December 6, 2015
Updated: December 6, 2015 19:33 IST
Ever since the discovery of X-rays, researchers have exposed fruit flies, mice and other living beings to radiation. An atom bomb survivor study and medical and industrial exposures to radiation have provided human data. These demonstrated that radiation at high doses will cause cancer. But at low doses there are uncertainties. As a matter of abundant caution, scientists assume that even low doses have a detrimental effect and there is no threshold below which the effect is zero. Presently, many scientists oppose this Linear No threshold Hypothesis.
A few years ago, a group of 26 scientists highly regarded in radiobiology research wondered what will happen if they grow living organisms in ultra low radiation levels. Quite unexpectedly, the study carried out on their recommendation showed that the experimental organisms (one radiation sensitive and the other radiation resistant) required radiation for their normal growth! On July 7, 2011, The Hindu reported the preliminary results of such a study.
Dr. Hugo Castillo and his colleagues reported the final details of their study in the September 2015 issue of the International Journal of Radiation Biology (IJRB).
It was difficult to carry out such an experiment, as natural radiation which primarily consists of gamma radiation emitted by radioactivity present in building materials and soil and cosmic radiation from outer space is present everywhere.
How to achieve ultra low levels of radiation? Researchers set up their laboratory in New Mexico, USA in the Waste Isolation Pilot Plant (WIPP). WIPP is located at a depth of 650 metres in the middle of a 610-metre thick ancient salt deposit that has been stable for more than 200 million years. The radioactivity content of the salt deposit (pure sodium chloride) is extremely low. The thick salt layers reduce cosmic radiation to negligible levels. To reduce the radiation background still further the scientists used a 15.24 cm thick vault made from pre-World War II steel in which there are no fallout radioactive elements.
The typical background radiation level in the vault where they did the experiment was 0.16 nGy per hour, which is about 400 times less than the normal background radiation. (Gy is a unit of dose; in a Gy of radiation, the energy absorbed per kg of material is one Joule; nGy -nanoGy- is one thousandth of a millionth of a Gy.)
The researchers used Shewanella oneidensis, a radiation sensitive organism and Deinococcus radiodurans, a radiation resistant microbe, in their study. They grew them in separate incubators at carefully controlled humidity (48 per cent) and temperature (30 degree Celsius).
They exposed the microbes to two radiation levels 0.2 nGy per hour (at ultra low background radiation level) and 71.3 nGy per hour (at background radiation level). They maintained the latter level by using known amounts of potassium chloride in sacks (totally 11.5 kg) around the incubators which contained the microbes to be irradiated to background radiation levels. Potassium chloride, which contains potassium-40 emits high-energy gamma rays and is an ideal source of radiation.
After exposure, the scientists extracted the RNA from the microbes and demonstrated the molecular evidence that ultra low background radiation inhibited growth and elicited a stress response in the two species of bacteria. Interestingly, when the researchers returned the cells to background radiation levels, growth rates recovered and the stress response dissipated.
“The presence of radiation isn’t necessarily bad and the absence of radiation isn’t necessarily good”, Mr James Conca, a regular columnist of Forbes (Sept 23, 2015) commented in his inimitable style, after lucidly reviewing the IJRB paper.
The organisms showed quantifiable stress response, identifiable to specific genes in their DNA, when they remained in the absence of radiation. Radiation seems to be necessary for normal growth for these two lowly organisms!
(The writer is former Secretary, Atomic Energy Regulatory Board)