A
new study from Massachusetts Institute of Technology (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 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.
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 okay 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 cGy/min,
or 105 cGy/year).
“Almost
all radiation studies are done with one quick hit of radiation. That would
cause a totally different biological outcome compared to long-term conditions,”
says Engelward, an associate professor of biological engineering at MIT.
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/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.
DNA
damage occurs spontaneously even at background radiation levels, conservatively
at a rate of about 10,000 changes per cell per day. Most of that damage is
fixed by DNA repair systems within each cell. The researchers estimate that the
amount of radiation used in this study produces an additional dozen lesions per
cell per day, all of which appear to have been repaired.
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.
Doug
Boreham, a professor of medical physics and applied radiation sciences at McMaster University, says the study adds to
growing evidence that low doses of radiation are not as harmful as people often
fear.
“Now,
it’s believed that all radiation is bad for you, and any time you get a little
bit of radiation, it adds up and your risk of cancer goes up,” says Boreham,
who was not involved in this study. “There’s now evidence building that that is
not the case.”
Conservative estimates
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.
However, “when you’ve got a contaminated environment, then the source is no longer
controlled, and every citizen has to pay for their own dose avoidance,” Yanch
says. “They have to leave their home or their community, maybe even forever.
They often lose their jobs, like you saw in Fukushima. And there you really want to call
into question how conservative in your analysis of the radiation effect you
want to be. Instead of being conservative, it makes more sense to look at a
best estimate of how hazardous radiation really is.”
Those
conservative estimates are based on acute radiation exposures, and then
extrapolating what might happen at lower doses and lower dose-rates, Engelward
says. “Basically you’re using a data set collected based on an acute high dose
exposure to make predictions about what’s happening at very low doses over a
long period of time, and you don’t really have any direct data. It’s
guesswork,” she says. “People argue constantly about how to predict what is
happening at lower doses and lower dose-rates.”
However,
the researchers say that more studies are needed before evacuation guidelines
can be revised.
“Clearly
these studies had to be done in animals rather than people, but many studies
show that mice and humans share similar responses to radiation. This work
therefore provides a framework for additional research and careful evaluation
of our current guidelines,” 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.