The LNT Hypothesis

When I get involved in discussions about radiation and radioactivity, I keep hearing the initials "LNT" mentioned. What do these initials mean?
The underlying basis for the LNT, or "Linear No Threshold" hypothesis is the worthwhile objective of controlling radiation doses in humans. The hypothesis states that since scientists have observed a linear relationship between radiation dose and effect at high doses, and since we cannot create a "radiation free" environment to test the theory at low doses (taken to be 20,000 millirem or less), we will assume, for radiation protection purposes, that the relationship is indeed linear at low doses as well. This leads one to the obvious conclusion that any dose, no matter how small, may be capable of causing some biological damage or detriment. Nonetheless, it has been considered for the past 40 years or so to adopt the philosophy that radiation exposure is harmful at any level.

Why is it referred to as a hypothesis?
There is no real evidence to support the assumption that irrepairable biological damage occurs at low doses. Therefore, it must remain a theoretical concept. By the way, some refer to the LNT as a theory rather than a hypothesis. In this discussion, the two will be used interchangeably to avoid the subtle distinctions between these two words.

Who has adopted the LNT theory?
The LNT hypothesis has been adopted by every national and international body that offers radiation protection recommendations or interprets scientific data. These include, but are not limited to, the National Council on Radiation Protection and Measurements (NCRP), the International Commission on Radiological Protection (ICRP), the National Academy of Sciences (NAS) Biological Effects of Ionizing Radiation (BEIR) Committees, the International Atomic Energy Agency (IAEA), and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). In addition, the Health Physics Society (HPS), in their 1993 position statement entitled "Radiation Dose Limits for the General Public", endorses the "As Low As Reasonably Achievable" (ALARA) philosophy (i.e., that there is no absolutely safe dose threshold), as a reasonable basis for radiation protection programs.

Are these regulations?
No. These are recommendations by committees and organizations that were set up to offer standards and guidance in the areas of radiation safety and dose control. However, regulatory agencies almost always adopt the recommendations of these organizations, in one form or another. For example, in the United States, federal and state regulatory agencies overseeing the safe use of radiation and radioactivity require the application of the ALARA principle, with its LNT basis, in licensee or contractor radiation protection programs.

It sounds like a wise idea. However, I get the sense that there may be some negative aspects to the LNT theory. Am I right?
Yes. It doesn't take much in the way of common sense to observe some of the flaws in the LNT hypothesis.

Why don't you give me a couple of examples?
Well, for starters, if you read the "Radioactivity Basics" chapter entitled "It's Everywhere!", you already know that natural background doses vary greatly over the world. However, even in populations where background levels are much higher than those typical of the United States, and even much higher than radiation doses typical of occupational radiation work, no ill effects have ever been observed.

So?
If the LNT theory was true in these dose ranges, we would expect to see increased incidence of radiation-related health effects. We don't. To cite the National HPS once more in another position statement entitled "Compatibility in Radiation Protection Regulations" (January, 1992), "...while detrimental effects in man are known to be associated with high exposures to radiation, no ill effects are observed at levels within the range of exposures to natural sources." Taking this statement as fact leads one to conclude that the "no dose, no matter how small, has some risk associated with it" philosophy is overly conservative and cannot be defended.

Can you provide some more population-specific examples to support the lack of adverse effects?
Certainly. In the case of natural background, there are certain areas of the world where levels of natural radioactivity are quite high relative to what we encounter here in the United States. Particular examples include Ramsar, Iran and the Kerala region in India. Even so, no adverse health effects of natural radiation have been found in either these regions (or any other areas of elevated background for that matter). In the workplace setting, no evidence of detriment to the workers occupationally exposed to radiation within the established regulatory limits have been shown.

Alright. What other difficulties are associated with the LNT?
Unlike other areas of study, such as health risks from exposure to air pollutants, where there is a great deal of direct data in the region of interest, radiation effects have been extrapolated far below the region where any meaningful data exist. This has generated no end of controversy in the radiation safety community regarding its continued applicability.

Why is that?
Because it has not been proven, use of the LNT hypothesis requires that latent cancer fatalities be projected from accumulated exposures to very small levels of ionizing radiation. And because they are nothing more than projections, there is no assurance of their validity. Nonetheless, these projected risks are often treated as "fact", with all of the resulting adverse action (i.e., worried and confused people, expensive upgrades to safety programs, and often litigation).

What are you actually referring to when you talk about radiation risks?
When we speak of risks from radiation exposure, we often focus in on either cancer or genetic effects. In the case of cancer, quantitative estimates of "risk" are reported as the number of cancers per unit dose (e.g., rems or sieverts). If the LNT hypothesis is incorporated into the development of risk factors, they are assumed to be valid for all doses, and dose rates, even those approaching zero.

And I take it that is not very scientific, right?
Yes. These risk factors arise from epidemiological data from the atomic bomb blasts of Hiroshima and Nagasaki. The argument can be made that these risk estimates are not relevant to "normal" radiological protection situations, where individuals are irradiated with very low doses (on the order of 100 mrem or 1 mSv) delivered over an extended period (i.e., a year). In Japan, for example, a certain proportion of the population was irradiated "acutely", that is, in a fraction of a second or a few seconds with near lethal doses (up to 500,000 mrem or 500 rem). These dose rates were orders of magnitude higher than those commonly encountered in radiological protection. Extrapolating over such a vast dose-rate span is certainly grounds for arguing the scientific merits of the LNT assumption.

Time out! You're talking risk, cancer, genetic effects, acute and chronic doses, etc. Where can I refresh my memory on these issues?
For starters, you might want to go back and review Section 1, "Radiation Risk Basics" of this Radioactivity Basics chapter on "Radiation Risks". Then, followup with a review of basic health physics texts, examples of which are included at the end of this chapter.

Ok, I'll do it! What else?
Keep in mind that the LNT hypothesis states that even the lowest, close to zero, radiation dose is detrimental, and can produce a cancer or a hereditary effect. However, no hereditary effects have been discovered in the progeny of survivors of Hiroshima and Nagasaki irradiated with high sub-lethal doses.

Well that certainly is telling. What about cancers?
Radiation carcinogenesis is often viewed as a straightforward process, meaning that if the DNA in one cell in the body absorbs one photon of radiation resulting in a single mutation, a cancer will inevitably result. However, the notion that cancer induction is caused by a single mutation in one cell is probably erroneous based on ongoing studies which describe the complexity of this process. In actuality, a cell may have to divide billions of times before a cancer is formed. If true, predicting cancer as an "outcome" of radiation exposure is impossible in light of our present state of knowledge. This presents a clear lack of credibility and scientific basis for the LNT hypothesis.

But isn't it better to be "safe than sorry" in the case of radiation safety?
Up to a point, that is true. However, adoption of the LNT hypothesis as the basis for radiation protection regulations means that significant sums of money are spent in the United States and in other countries around the world to remediate or clean-up residual radioactivity in soils, in buildings, on equipment, and in potentially useful feed materials even though there is no demonstrable health effect associated with the use of those materials. There are certainly grounds for contesting these expenditures.

So what is the alternative to the LNT hypothesis?
Actually, there is one really interesting one that is being discussed. There are proponents of the position that radiation protection should be based on the principle of practical thresholds - radiation doses at which "harm" cannot be detected. These thresholds would be different for various dose rates, and for various effects, such as an increase in cancer incidence, hereditary changes, or acute radiation sickness. The existing range of natural doses, and epidemiological observations of both adverse and beneficial effects of radiation would provide guidance as to the numerical values of these thresholds. This method of control is similar to that used for the control of exposures to chemicals.

Do you think it will fly?
Only time will tell. However, it is an intriguing possibility.

Do you have any final words on the LNT hypothesis?
This theory is currently very much entrenched with national and international organizations that provide radiation protection recommendations. In turn, regulatory bodies have adopted the LNT hypothesis in its entirety in order to maintain the incidence of somatic and genetic effects to a practical minimum. The conservative nature of the LNT hypothesis is obviously appealing to these groups. Furthermore, even though there is no scientific basis to support it at the total exposures and exposure rates routinely received by the general public and radiation workers, the LNT hypothesis will likely remain a fundamental tenet of radiation protection "policy" for some time to come. We believe it would be naive to think that regulatory officials will alter their views on this issue any time soon.

That's too bad, isn't it?
Maybe not . . . at least for now. We can't lose sight of the fact that many members of the general public suffer to some extent from a fear of radiation that has been created by a lack of understanding and education in this sometimes highly technical area. The situation has definitely not been helped by our "no dose is safe" regulatory basis. In addition, years of anti-radiation and anti-nuclear sentiment from a variety of groups, along with the generally anti-radiation messages often presented by the media have not resulted in a fair and balanced discussion of radiation-related effects. Therefore it will certainly take time before people will accept a different regulatory basis than the one we have now.

Will we ever be able to prove whether the LNT theory is a fact rather than a supposition?
It appears doubtful at this point that we will ever really be able to prove the legitimacy of the LNT hypothesis.

Where can I get further information on the LNT theory and the controversy that surrounds it?
In this age of ready access to essentially any topic, several sources are available. First, any basic health physics textbook discusses this theory. Examples include Dr. Herman Cember's "Introduction to Health Physics" and Dr. Daniel Gollnick's "Basic Radiation Protection Technology". Visit a technical library to find these and other radiation protection texts, and jump to the sections on radiation-related health effects. For those individuals that can never learn too much, the NCRP, ICRP, National Academy of Sciences' BEIR and UNSCEAR technical reports should also be examined. And of course, this is just for starters!

Are there any on-line resources?
Absolutely. The HPS website at http://www.hps.org should be consulted for background information including its public information section and position statements. You can also conduct a search of items available on the internet by using key words like "linear, no threshold hypothesis" or variations on this theme. Finally, the U. S. Department of Energy (DOE) Low Dose Radiation Research Program site at http://lowdose.org discusses radiation interactions on the genomic level . . . even at very low doses. The work of this program is not one to be missed.

Anything else?
Don't overlook the other chapters in the "Radioactivity Basics" section of the IEM web page. In particular, check out the Chapter 4 write-ups on the ALARA principle, hormesis, and other interesting, but often controversial topics.

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