Why Low-Level Radiation Can't Cause Cancer

Ted Rockwell (301) 652-9509
3403 Woolsey Drive
Chevy Chase, MD 20815

The biological justification claimed for the Linear Non-Threshold (LNT) model is that when a single ionizing photon or particle hits a living organism, DNA in the cell may be damaged. This may impair the cell's function and lead to cancer. But to understand the significance of this event, we have to look at the numbers of such events. The first consideration is that we all live in a sea of natural radioactivity -- cosmic radiation pouring in from outer space and naturally radioactive materials in our soil, building materials, food and water. The body cannot distinguish "man-made" radiation from "natural" radiation; in both cases the nuclear particles arise from the same processes and have the same characteristics. And our bodies are impacted by some 15,000 nuclear rays or particles every second -- over a billion such events every day of our lives from these natural sources.

But our bodies face even greater challenges: About 5000 purine bases are lost daily from the DNA in each human cell because the body's normal heat breaks their linkages to deoxyribose. Even more damage is caused by normal cell division and DNA replication. But the most damage--a million DNA nucleotides in each cell damaged each day -- is caused by free radicals created in the normal process of metabolism resulting from routine eating and breathing. Radiation causes more double breaks per event in the DNA than metabolism does, and these are harder to repair than single breaks; but even after making generous allowance for this difference, the mutations (unrepaired or misrepaired damage) from metabolism outnumber those caused by natural radiation by ten-million-fold.

The table below, from work in progress by Myron Pollycove and Ludwig Feinendegen et al., summarizes these numbers for each of the body's 100 million million cells. Some ten billion free radicals are created each day in each cell, and about 1% if these are within striking distance of the DNA. These free radicals near the DNA are shown in the first row of the first column below. (There is no really comparable figure for radiation.) About 99% of these are gobbled up before they can cause any harm, and the remaining free radicals damage the DNA, as shown in the second row. The corresponding number of DNA alterations caused by background gamma (low LET) radiation is only 0.005 per day, or one DNA alteration every 200 days. About 99.99% of these alterations are repaired in the metabolic case, and about 1 in 500 in the radiation case (because of the greater difficulty in repairing the double breaks). Finally, about 99% of the damaged cells are removed, leaving 1% of the damaged cells to persist as mutations, as shown in the last row of the table. 

  These facts raise two questions: first, how can any living organism withstand such an onslaught? How do any of us survive? The answer is that the body has a large variety of anti-oxidants that prevent damage, enzymes that continually scan the DNA to repair damaged nucleotides, and processes that remove those it cannot repair. The second question is: How does chronic exposure to high-level radiation harm an organism? Clearly, adding a few more mutations to the millions already occurring from metabolism is not significant. But high radiation levels can overwhelm the organism's normal biological functions and repair processes, and leave the organism damaged and vulnerable to the mechanisms that initiate and progress to cancer and other adverse consequences.

This raises another question: What is the effect of low-level radiation that is not strong enough to degrade the body's tissue repair capacity? The answer is suggested by how the body reacts to low levels of other potential toxins: when we inject small quantities of disease bacteria or toxic metals into the body, the result is to stimulate the immune system, so that subsequent attacks by this toxin, in larger amounts, are effectively countered. Some research indicates that radiation may work just the same way. Numerous studies have shown that cellular and DNA repair mechanisms are stimulated by low- to moderate-levels of radiation. Japanese studies indicate that several 5- to 10-rad full-body or half-body doses delivered in one- or two-minute bursts several days apart seem to provide a high enough dose rate to stimulate the defense mechanisms without adding significantly to the total cell damage over the long run. The intervals between irradiations are timed to optimally match normal cell cycles. There are also reports that organisms kept in a below-normal radiation background are affected adversely, and recover when returned to normal.

To summarize: our body temperature and normal eating and breathing cause millions of times more mutations in our bodies than the natural level of ionizing radiation. And the low-level radiation being regulated under current policy is as low as 1% of the natural radiation background we all live in. Scientific theory and data, as well as our actual experience, show that low-level radiation does not harm us.

References:
1. "Cancer," in Molecular Biology of the Cell, Bruce Alberts, J.D. Watson, et al., editors, Chap. 21 pp. 1187-1218, (Garland Pub, 1989).
2. Dan Billen, "Spontaneous DNA Damage and Its Significance for the 'Negligible Dose' Controversy in Radiation Protection," Radiation Research, Vol. 124, pp 242-245 (1990).
3. Harold Varmus and R.A. Weinberg, Genes and the Biology of Cancer,. (Scientific American Library, 153, 1993).
4. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 1994), "Sources and Effects of Ionizing Radiation; Report to the General Assembly, with Scientific Annexes," New York, N.Y., United Nations, Annex B.