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There is no Scientific Basis for the Linear Hypothesis' Extension to Low Doses

A. N. Tschaeche, CHP

Abstract

The history of the development and use of the Linear Hypothesis is presented. Some of the stated bases for the Hypothesis are examined and evidence for dismissal of those bases as valid for use with the Hypothesis is given. A brief discussion of the consequences in the United States of the use of the Hypothesis is made. An alternative hypothesis for low dose effects is developed and its probable consequences are posited.

Introduction

The basis for radiation protection standards in the world as set forth in recommendations of the International Commission for Radiological Protection (ICRP) is a linear extrapolation of a dose and effect curve from high dose, high dose rate data for humans to zero dose and zero effect. Other countries have either accepted the ICRP recommendations or have developed their own. The United States has the National Committee on Radiation Protection and Measurements (NCRP). The NCRP uses the same basis for its recommendations, but currently contains numerical values and ideas that are different from those of the ICRP. However, the NCRP rationalizes the differences and states that the protection afforded both systems is identical.

The basis is called the Linear, No-Threshold Hypothesis or LNTH and is shortened sometimes to just the Linear Hypothesis. The use of the word "hypothesis" was an attempt by the ICRP and NCRP to indicate that any use of the idea was to use something that was not known as a fact, but was only a way of guessing what the dose-effect curve looked like at low doses where there were no data of the same value as for high doses. The idea of "hypothesis" has been largely lost today and many if not most of those working in the field of radiation protection accept the hypothesis as fact. Large sums of money are spent to reduce doses because of the hypothesized harm those low doses are believed to cause. The public in many, if not all countries, believes that any dose, no matter how small, is harmful. Those publics are, in general, afraid of radiation and that fear affects the ability of the nuclear industry to provide fully, the enormous benefits of the use of radioactive material and radiation.

The time has come to examine whether the LNTH should continue to be the basis for radiation protection standards in the world.

History

ICRP Publication 1, published in 1958, contains many statements that, while not called the LNTH, clearly embody the idea. For example: "The most conservative approach would be to assume that there is no threshold and no recovery, in which case even low accumulated doses would induce leukemia in some susceptible individuals, and the incidence might be proportional to the accumulated dose. The same situation exists with respect to the induction of bone tumors by bone-seeking radioactive substances." And "Any departure from the environmental conditions in which man has evolved may entail a risk of deleterious effects. It is therefore assumed that long continued exposure to ionizing radiation additional to that due to natural radiation involves some risk." Those statements are the earliest I could find that propose such a basis for radiation protection standards. At the time, 1958, it was not unreasonable to use that basis because of the lack of information on the long term effects of any doses of ionizing radiation. Even the high dose effects from the two atomic bombs dropped on Japan were not well known. They are still not well known 40 years later. There was no information on the effects of low doses and there still are no human data that demonstrate measurable harmful effects of low doses.

The radiation protection community was the only community to use a LNTH for projecting low dose effects. The industrial hygiene community demanded that harmful effects be expressed and measured to the extent practical before it would assign limits for intakes of chemicals. In other words, the radiation protection community assumed radiation was guilty of harm at low doses without any data demonstrating that it was and the industrial hygiene community assumed an agent was harmless until harm was demonstrated and quantified. Many other safety and health decisions were made using the "innocent until proven guilty" approach. Only in the radiation protection community was the "guilty until proven innocent" approach used.

One might wonder why radiation protection was different from other hazardous agents. The book "Nuclear Fear" by Spencer R. Weart contains a discussion of why radiation has been looked on with great fear by the public and even by those who work with it. Weart states "Fifteen years ago when I began studying the history of nuclear energy I did not think that images were important in themselves. I was wrong. Radioactive monsters, utopian atom-powered cities, exploding planets, weird ray devices, and many other images have crept into the way everyone thinks about nuclear energy, whether that energy is used in weapons or in civilian reactors. The images, by connecting up with major social and psychological forces, have exerted a strange and powerful pressure with history." The ICRP members, being human, undoubtedly were affected by their images and by the lack of scientific data on radiation effects. Therefore, at the time, their use of the LNTH is understandable. Also, their earlier recommendations had been based on observable effects. Not having been there, I can only guess that the ICRP members of those days wanted to be sure not to make a mistake and so were very careful and conservative in their thinking about radiation protection standards. Giving them the benefit of the doubt, they also wanted the benefits of radiation to be realized and knew the public would never forgive them if their radiation protection standards were, in the long run, demonstrated to cause harm.

ICRP Publication 6, in 1962, continued the idea of the LNTH by stating: "The basis of the Commission's recommendations is that any exposure to radiation may carry some risk. The assumption has been made that, down to the lowest levels of dose, the risk of inducing disease or disability in an individual increases with the dose accumulated by the individual, but is small even at the maximum permissible levels recommended for occupational exposure."
ICRP Publication 9, in 1965, repeated and amplified the idea of the LNTH by stating: "As the existence of a threshold dose is unknown, it has been assumed that even the smallest doses involve a proportionately small risk of induction of malignancies. Also, because of the lack of knowledge of the nature of the dose-effect relationship in the induction of malignancies in man -- particularly at those dose levels which are relevant in radiological protection -- the Commission sees no practical alternative, for the purposes of radiological protection, to assuming a linear relationship between dose and effect, and that doses act cumulatively. The Commission is aware that the assumptions of no threshold and of complete additivity of all doses may be incorrect, but is satisfied that they are unlikely to lead to the underestimation of risks. Information is not available at the present time which would lead to any alternative hypothesis." And "A basis of the Commission's recommendations is the cautious assumption that any exposure to radiation may carry some risk for the development of somatic effects, including leukemia and other malignancies, and of hereditary effects. The assumptions made that, down to the lowest levels of dose, the risk of inducing disease or disability increases with the dose accumulated by the individual. This assumption implies that there is no wholly ‘safe' dose of radiation. The Commission recognizes that this is a conservative assumption, and that some effects may require a minimum or threshold dose. However, in the absence of positive knowledge, the Commission believes that the policy of assuming a risk of injury at low doses is the most reasonable basis for radiation protection." And "Any exposure to radiation is assumed to entail a risk of deleterious effects. However, unless man wishes to dispense with activities involving exposures to ionizing radiations, he must recognize that there is a degree of risk and must limit the radiation dose to a level at which the assumed risk is deemed to be acceptable to the individual and to society in view of the benefits derived from such activities."
Note the clear idea that the basis is an assumption. Also that it is the only "reasonable" assumption. Also that it is "conservative." Again, at the time, 1965, the idea is probably not wrong, if it were fully understood by the public and by those in the nuclear industry. Actually, up to about 1968 in the United States, there was little public fear of low doses, except for that of doses from fallout from nuclear weapons testing. In 1958 the US stopped testing nuclear weapons in air so the fallout fear went away in large part. In 1968 the anti nuclear movement began in ernest and the anti nuclear people began to use a corruption of the idea that "a little radiation MAY be harmful" into "a little radiation IS harmful" to frighten the US public into rejecting nuclear power and other benefits of the nuclear industry.
In 1966 ICRP Publication 8, The Evaluation of Risks from Radiation, states: "In defining its basic concepts the Commission has stated that the permissible doses for occupational workers can ‘be expected to produce effects which could be detected only by statistical methods applied to large groups.' This circumstance would by itself create great difficulty in evaluating, by direct observation, the risks attendant on these levels of exposure; in practice the problem is made considerably more difficult because the same types of injury arise frequently from other and unidentified causes. The only practical approach is an indirect one, namely, to assume that the response per unit dose at very low doses is the same as that observed at the considerably higher levels of exposure for which reasonably quantitative observation is possible." Note that each quotation reiterates essentially unchanged the LNTH idea.
One very interesting statement is also contained in Publication 8: "Thus, even if risks from low levels of radiation could be accurately evaluated, it would be impossible to balance them exactly against the risks of possible remedial action. The only reasonable course, as in dealing with other problems of public health, is for decisions to be based on a judicious evaluation of all aspects of the problem." I submit that the ICRP has not been following that reasonable course in the past few years as it continues to espouse the LNTH as the only basis for its recommendations. I will expand on that statement later on.
The 1977 ICRP Recommendations in Publication 26 state: "For radiation protection purposes it is necessary to make certain simplifying assumptions. One such basic assumption underlying the Commission's recommendations is that, regarding stochastic effects, there is, within the range of exposure conditions usually encountered in radiation work, a linear relationship without threshold between dose and the probability of an effect." Again the LNTH.
However, here is another statement in Publication 26 that is not generally appreciated by either the radiation protection community or the public: "The use of linear extrapolations, from the frequency of effects observed at high doses, may suffice to assess an upper limit of risk, with which the benefit of a practice, or the hazard of an alternative practice -- not involving radiation exposure -- may be compared. However, the more cautious such an assumption of linearity is, the more important it becomes to recognize that it may lead to an overestimate of the radiation risks, which in turn could result in the choice of alternatives that are more hazardous than practices involving radiation exposures. Thus, in the choice of alternative practices, radiation risk estimates should be used only with great caution and with explicit recognition of the possibility that the actual risk at low doses may be lower than that implied by a deliberately cautious assumption of proportionality." Regulatory agencies in the US have taken the ICRP and NCRP risk estimates as the gospel truth and have not used ‘great caution' with the result that huge sums of money are being spent to decontaminate nuclear sites to levels for which no measurable benefit exists. Further, the US regulatory agencies do not tell the public that the risk of low doses may be zero or that low doses may be beneficial or even essential to life. The result is public fear that many believe is completely irrational.
The most current ICRP recommendations appeared in 1990 in ICRP Publication 60. Those recommendations are based on numerous assumptions, one of which continues to be the LNTH. The LNTH is modified to a linear-quadratic form in the following statement: "The Commission has concluded, after reviewing the available experimental information on dose-response relationships and the influence of dose and dose rate, that the most probable response is linear quadratic in form for low LET radiation. The linear coefficient at low doses or low dose rates is obtained from the high dose, high dose rate estimates of risk by dividing by a DDREF (dose and dose rate effectiveness factor) of 2." Here is a little modification of the LNTH, but not much.
The same publication states: "There is some experimental evidence that radiation can act to stimulate a variety of cellular functions, including proliferation and repair. Such stimulation is not necessarily beneficial. In some circumstances, radiation appears also to enhance immunological responses and to modify the balance of hormones in the body. In particular, radiation may be able to stimulate the repair of prior radiation damage, thus decreasing its consequences, or may be able to improve immunological surveillance, thus strengthening the body's natural defense mechanisms. Most of the experimental data on such effects, currently termed ‘hormesis', have been inconclusive, mainly because of statistical difficulties at low doses. Furthermore, many relate to biological endpoints other than cancer or hereditary effects. The available data on hormesis are not sufficient to take them into account in radiological protection."13
For the first time the ICRP mentions beneficial effects and hormesis, but immediately dismisses them in considering radiation protection standards. This brings us to the end of the historical part of this talk. Now, on to examination of why, in light of today's experience, the basis for the LNTH should be reexamined and dismissed.

Bases

The stated bases quoted above are:
  1. Most conservative approach
  2. Long continued exposure to ionizing radiation additional to that due to natural radiation involves some risk
  3. Down to the lowest levels of dose, the risk of inducing disease or disability in an individual increases with the dose accumulated by the individual
  4. The existence of a threshold dose is unknown
  5. Because of the lack of knowledge of the nature of the dose-effect relationship in the induction of malignancies in man, the Commission sees no practical alternative to assuming a linear relationship between dose and effect.
  6. Cautious assumption that any exposure to radiation may carry some risk
  7. Must limit the dose to a level at which the assumed risk is deemed acceptable in view of the benefits.
  8. The policy of assuming a risk of injury at low doses is the most reasonable basis
  9. Only practical approach
  10. No threshold and no recovery
  11. Only deleterious effects considered; beneficial or zero effects are ignored
  12. Information is not available which would lead to any alternative hypothesis
Let's examine each basis.
  1. The LNTH is certainly not the most conservative approach. A superlinear model where low doses are more harmful percentage wise than high doses is far more conservative.
  2. Natural background varies over the earth by a factor of ten or more. Therefore, humans must be able to "handle" such doses without trouble or we would not be here as a race. The addition of a little more radiation to background, where the background is in the range of 3 mSv cannot be harmful.
  3. This is the unproved assumption.
  4. Just because we don't know the numerical value for the threshold, doesn't mean a threshold doesn't exist. Why not assume a threshold and go from there?
  5. Lack of knowledge of the shape of the low dose-effect curve doesn't necessarily mean a linear curve is the best.
  6. Why must we use cautious assumptions? Men have always taken risks. Why is taking risks with radiation so much worse than taking risks with other things. Using cautious assumptions is not always the best procedure. Certainly we are far too cautious with Saddam Hussein and clearly were with Hitler.
  7. Why must the benefit always be greater than the risk in radiation protection uniquely? If such a philosophy had been applied to flying, we'd still be taking trains, unless they would have been declared too risky. We clearly would not smoke cigarettes if the benefit had to be greater than the risk.
  8. Maybe the most reasonable at the time, but not now.
  9. The LNTH is clearly not the only practical approach. In light of what we now know, a threshold is much more practical.
  10. The assumption of no recovery remains firm in the minds of the ICRP members (except for the "dismissal statement" in ICRP 60). However, there are now data that clearly demonstrate complete repair of radiation damage in humans, or of apoptosis that removes a damaged cell by cell death before it can replicate.
  11. The ICRP has a very pessimistic slant to its thinking. It only considered evidence for deleterious effects. It ignores any suggestion of beneficial or zero effects. In so doing its recommendations are hugely unbalanced and biased in favor of the bad while ignoring the good. Such bias is not appropriate in today's climate.
  12. There was and is information that would lead to a threshold hypothesis. That information is that there are no human data that demonstrate any harm at low doses and there never have been. The existence of data demonstrating no effect is an excellent basis for a threshold hypothesis.
So, today, I submit that the LNTH has no basis except in the minds of the ICRP. I submit there is no "scientific" basis for the LNTH. I submit all the bases are opinions, guesses, or wishes of well-meaning men.

Consequences

The consequences of the LNTH have been horrendous in the US. The corruption of the idea that "a little radiation MAY cause harm" into "a little radiation WILL cause harm" by the media, the regulators and the nuclear industry itself has created fear of anything having to do with radiation in the minds of the public. That fear has resulted in the United States in: In my opinion the LNTH is the major reason why the result is observed. The reasoning behind that statement has already been stated. The LNTH is only that: a hypothesis. But the anti nuclear people, the regulators, the media and the nuclear industry in large part have either forgotten or choose to ignore the hypothetical aspect and have corrupted the LNTH into fact. A little radiation will, really and truly, hurt you. The public has been brainwashed into believing the corruption and has been made to fear even tiny doses, far below natural background. That fear forces the US Congress (and perhaps other governing bodies in the world such as the NRPB in the UK) to pass laws that result in spending billions of tax dollars for no measurable benefit. It is time for the ICRP to follow its own words and make its decisions "based on a judicious evaluation of all aspects of the problem." For the ICRP to ignore how its recommendations are taken by the public and implemented by regulatory bodies and the resulting loss of the benefits of nuclear industry is an abrogation of responsibility.

Alternative Approach

The ICRP and NCRP should abandon the LNTH as the basis for estimating low dose effects. One possible alternative is to establish a "threshold" below which no harm is observed based on whole human data. I say "whole human" so that animal data and data obtained in a laboratory, but not on whole human beings would not be considered. The recovery ability of the human body must be accounted for in any reasonable current standards for radiation protection. The fact that the millions of exposures to medical and dental radiation have not produced measurable deleterious effects must be considered. The fact that people in high background areas do not suffer deleterious effects must be considered. The harm that the LNTH has produced must be considered. Even the latest UNSCEAR Report states: "The scientific community has been aware for many years of the possibility that low doses of radiation may result in changes in cells and organisms, which reflect an ability to adapt to the effects of radiation." And: "In conclusion, there is substantial evidence of an adaptive response in selected cellular systems following acute exposure to conditioning doses of low-LET radiation."

So, to me, the time has come to change the paradigm. The costs of not doing so are too great.

Prediction

If the paradigm is changed and a threshold is used as the basis, and the threshold is sufficiently high, such as probably not more than 50 mSv per year or single doses of 100 mSv, and if the ICRP and NCRP make clear statements that such doses are safe, the regulatory agencies will comply, the media will get the message out, the public will stop being afraid, and the benefits of the nuclear industry will be available. Of course the anti nuclear establishment will fight tooth and nail to prevent any of this from happening. But, if the ICRP and NCRP are willing to bite the bullet, I predict the paradigm will change.

Having said all of this, there may be other paradigms available, but I don't know what they are. All I know is the current paradigm is killing the nuclear industry.

References

  1. Recommendations of the International Commission on Radiological Protection, Publication 1, September 9, 1958, 5
  2. Ibid 29
  3. Nuclear Fear, A History of Images, Spencer R. Weart, Harvard University Press, 1988, preface
  4. Recommendations of the International Commission on Radiological Protection, Publication 6, 1964, 32
  5. Recommendations of the International Commission on Radiological Protection, Publication 9, September 17, 1965, 7
  6. Ibid 29
  7. Ibid 34
  8. The Evaluation of Risks from Radiation, ICRP Publication 8, 1966, pg 1, Chapter I Introduction
  9. Ibid
  10. Annals of the ICRP, Recommendations of the International Commission on Radiological Protection, ICRP Publication 26, 1977, 27
  11. Ibid, 30
  12. 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publication 60, 1990, S9
  13. Sources and Effects of Ionizing Radiation, United Nations Scientific Committee on the Effects of Atomic Radiation, UNSCEAR 1994 Report to the General Assembly, with Scientific Annexes, United Nations, 1994, Appendix B, pg 186, 1
  14. Ibid, pg 223, 272

Acknowledgement:
    This is the text of a lecture at the Annual Congress of the South African Radiation Protection Association  , held 12-15 May 1998. I should thank Dr. Ado van Rensburg for his kind permission for reproducing this aricle.

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