Answer to Question #9694 Submitted to "Ask the Experts"

My question has to do with health effects of low levels of radiation exposure associated with the Fukushima reactor accident. Fox News interviewed a person who said that any level of radiation is harmful. Does the Health Physics Society (HPS) endorse this position? Does the HPS endorse the linear no-threshold (LNT) model for realistically calculating latent cancer deaths from low levels of radiation (i.e., below 100 mSv)? I have been following the debate over these issues and have found different opinions. At one extreme, there is a theory (hormesis) that asserts that low radiation levels are beneficial and actually produce health benefits. There have been papers by the Electric Power Research Institute and the University of Massachusetts School of Public Health (BELLE newsletter) asserting that a hormetic effect may exist with low levels of radiation exposure. A recent paper by Professor Bernard Cohen, University of Pittsburgh, claims that data show that people exposed to high levels of radiation from radon gas actually have lower cancer death rates than those exposed to low levels. There was also a recent presentation by a French scientist to the Advisory Committee on Nuclear Waste in which he argued that the LNT model overstates health effects (i.e., cancer) at low radiation exposures. The other extreme is the opinion stated on Fox News that any dose, no matter how small, is harmful.

The Health Physics Society (HPS) does not support the position that any level of radiation is harmful, nor does it endorse using the linear no-threshold (LNT) model to calculate latent cancer deaths from low levels of radiation. The Health Physics Society recommends against quantitative estimation of health risks below an individual dose of 50 mSv in one year or a lifetime dose of 100 mSv above that received from natural sources (see Radiation Risk in Perspective). The confusing and often conflicting statements made about the health effects, or risks, of low levels of radiation exposure arise because of the current lack of knowledge in this area. It has been clearly established that very high doses of radiation, in the range of hundreds of rads delivered over short time intervals, could lead to severe, prompt effects and even death, the so-called acute radiation syndromes. It has also been clearly established that radiation doses of smaller magnitude, but above about 100 mSv, do not lead to the acute radiation syndromes, but do increase the risk of developing radiogenic cancers following a latent period that may extend from 5 years to over 20 years. It is not now known what the effects of doses below 100 mSv might be. Some believe that there are no effects (that is, there is a threshold); some believe that because humans evolved in a background radiation field, they are adapted to that level of radiation and the radiation may even serve a useful protective function (hormesis). 

The question, however, remains, How do we regulate radiation exposures below 100 mSv, which are the levels typical of occupational exposures, as well as exposures of members of the public that result from these operations? Organizations such as the International Commission on Radiological Protection, the National Council on Radiation Protection and Measurements, and others, as well as regulatory bodies in all countries, including the Nuclear Regulatory Commission and the Environmental Protection Agency in the United States, have opted to be guided by the precautionary principle. This principle states that if you don't know if an agent is harmful, assume that it is and regulate accordingly until you know better. This is more popularly known as better safe than sorry. Since little is known about the effects of radiation below about 100 mSv, it is assumed that the harm at these low levels is directly proportional to the dose, with no threshold dose below which there is no effect. Hence LNT. Much of the controversy about the effects of low-level radiation arises because people forget that LNT is a hypothesis, that is, it remains to be validated. However, adopting the position that one should not quantify health risks below 100 mSv leaves regulatory bodies with few options on how to regulate radiation-generating activities. LNT is a necessary tool to permit a reasonable foundation on which to base radiation-protection regulations, until future knowledge determines otherwise.

Finally, it should be noted that although there is no direct evidence to indicate what effects low levels of radiation might have, there is substantial evidence supporting the idea that cancer may be initiated by a single gene mutation. There is also substantial evidence to suggest that the type of chromosomal damage caused by radiation is of a kind (double strand breaks) that is prone to errors during repair. There are also physical reasons why any radiation effect at very low doses should be proportional to the dose (unless one assumes that repair mechanisms are always error free at low doses). These indications, combined, suggest that LNT is a reasonable position, although it should be used with caution, especially in estimating numbers of deaths that may result from low-level radiation exposures. Such use is not justified by our current state of knowledge, nor is it justified by LNT. An unproven hypothesis may be useful in a pragmatic way to permit regulatory oversight, but not to arrive at quantitative conclusions about effects. This is consistent with the HPS position linked at the beginning of this answer.

Sami Sherbini, PhD