IN THE early 80's if anyone claimed that low-level exposure to ionising radiation has a beneficial effect, the response would have been a bland smile followed by a smirk! But currently many scientists are pursuing the concept with evangelic zeal. Hormesis is defined as the stimulating effect of small doses of substances, which in larger doses are inhibitory.

In 1981, T.D. Luckey revived `hormesis' with reference to ionising radiation backing it up with 1250 articles. The effects observed included the growth of algae under X-irradiation, growth of peas, increase in life span of invertebrates and insects and seedling stimulation by X-rays. Luckey wrote a very interesting book titled Radiation Hormesis.

John Cameron, emeritus professor, University of Wisconsin published in the Forum on Physics and Society (2001).

He argued that the U.S. Gulf States have a high cancer death rate compared to the mountain states although background radiation is much lower in the Gulf States. He suggested that they are suffering from "radiation deficiency"! Mainstream scientists have questioned the results of such ecological studies.

Cameron referred to an unpublished study, which showed that nuclear shipyard workers who were exposed to more than 5mSv of dose had a death rate from all causes, which was 24 per cent lower than the control group. Writing in the Environmental Pollution Journal (2005), Edward J. Calabrese, Professor at the University of Massachusetts at Amherst provided substantial evidence that hormesis is real, reproducible and in properly designed studies very common, in fact more common than any other dose-response model.

Genetic effects

For instance, H. Muller who demonstrated genetic effects by irradiating fruit flies in 1927, predicted a genetic catastrophe from atomic bomb explosion. This lead to feverish publicity. But there was no publicity for the disproof 35 years later. The story of double-headed babies in Hiroshima and Nagasaki was fiction; but it made good copy.

Repair mechanisms

The interaction of radiation with tissue is not just a purely physical mechanism. We cannot ignore the profound biological defence mechanisms. B.L Cohen (Risk Analysis, 1995) reviewed the evidence that biological defence mechanisms are stimulated by low-level radiations.

In 1991, the International Commission on Radiological Protection stated: "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 balance of hormones.

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 mechanisms."

ICRP stated that the data on hormesis are not sufficient to take them into account in radiological protection. The Committee on the Biological Effects of Ionizing Radiation has stated that the assumption that any stimulatory hormetic effects from low doses of ionizing radiation will have a significant health benefit to humans that exceeds potential detrimental effects from radiation exposure is unwarranted at this time".

K.S. Parthasarathy

Department of Atomic

Energy, Mumbai

ksparth@yahoo.com

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