under the name of National Council on Radiation Protection And Measurements (NCRP). Its responsibilities have broadened over the years; in 1989 it celebrated its 60th anniversary and the 25th anniversary of the Charter given to it by the United States Congress in 1964.
By mid 1931, physical standards for the measurement of x rays had been developed in four countries, including the United States, and were found to be in good agreement. The remaining task was to determine an acceptable level of radiation to which radiation workers might be on a continuing basis with reasonable safety.
This level was initially described as a tolerance dose, which is another way of saying a dose that is acceptable, or can be tolerated, by individuals in relative safety. In 1934, the amount of dose adopted by the NCRP as tolerable was 1/10 unit a day (30 units in a year). Later that year 1934, the ICRP proposed a value of 2/10 unit a day (60 units in a year). Both were based on working 300 days a year. Observations during the 1930's and later appeared to confirm that both of the tolerance doses which had been proposed were within reasonable safety limits.
A major test of these standards came with their use at the start of the atomic bomb project in the early 1940's. During the three years of the project, a great amount of biological research was conducted. Several men were killed in a laboratory accident that exposed them to very large amounts of radiation. No one else was seriously injured by radiation during that entire intensive development - a clear demonstration that the Advisory Committee standards were indeed effective
After World War II, the NCRP was reorganized; it greatly expanded its study programs to include all of the newly discovered radiations and their accompanying problems Because it was realized that the use of radiation would soon be much more widespread than it had been before the war, in 1946 NCRP recommended a reduction of the tolerance dose for radiation workers from 1/10 unit in a day to 3/10 unit in a six-day week. This was half of the previous allowance, and permitted up to fifteen units in a year.
By this time it was recognized that there were two major classes of radiation exposure conditions, and they could not be treated in the same way. First were acute exposure conditions; these would involve large radiation exposures in short periods of time and could result in serious injuries, recognizable within hours or a few days. In peacetime they could be caused only by a major accident (which fortunately is likely to be very rare) or by a large therapeutic radiation exposure. The second class, the more common exposures, often referred to as protracted exposures, involve very small amounts of radiation, generally delivered over long periods of time - months to years. For these exposures there is almost complete uncertainty as to any effect in man. If an effect occurs at all, it might not be clinically observable for many years and would probably be indistinguishable from same effect caused by any of a large number of other agents.
The permissible levels of radiation exposure for radiation workers and the general public fall within the category of protracted exposures. They are frequently referred to as "low dose". In fact, the tolerance dose level, and later permissible dose levels, were originally chosen in the belief that no injuries would ever be found at or below these levels -- and that may be true. The discussions in this paper apply only to exposures below the permissible dose limits.
The reduction in permissible dose in 1946 was not made because of any observed effects or new knowledge that radiation was more hazardous than previously thought. It was done because it was practical and prudent and could be accomplished without an unacceptable increase in cost. Also in 1946, an upper limit of exposure for children in the population was recommended. The limit was to be not more than 1/10th of that for radiation workers. Since children could not be separated from adults as far as radiation control practice is concerned, this meant that all members of the public would be equally protected with an extra safety factor of ten.
By the mid-1950's our radiation technology had increased extensively, as had also the uses and applications of radiation by the public and our knowledge of the biological effects of radiation. Our capabilities in protection had also increased, so at that time it was decided that the allowable, or permissible, dose for radiation workers could again be further reduced, this time to 5 units in a year - and 1/10th of that amount for the public. Again, no evidence of increased hazard prompted this change. Subsequent studies have shown that radiation at moderate and high doses is a weak carcinogen -- it can increase the risk of cancer. It is not known if low doses are also carcinogenic, but the assumption is often made, for planning and design purposes, that there could be some small risk associated with low doses of radiation.
The point of this discussion is to make it clear that over the last 80 years there has been a continuing study of ionizing radiation to enable its continued use within acceptable limits of safety and with freedom from fears of injury. Because of the confidence we now have in our understanding of the biological effects of radiation and in our capabilities for properly measuring it, ionizing radiation today is among the least threatening of the carcinogenic agents to which people are exposed.
All of the actions discussed above were carried out by private organizations completely outside of any government structure or controls, although several government agencies were participants in the committee activities on the same footing with the professional organizations. This continued until the late 1950's when some Congressional Hearings brought to light the fact that the federal government had never developed any radiation protection standards of its own. It had simply adopted those recommended by the NCRP in 1934, 1946, and again in 1956. In 1959, a Federal Radiation Council (FRC) was established to officially watch over government radiation protection standards and practices. After about a year of study, the Federal Radiation Council adopted the standards proposed by the NCRP. Today the functions of the FRC are performed by the US Environmental Protection Agency.
It is worth noting that the acceptable, or permissible, doses in 1934 were 12 times higher than in 1990. Nevertheless, it has not been possible to find any differences between the kinds and patterns of cancers seen in people who might have been exposed to radiation at those earlier levels and the cancers that normally occur among the general public today.
Furthermore, it has never been possible to establish an unambiguous link between and radiation exposure and cancer incidence in groups of workers exposed to radiation while working within the accepted standards of safety (in the range that is commonly referred to as "low-level radiation")
1-7. Is radiation a part of the natural world ? We live in a sea of radiation. Radiation has always been a part of our lives -- not just since the release of atomic energy half a century ago. Radiation has been a part of our natural environment since our world began. Different kinds of radiation touch us every moment of our lives, and some kinds will play an increasingly important role in our ability to continue to live on this planet. In this book, discussions will be focused on ionizing radiation. These radiations are of growing importance today, and the continued development of their uses will depend, to a considerable extent, upon decisions that will be made by all of us, in times that are not too far ahead.
It is unfortunate that the first widespread attention to any kind of atomic radiation came in connection with warfare. The military use of radiation is a subject that will not be dealt with in this document. We are looking at nuclear radiation as it is used in many peaceful ways. It is something that we need and can utilize for the benefit of mankind, as long as it is properly controlled. Part 2 will discuss some examples of radiation uses in research, industry, and medical diagnosis and treatment
There are many aspects of radiation that are critically important to our lives, and it is important that we all know the facts about radiation. Sooner or later, it will be necessary for us to exercise our own individual judgment in interpreting things that are said or written about radiation. This judgment must be based on factual knowledge.
The study of radiation, how we produce it, how we measure it, and how we protect ourselves from its possible harmful effects, involves a mixture of many different scientific and technical disciplines. These include biology and medicine, chemistry, physics, mathematics, and engineering.
There are a number of different parts of the problem of understanding radiation. Information on the negative aspects of radiation is widely distributed and poorly described. In this book, an attempt will be made to undo the effects of such incomplete and misleading information, by pointing out some of the facts we need to know about radiation. These facts will relate to the effects of small amounts of radiation. As is the case for any physical, chemical, or biological agent we know of, radiation, if excessively or carelessly used, can be harmful. But in this country over the last 40 years, we know of no deaths caused by exposure of the public to nuclear radiation.
The statement immediately above applies to the accident at Three-Mile Island. Following that event, some writers predicted that as many as a thousand deaths would follow. Actually, there was not a single biological injury found, let alone death, that can be definitely related to radiation from that accident. The statement also applies to the production of electricity by other nuclear power plants in the United States. A study by the National Institutes of Health of the populations surrounding each of the nuclear power plants in the U. S. was completed in 1991. It found that there were not significantly more cancers or other radiation-related effects in areas near the plants than in areas far removed from nuclear activities.
The discussions to follow will be limited to what is commonly referred to as ionizing radiation. We will simply use the term "radiation" to mean "ionizing radiation" unless otherwise indicated. Our aim, overall, is to show that properly controlled applications of ionizing radiation are critically important and are far less dangerous than is claimed by their critics.
1-8. What radiation exposures do people get ?. Up to this point, several different kinds of radiation and their sources have been mentioned. It is now appropriate to examine how the different radiations are divided among natural sources -- the ones that exist essentially everywhere in the world -- and man-made sources -- those used in a variety of applications and subject to man's control.
Up to this point we have not expressed amounts of radiation in any particular units -- the "feet and inches" of radiation exposures. One of the units used for many years, and frequently mentioned in the media, is the "rem". (This is a name, not an acronym). Most of the radiation doses we discuss in reference to exposures of the pubic are less than one rem, so we often speak in terms of 1/1000th of a rem -- a millirem. Rem and millirem have the same meaning for radiation as gram and milligram have as units for measuring a dose of common drugs. In the comparisons to be discussed below, we will use the millirem. The pie chart in Figure 4, taken from NCRP report No. 93, illustrates the sources of radiation exposure to people in the U. S.; it indicates that the average exposure of persons in this country is about 360 millirems in a year.
The clear area on the chart indicates that about 82% of our exposure to ionizing radiation is from natural sources in the environment. The largest contributor to the total, about 55%, is radon, a radioactive gas which originates in the radium which is a component of practically all soil and rock. The other exposures from natural radiations are those from cosmic radiations from outer space -- about 8% ; terrestrial sources -- about 8% ; and internal radiation, primarily from radioactive potassium in our bodies -- about 11%. The sum of all of these is an average exposure of almost 300 millirem in a year.
Contrasted with this, the shaded portion of the pie chart shows that medical x-rays account for an average of only about 11%, and nuclear medicine, another 4%, of the average person's radiation exposure, for a total of about 54 millirem. These medically related exposures generally have clear-cut benefits to our health
Radiation exposure from commercial applications of ionizing radiations contributes only about 3%, or 11 millirem, to our total exposure. Among the most important of these sources of radiation are our domestic water supplies, building materials, mining and agricultural products, and fuels -- particularly coal. Each of these applications is generally associated with some net health benefit. The average exposures from these sources are small and are often expressed in microrem (millionths of a rem) rather than millirem. There is a wide variety of lesser sources -- television sets, for example. Luminous watches and clocks, airport inspection systems, and smoke detectors, contribute to the average person's exposure less than a hundredth of the radiation from TV receivers. And the radiation from television sets is, itself, quite small.
It is worth noting that the present production of electricity by nuclear power contributes, on average, only about 1/1000 (.001) of our radiation exposure; that includes mining, milling, reactor operation, transportation, and waste disposal. Electricity production from coal-fired power plants, on the other hand, contributes, on average, up to about 5/1000 (.005) of our radiation exposure. In other words, up to five times as much radiation exposure of the population results from coal-fired plants as from nuclear power generation. It should be noted that there are other types of environmental pollution, e.g., from toxic chemical emissions, waste, and mining, associated with coal-fired power plants.