Radiation exposure is not too good for one's health and well being. But how much is enough and how much is deadly? A new study from MIT scientists suggests that the guidelines governments use to determine when to evacuate people following a nuclear accident may be too conservative. The study, led by Bevin Engelward and Jacquelyn Yanch and published in the journal Environmental Health Perspectives, found that when mice were exposed to radiation doses about 400 times greater than background levels for five weeks, no DNA damage could be detected.
Radiation exposure is not too good for one's health and well being. But how much is enough and how much is deadly? A new study from MIT scientists suggests that the guidelines governments use to determine when to evacuate people following a nuclear accident may be too conservative. The study, led by Bevin Engelward and Jacquelyn Yanch and published in the journal Environmental Health Perspectives, found that when mice were exposed to radiation doses about 400 times greater than background levels for five weeks, no DNA damage could be detected.
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Current U.S. regulations require that residents of any area that reaches radiation levels eight times higher than background should be evacuated. However, the financial and emotional cost of such relocation may not be worthwhile, the researchers say.
"There are no data that say that’s a dangerous level," says Yanch, a senior lecturer in MIT’s Department of Nuclear Science and Engineering. "This paper shows that you could go 400 times higher than average background levels and you’re still not detecting genetic damage. It could potentially have a big impact on tens if not hundreds of thousands of people in the vicinity of a nuclear power plant accident or a nuclear bomb detonation, if we figure out just when we should evacuate and when it’s OK to stay where we are."
Until now, very few studies have measured the effects of low doses of radiation delivered over a long period of time. This study is the first to measure the genetic damage seen at a level as low as 400 times background (0.0002 centigray per minute, or 105 cGy in a year).
The health hazards of low doses of ionizing radiation are unknown and controversial, because the effects, mainly cancer and genetic damage, take many years to appear, and the incidence due to radiation exposure can't be statistically separated from the many other causes of these diseases. The average dose of background radiation, mostly from natural sources, that every human unavoidably receives during daily life. Background radiation level is widely used in radiological health fields as a standard for setting exposure limits. Presumably, a dose of radiation which is equivalent to what a person would receive in a few days of ordinary life will not increase his rate of disease measurably.
How much is too much?
Background radiation comes from cosmic radiation and natural radioactive isotopes in the environment. These sources add up to about 0.3 cGy per year per person, on average.
“Exposure to low-dose-rate radiation is natural, and some people may even say essential for life. The question is, how high does the rate need to get before we need to worry about ill effects on our health?†Yanch says.
Previous studies have shown that a radiation level of 10.5 cGy, the total dose used in this study, does produce DNA damage if given all at once. However, for this study, the researchers spread the dose out over five weeks, using radioactive iodine as a source. The radiation emitted by the radioactive iodine is similar to that emitted by the damaged Fukushima reactor in Japan.
At the end of five weeks, the researchers tested for several types of DNA damage, using the most sensitive techniques available. Those types of damage fall into two major classes: base lesions, in which the structure of the DNA base (nucleotide) is altered, and breaks in the DNA strand. They found no significant increases in either type.
Though the study ended after five weeks, Engelward believes the results would be the same for longer exposures. “My take on this is that this amount of radiation is not creating very many lesions to begin with, and you already have good DNA repair systems. My guess is that you could probably leave the mice there indefinitely and the damage wouldn’t be significant,†she says.
Most of the radiation studies on which evacuation guidelines have been based were originally done to establish safe levels for radiation in the workplace, Yanch says — meaning they are very conservative. In workplace cases, this makes sense because the employer can pay for shielding for all of their employees at once, which lowers the cost, she says.
Those conservative estimates are based on acute radiation exposures, and then extrapolating what might happen at lower doses and lower dose-rates, Engelward says.
"It is interesting that, despite the evacuation of roughly 100,000 residents, the Japanese government was criticized for not imposing evacuations for even more people. From our studies, we would predict that the population that was left behind would not show excess DNA damage - this is something we can test using technologies recently developed in our laboratory," she adds.
The definitions of any exposure risk tend to be conservative. What this study shows is that current standard is based on acute exposure as opposed to long term chronic levels which are entirely different.
For further information see Radiation.
Dressed image via EPA
