SierraScape April - May 2006
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by Ken Schechtman, PhD
Since the dawn of the nuclear age, ionizing radiation has had a primeval niche in the pantheon of environmental fears. From the horror films of the 1950s to the evacuations following Three Mile Island and Chernobyl, one legacy of Hiroshima is a morbid fascination with radiation and the grotesque images it evokes.
In one sense, the fear is justified. By its very definition, ionizing radiation produces ions. It transforms electrically neutral atoms into charged particles. It can change the structure and behavior of molecules and, as has been demonstrated in animal experimentation, it can change your DNA producing mutations that are the potential precursors of birth defects and cancer.
But there is a circuitous distance between the potential of laboratory findings and the multifaceted complexity of disease etiology. Indeed, despite 60 years of research, much of what we know about the human effects radiation exposure comes from data gathered in the immediate and long-term aftermath of the atomic bombings of Hiroshima and Nagasaki.
Detailed studies of 90,000 survivors performed by the Hiroshima-based Radiation Effects Research Foundation (RERF) have collected information about exposure as a function of distance from the bomb epicenters and movements following the attacks. Follow-up RERF studies have correlated cancer rates with the magnitude of exposure. The 3000 children born to pregnant survivors of the attacks have produced a wealth of data about the consequences. And with equivocal science to support it, linear extrapolations using known high dose disease effects in Japan have been used to estimate the consequences of routine low level exposure to medical, environmental, and workplace radiation.
The basic message of the A-bomb survivor cancer studies is that ionizing radiation is carcinogenic, but that it is a weak carcinogen. The most compelling evidence of the etiologic connection comes from the clear delineation of a dose response relationship which has established that people closest to the epicenter have higher cancer rates than people whose radiation dose was lower because they were further away when the bombs detonated. One study reported that exposed survivors of Hiroshima and Nagasaki had experienced 8000 cancer deaths, but that 7500 would have been expected had there been no bombings. This estimated 6.7% increase seems surprisingly low, especially since the radiation dose was so high in many victims, and especially when measured against other disease risk factors. Smoking, for example, causes 90% of lung cancers and can increase risk as much as 30-fold. And many measures of cardiovascular risk are associated with a doubling of risk status.
A more recent study published in the British Medical Journal in 2005 confirms the weak carcinogenic impact of radiation exposure. This multinational study involving 15 countries reported on cancer rates among 407,391 nuclear industry workers who provided 5.2 million person years of data. The workers, who were carefully monitored for exposures that were several times greater than what we routinely receive from cosmic, terrestrial, and medical sources, confirmed the dose response relationship observed in Japan. But the authors estimated that although the cancer rates they measured were at the high end of what would be predicted by the Japanese studies, only "1-2% of deaths from cancer among workers in this cohort may be attributable to radiation."
As an addendum to these studies of mega radiation doses in Japan and high doses among radiation workers, we note finally that other researches have focused on the potential carcinogenic effects of more routine exposure. For many reasons, these very-low dose studies have produced equivocal results at best. For example, the Journal of Radiation Research published a report in the year 2000 which found that cancer rates among 106,517 subjects living in areas of Yanbgjiang China that have high levels of background radiation were slightly lower than among Chinese living in lower exposure areas. And 20 years after the Three Mile Island accident produced marginal increases in radiation exposure, a Pennsylvania state health registry that tracked 30,000 area residents was disbanded because it found no health effects.
Despite these negative very low dose studies, there are two messages in the above research. The first and clearest is that exposure to ionizing radiation at levels that exceed medical and standard background concentrations is associated with a small but very real increase in cancer incidence and mortality. The second message provides a compelling challenge to environmentalists. As we formulate our policies and define our priorities, we have an obligation to perform our own risk-benefit assessments.
Environmental perils were not created equal. And sometimes, there is a Hobson's choice under which viable solutions to one environmental challenge imply the exacerbation of others. With these final musings in mind, the next article in this series will wade tentatively into the murky waters of which ox gets gored when we weigh the risks and balance the choices we might prefer not to face.