Dr. K.S. Parthasarathy
Exposure to high levels of ionizing radiation can harm the tissues of the human body. This has been demonstrated unequivocally. Many who handled X-ray Units carelessly,suffered extensive skin damage and became marteers. They suffered clinically identifiable effects. Long term studies of exposed populations such as the survivors of the atomic bombings at Hiroshima and Nagasaki demonstrated that radiation exposure has a potential for inducing cancers in the exposed populations. These can be inferred from epidemiological studies. But there is a group of scientists who firmly believe that low doses of radiation can be beneficial. Dr. T. Luckey is an eloquent proponent of this theory. He wrote a very interesting Book titled "Radiation Hormesis" .
In 1987, Dr. Marshall Brucer wrote an eminently readable feature article titled "Radiation Hormesis after 85 years" in the American Health Physics Society's New letter. Brucer introduces the topic by citing the developments chronologically. Our pancreas secretes only when food passes through the stomach. In 1902, E. Starling, an English Physiologist discovered that an acid extract of duodenum contained 'secretin' that when discharged into blood, stimulated the pancreas to secrete. In 1904 Starling was the first to coin the term "hormone" (hormo in Greek mean to excite) to designate any substance produced in small amounts but carried in blood to influence some other organ.
"Weak stimulus might stimulate what the same, but stronger stimulus inhibits". Based on this pharmaceutical principle practiced by ancients, Brucer argues that toxicity is a matter of dose. A peg of whiskey, a whiff of nicotine from a cigarette, caffeine in a cup of coffee all these stimulate. A single intake of few liters of alcohol, half a gram of nicotine or caffeine will kill. They stimulate at low doses and kill at high doses.
Brucer pointed out that the word "hormesis" was coined by C Southam and J Ehrlich. They found that concentrations of oak bark extract inhibited fungal growth while in low dose, stimulated fungal growth. According to Brucer Starling's 'hormone' was modified to "hormesis" (phytopathology 33:517,1943)
During the 1950s, some Agricultural Chemists fed antibiotics to live stock. They thought that suppression of flora in the gut would decrease growth. But poultry, pigs and cattle fattened enormously on a diet containing low doses of antibiotics. T. Luckey who was engaged in such studies surveyed the literature later. In 1981, Luckey revived the term 'hormesis' with reference to ionizing radiation backing it up with over 1250 articles. The effects observed include growth of algae under X irradiation (1898), growth of peas (1908), increase in life span of invertebrates (1918) and insects (1919), and seedling stimulation by X-rays (1927).
Brucer observed that before World War I and continuing into 1930s about ten articles a year referred to a hormetic effect. After World War II, twenty articles per year mentioned a hormetic effect. He implied that hormesis theory did not thrive because all new paper head lines highlighted negative effects!!. For instance, H. Muller who demonstrated genetic effects by irradiating fruitflies in 1927, predicted a genetic catastrophe from atomic bomb explosiion. This lead to feverish publicity. Brucer bemoans the fact that no publicity was given the disproof 35 years later. The story of double headed babies in Hiroshima & Nagasaki was fiction; but it made good copy. According to Brucer "Health Physics and Genetics were supported lavishly by radiation hysteria and radiation biology became the ]most intensely researched science in history."
If the interaction of radiation with tissue is a purely physical mechanism, the probability for initiation of an effect in tissue will be is proportional to the number of initiating particles and hence to dose. But this picture is too simple as it ignores the profound biological defence mechanisms.B.L Cohen (Risk Analysis,1995) reviewed the evidence that biological defence mechanisms are stimulated by low level radiations. When radiation interacts with cells they may induce chromosome aberrations. Several studies have shown that frequency of chromosome aberrations induced by a high dose of radiation is substantially reduced if the cells are pre-exposed to low level dose.
Cohen cites a few cases of protection afforded by pre- exposure to low doses. Low level radiation enhances production of repair enzymes. Linear no threshold concept is a simple theory. When the dose is low, the time interval between irradiation and appearance of tumor is observed to be large.In view of the latency period, an exposed person may die of other causes before cancer can develop. Cohen argues that there is a "practical threshold" for radiation induced cancer. Based on the radon-lung cancer relationship, Cohen demonstrated that the linear no threshold theory fails very badly in the low dose, low dose rate region.
The International Commission on Radiological Protection in its publication no 60 stated thus: " There is some experimental evidence that radiation can act to stimulate a variety of cellular functions including proliferation and repair. Such simulation is not necessarily beneficial. In some circumstances,radiation appears also to enhance immunological responses and to modify 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 strenghthening the body's natural mechanisms" .ICRP went on to suggest that the experimental data on such effects have been inconclusive, mainly because of statistical difficulties at low doses. The Commission unequivocally stated that the available data on hormesis are not sufficient to take them into account in radiological protection.
Since this report was published, some more work has been completed. Whether these will make any impact to force rethinking of ICRP's conclusions will have to be seen. The answers to the important questions will have to probably come from molecular biologists. They have also to find explanations to the new findings of 'genomic instability'. The debate on effects of low level radiation appears to generate more heat than light. In many m]eetings, the debates generate lot of sound and fury signifying nothing. Living tissue with DNA packed with genetic information will continue to be an enigma at least for a few more decades.
This article originally appeared in AERB newsletter, the quarterly publication from the Indian atomic energy regulatory board. I wish to thank Dr. Parthasarathy for permission of reproducing this article.
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