Radiation Exposure: Is it Harmful or Beneficial?
Published on: February 4, 2012 – 00:10
BY K S PARTHASARATHY
Based on epidemiological studies of over fifty groups of about two million radiation exposed persons over the past several decades, specialists concluded that radiation may induce cancer at high doses.
At low doses typical in medical x-ray or nuclear medicine procedures we cannot prove the cancer inducing potential of radiation. This is because it is difficult to identify radiation induced cancers against the large number of normal cancers.
There are specialists who believe that radiation exposure at low levels is beneficial. A growing number of specialists think that low dose radiation exposure is not as harmful as was thought of so far.
No one knows whether there is a threshold dose above which only harmful effects manifest? Radiation protection depends on assumptions. We have to carry out experiments at low doses to resolve the issues scientifically.
Recently, Italian researchers claimed that low dose radiation exposure can induce biological and cellular changes that might offset the hazards of radiation. Probably, at low doses there may be protective mechanisms at work.
They studied interventional cardiologists – a group most exposed to ionising radiation among health professionals. They have a per capita exposure 2 to 3 times higher than that of radiologists.
Dr Gian Luigi Russo, Institute of Food Sciences, National Research Council, Rome and his coworkers chose ten healthy interventional cardiologists with an average age of 38 years, with a median radiation exposure of 4.7 mSv per year and lifetime exposure ranging between 20 to 100 mSv, working in the cardiac catheterization unit and
10 matched unexposed controls recruited from among the hospital and laboratory workers who did not have radiation exposure. (mSv is a unit of radiation dose; AERB prescribes an annual dose limit of 30mSv for
The researchers measured many markers of oxidative metabolism in plasma, and in red blood cells (erythrocytes) and white blood cells (lymphocytes) in the two groups. In the exposed group, there was a threefold increase in hydrogen peroxide, a biochemical marker indicative of oxidative stress. When hydrogen peroxide is formed in the body, the body releases antioxidants to counteract the effect.
The researchers did not observe any significant change in the level of antioxidants in the two groups, may be because the bodies of the exposed groups generated adaptive response by activating antioxidant defence mechanisms to counteract the increase in hydrogen peroxide levels.
In interventional cardiologists, chronic exposure to low dose radiation may induce two
specific types of cellular defences against oxidative stress, the researchers argued in the August 23, 2011 issue of the European Heart Journal.
Firstly, the oxidative stress was found to be counter balanced by a 1.7 fold increase in glutathione, a measure of antioxidant response in the exposed group.
Secondly, the cardiologists exposed to radiation had significantly higher levels of Caspase-3 activity in their white blood cells. Capase-3 is a biochemical which is implicated in programmed cell deaths.
Capase-3 helps to eliminate damaged cells. This is beneficial as damaged cells may become cancerous, if they survive.
Both mechanisms may compensate for the unbalanced reactive oxygen species and contribute to maintain relatively stable equilibrium in the cell.
Other experiments carried out at the Waste Isolation Pilot Plant (WIPP), an underground lab in New Mexico and at the Lovelace Respiratory Research Institute, Albuquerque gave interesting results (Health Physics, March 2011).
WIPP is located at a depth of 650 metre 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 is extremely low. The radiation levels in the lab are ten times lower than the normal natural background radiation levels. The contribution to the background from potassium-40, the only identifiable radionuclide present in the lab can also be reduced further by using a modest amount of shielding. Massive, 650 metre thick, salt reduced the cosmic ray background.
Researchers incubated Deinococcus Radiodurans, a bacterium which is highly resistant to radiation, above-ground and in WIPP in a 15 cm thick pre-World War II steel chamber; that steel is not contaminated by traces of radio-nuclides from nuclear weapons fallout.
The radiation level underground was 20 per cent of the surface radiation levels and in the pre WW II chamber it was 30 per cent of those underground. Overall reduction was about 15 times.
Scientists monitored the bacterial growth by assaying for protein, optical density of the cultures and cell agar plate counts. Though data had relatively high variability, the three indicators of cell growth demonstrated that the cells grown underground were inhibited and grew increasingly so with increasing time underground (Health Physics, 2011).
In the second experiment, researchers exposed a type of human lung cells at 1.75 mGy per year; another sample of cells to 0.3 mGy per year by using a 10 cm lead shield. The former corresponds to a typical background radiation level. (Gy is a unit of radiation dose It is equal to an energy absorption of one joule per kg.). They controlled the temperature, carbon dioxide and humidity levels in the two incubators in which the cells were placed ensuring that these parameters were statistically the same. They analyzed the exposed cells directly by standard methods for the presence of heat shock proteins or by exposing the cells to a single x-ray dose of 0.10Gy and then assayed for heat shock proteins.
Shielding cells from natural radiation up-regulated (initiated the process of increasing the response to a stimulus) the expression of two out of three stress proteins and follow on x-ray exposure further up-regulated expression.
Results were similar with the bronchial epithelial cells. Both studies demonstrated a stress response when cells were grown under reduced radiation conditions. Does it show that radiation is necessary for normal growth?
It may lead to increasing the levels of radiation considered safe; it will have a profound impact on the economics of decommissioning nuclear facilities, long term storage of radioactive waste, construction of nuclear power facilities among others.
This was part of a $150 million, five-year long, low-dose research project recommended by 26 scientists highly regarded in radiobiology research community and representing competing radiation effects hypotheses. Let us hope that they will develop a model based on sound science.