LST 2-3

Part 2


2-1 The development of radiation protection standards. Part I showed, in simple terms, what ionizing radiation is and some of the factors that have been considered in establishing the standards of radiation safety used in the world today. It is strictly a non-technical overview. For those who may want to better understand the basic aspects of radiation protection, this Part will add more detail. The few special terms necessary to understand the explanations, including the names and use of the radiation quantities and their units, will be dealt with as simply as possible.

Unfortunately there is a complicating factor which cannot be avoided. In the late 1970's, an international agreement was made concerning physical quantities and scientific units, including those for ionizing radiation, to be used in future years. Many terms, to which the scientific community had become accustomed were changed, both in name and in size. Even though there is an international agreement on paper, it is impossible to have the names and sizes of physical quantities change and expect the new formulations to fall into immediate use. To ease the transition, technical publications, in 1979, adopted the practice of using the old units (which had been in use for many years) followed immediately by the new units in parentheses. Five years later the practice was to be reversed to use the new units, followed by the old units in parentheses. Eventually all publications will be using the new system of quantities and units exclusively. This has already been done in the rest of the world, without exception. In the United States, both the old and new units are still used by government agencies and the non-technical media.

Most material written in this country about ionizing radiation, in such non-technical sources as newspapers or magazines, continues to use the old units. Therefore you will need to recognize both the old and the new units. After a short discussion about the units, a small list of equivalent units will be given. This appears to be the best compromise to deal with an otherwise unfortunate situation, as far as the public reader is concerned.

In Part 1, the term "dose" was used in a very general sense to mean the amount of radiation a person is exposed to or the amount of radiation absorbed by or in the body. For purposes of protection standards, a dose is the energy absorbed in the organs of the human body. To be exposed to radiation, that is to absorb some radiation energy, is to receive a radiation dose. The terminology "dose" is a carryover from the early medical use of x rays, comparable to the dose of medicine measured in grains or ounces. For medicine, a full prescription includes other information besides the number of grains or ounces of the drug; it includes the time and amount of each administration; whether it should be taken with, before, or after meals; possible interactions; and other data, so that the desired effect could be achieved. For radiation, the beneficial effects can be best achieved only if the quantity of radiation, as well as the rate and manner of delivery to specific organs, are specified, and its physical characteristics are described. All of these together - and more - constitute the prescription of a radiation dose.

An example of the need to consider all of the factors involved in the description of a dose of radiation, we might look at the prescription of a medicinal tablet. A prescribed tablet taken once a day for twenty days may be very effective in treating a particular disorder, but to take all 20 in one day could be lethal. A dose of radiation to individuals must make allowance for the fact that individual body organs may respond differently to equal amounts of the same radiation delivered over different time intervals. Moreover, the same organ may respond differently to different kinds of radiation. A dose is commonly expressed in "rem", a unit which allows for the radiation differences. Because, in most radiation work, the amounts of radiation to which workers are exposed are usually very small, the dose is often expressed in millirem, or thousandths of a rem. The rate of delivery of the dose would be expressed in millirem per hour (or day or year)

The units for dose are as follows:

New System Old system

1 sievert (Sv) = 100 rem

0.010 sievert (Sv) = 1 rem = 1000 mrem

10 millisievert (mSv) = 1 rem

1 millisievert (mSv) = 0.1 rem = 100 millirem

To help readers to understand the significance of some radiation incident or situation, writers frequently make comparisons between the dose in question and a dose to a typical individual from a chest x-ray examination. This is done in the honest belief that since almost everyone has had an uneventful chest x ray, such a comparison will be helpful to understanding radiation quantities. Helpful or not, it is a poor comparison and under some circumstances may be misleading. Such a misuse of the dose concept occurs in comparisons between, say, a 20 millirem dose given to a person for a chest x-ray examination for medical purposes, and a 20 millirem dose received by a radiation worker in a nuclear power plant over a 30-day period. The first is a dose to a small portion of the body in a fraction of a second; the second is a dose to the entire body, distributed irregularly over many days. Obviously it is improper and may be misleading to directly compare such different events.

With this background it might be noted that the official Government standards for radiation protection in current use today, except for a few refinements which will be noted as we go along, are essentially the same as those proposed by the International Commission on Radiological Protection (ICRP) and the U.S. National Committee on radiation Protection and Measurement (NCRP) in 1956-1957. At that time, the people who might be exposed to ionizing radiation at levels that appeared to be critical were primarily radiation workers, for whom it is practically impossible to reduce radiation exposure to zero. The value for the basic maximum permissible dose chosen for them was 5 rem (50 millisievert) in a year.

It was recognized in the early 1950's that some members of the general population might occasionally be exposed to radiation resulting from the nuclear operations and weapons tests being carried out during that period. Since it was not practical to exercise direct control over the receipt of exposure by individual members of the population an additional "safety factor" was provided. Their permissible dose limit was set at one tenth of that allowed for radiation workers, or 500 millirem (5 mSv) in a year. Later, because of the impracticality of controlling, or keeping track of individual exposures, an average permissible effective dose for a large population group was set at one third of the individual permissible effective dose, or 170 millirem (1.7 mSv) in a year.

The official permissible dose of 5 rem (50 mSv) in a year for radiation workers has remained essentially unchanged since 1956. The principal reason is the fact that since 1956 there has been no convincing evidence that individual workers who were exposed up to that level were injured by radiation. If 5 rem is considered acceptable for individual radiation workers, the degree of safety would be even greater for members of the general public, since they are allowed only one tenth of that acceptably safe dose. Furthermore, an overview of all of the radiation uses and practices that have been developed since 1956 has failed to disclose any reasonable likelihood that the patterns of exposure to man-made radiation will develop any new and unexpectedly dangerous aspects. Actually, there is probably less man-made radiation loose in the world today, since the atmospheric testing of nuclear weapons was discontinued in the early 1960's. The pie chart in Figure 4 shows that 99% of our radiation exposure is caused either by natural radiation or man-made radiation from identifiably worthwhile products or procedures. Of the remaining 1% , roughly one third is due to radioactive fallout from nuclear testing from 1945 to 1963, and that value today is probably only about half of what it was in 1956 when the current standards were introduced

2-2 What risk means to the public. A major problem that faces the radiation protectionist and radiation industries and that influences public opinion about radiation, is our society's general attitude toward risks. This fearful attitude is fed by the radiation-risk threats made by certain anti-war, anti-industry, and anti-nuclear activists. The subject of risk is so broad and complex that it deserves a whole book itself. In this book, we must limit the discussion to a few words about it that are necessary here.

Over the past few years many studies have been made of risks of all kinds and how they are regarded by the public. There are many data available on actual casualties or injuries in industrial activities and in conventional activities in which the general public engages. For example, consider automobile-related injuries and deaths. That close to 50,000 people are killed by automobiles during a year, is a fact -- not an estimate or extrapolation. Such a number does not depend upon theory. From such data surprisingly accurate predictions of automobile accident fatalities can be made and can be verified by counting the bodies.

In contrast with such risk studies, for which there are observed and recorded data, the available data for radiation effects cover only a relatively narrow range of radiation exposures. Two of the principal sources of data on the effects of radiation on human populations are the studies of patients treated for ankylosing spondilitis - a disease of the spine - and of Japanese survivors of atomic bombing. There are scattered sources of data from the radiation treatment of other diseases, but these serve more to verify the other two main sources rather than to widen the range of information. Most of these data are in the "high dose " range of acute exposures and are therefore outside of the "low dose" range that is the only one with which the public need be concerned. ("Acute exposure" means a single exposure, usually delivered in a short time period - from fractions of a second to a few hours). Extension of these data to obtain estimates of dose effects in the low dose range almost invariably leads to unnecessarily high risk estimates. Such extension is an improper procedure.

From the many studies of radiation effects that have been made over the past 50 years, virtually no significant effects on humans have been found when protracted doses are below about 20 rem (0.2 Sv). Nor are there any significant data for acute doses below about 5 rem.(0.05 Sv) This is not to imply that we know that there are no effects in the range below 5 rem; there may be. However, if there are any effects they are masked by the cancers caused by any of hundreds of other agents to which people are exposed daily. Unfortunately -- or fortunately -- the doses in the range of 5 rem or less, acute or protracted, are the ones about which the general public should be better informed, because they comprise the vast majority of doses that the public is likely to receive in health care or as a result of industrial activities.

The difference between the effects of occupational or public exposures, as from medical x rays of a part of the body, and the effects from instantaneous exposures of the whole body, as from a bomb, is further widened by the fact that the former is rarely of an acute nature and is more commonly distributed roughly evenly over time. For any dose of moderate size, (up to at least 50 rem) there is evidence that some degree of recovery of any damage to body cells caused by radiation takes place following each individual exposure. It is conservatively assumed that this recovery would occur in the dose range up to 5-rem . In other words a dose of 5 rem (0.05 Sv), distributed randomly over a year, is less effective than the same dose given all at once, e.g., within an hour.

It is now believed (based on biological theory and experiments) that for exposures below 5 rem, any effects would not exceed a maximum value and may be less than the maximum. The maximum value is proportional to the dose no matter how it is delivered. Thus, if a dose of 5 rem would cause 25 effects of some kind, 1 rem would cause no more than 1/5 as many, or 5 effects. This theoretical relationship is applied in the entire low-dose range. Since we do not have any data on effects on humans in this range, we have to accept the possibility that the actual number of effects may be anything between zero and this theoretical maximum.

To bring this relationship into the range of general interest in normal radiation exposures, we might examine what effect (let us say, deaths from cancer) would result from exposing each of a million people to a single dose of 1 rem (0.01 Sv) , excluding all natural radiation. By simple proportion, based on the effects of higher doses on the Japanese survivors, the 1 rem (0.01 Sv) would be expected to cause a maximum of 300 to 500 cancer deaths among that million people at some time in the future. In that population there would also be about 200,000 deaths from cancer from all causes. The difficulty in demonstrating the effects of radiation for individual exposures of less than 10 rem , is that this number of deaths will be too few to detect with any meaningful accuracy among all the other causes of deaths. Moreover, there is simply no way to find a million people who may have been exposed to a single dose of about 1 rem, and no other radiation exposure, other than that from natural sources, experienced over the rest of their lives.

The most that we can say with any moderate certainty is that if a million people are each exposed to a dose of one rem of radiation to their whole body, up to 500 of them may ultimately die of cancer as a result. (Of course everyone will be exposed to this much radiation every 2 or 3 years from natural sources, very little of which can be avoided). This problem of uncertainty is further compounded by the fact that in any situation involving cause and effect, the difficulty of proving that there is no effect is much greater than proving that there is an effect.

All of the arguments about the effects of very small radiation doses share a common weakness. In dealing with any uncertainty in our knowledge, and its application to the determination of the effects of small radiation exposures, our conservative tendency is to overestimate rather than underestimate the hazard. If there is a range of choices in interpreting the scientific findings, we incline to choose the more conservative position. That is, we make the hazard appear to be greater than it may actually be on basis of observable effects at high doses Thus our assumptions, if in error, lead to working dose limits that may be lower than actually necessary.

2-3 Radiation dose limits for people. As already noted, in 1934, it was believed in the U.S., on the basis of limited experience, that some amount of radiation could be accepted, in relative safety, by radiologists in the course of their clinical work. This did not mean that this amount was absolutely safe, but only that it was tolerably safe -- any adverse effect would be considered as minor, and would not impair the health of the professional radiologist or the radiation worker. It was then called a tolerance dose and would have permitted individual exposures up to 0.1 rem (0.001 Sv) every day for a working lifetime -- say, 50 years.

Studies of the deaths of radiologists who had been exposed to x rays before the time the first tolerance dose was adopted in 1934, showed that cancers among radiologists were appreciably more frequent than among physicians who had not worked with x rays. Later studies of a similar nature, covering radiologists who were exposed after 1934, have not shown any significant difference between the death rates of radiologists and non-radiological workers. There is no way of knowing exactly how much radiation was received by radiologists during either period because personally-worn radiation-measuring devices were not available in any quantity, until perhaps the late 1940's. However, the data clearly indicate that in spite of a substantial increase in activity in the field of radiology, the introduction of numerical standards resulted in a significant improvement in overall radiation safety

In 1946, immediately after the war, in the U.S., the National Committee on Radiation Protection reconvened and was reorganized. Its first objective was to review the basis for the establishment of a tolerance dose. This review took several important factors into consideration. First, it was recalled that in 1941, the Committee had determined that there was probably no level of exposure below which radiation would not produce some adverse genetic effects. This was an expression of the no-threshold concept, which says in effect, there is no level of dose below which effects do not occur.

Because of the huge amounts of radiation expected to be developed during the atomic bomb project, the government began a correspondingly large research effort into studying the effects of ionizing radiations of all kinds and at all levels of exposure. One major benefit of this effort was the development of a great many sophisticated dose-measuring instruments and devices so that individuals could be readily monitored.

By measuring the radiation exposure of individuals and studying the measurements in relation to their work, it was found that the necessary work could be carried out, at acceptable costs, without having to expose workers to as much as 0.1 rem in a day. Even though no adverse effects had been found at that level of exposure, the government-operated nuclear plants found it possible to conduct many of their operations at "a working tolerance dose" level as low as one tenth of the accepted tolerance dose or 0.01 rem (0.0001 Sv) in a day.

It should be noted that in 1948, the term "tolerance dose" was dropped and "maximum permissible dose " was substituted. Since the late 1960's the term "dose limit" has been frequently used. These changes were made primarily for semantic reasons and the numbers were not affected.

The world's radiation protection philosophy today is set forth by the International Commission on Radiological Protection in a statement outlining its system of radiation dose limitation, the main features of which follow: "(a) no practice shall be carried out unless its introduction produces a positive net benefit; (b) all exposures shall be kept as low as reasonably achievable, economic and social factors being taken into account; and (c) the dose equivalent to individuals shall not exceed the limits recommended for the appropriate circumstances by the Commission".

In common parlance, part (b) above has been reduced to the convenient acronym ALARA, for "as low as reasonably achievable", but without the essential limitations of "economic and social factors being taken into account". The omission implied in this abbreviation is partly responsible for some public misunderstanding of the full meaning of radiation protection standards.

The ALARA concept has been misinterpreted or misused by some as a byword, to mean "lower the limits, whatever they may be", even though low-level radiation exposures show no more evidence of hazards now than they did thirty years ago. It is true that a recent reevaluation of the Japanese bombing-effects data concluded that the effect of small doses -- less than 20 rem -- may be greater than previously estimated by a factor of two or more. To some that may appear to be significantly large, but it should be borne in mind that we don't really know any of the risk relationships in the very low dose region, as close as a factor of two. By "the very low dose region", is meant doses less than 5 rem in a year, received acutely or spread out more or less randomly over the year.

The concept of achieving a lowest practicable -- or achievable -- level of exposure is fundamental to the control, not only of radiation but of all potentially hazardous environmental agents. The ultimate goal of radiation protection, recognizing the impossibility of eliminating radiation exposure entirely, is optimizing radiation exposures to levels consistent with the needs and benefits of humanity and compatible with the other hazards to which man is exposed.