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There's a concept in radiation protection called ALARA, and it stands for
"as low as reasonably achievable." Within reason, you want to lower the levels as low
as possible. That's the acceptable answer, if you will, for an acceptable limit. Now,
at some point, you have to set an upper limit, right, because there are going to be times
when people are exposed, and you don't want them to be exposed any higher. NASA actually
has their own limits. They petitioned the Environmental Protection Agency years ago
to not be encumbered by the same limits you would have if you were a nuclear power plant
worker, for instance. Because first of all, your career wouldn't be very long because
the limits are much lower for a nuclear power plant worker. But the second reason is, their
petition made the claim that being an astronaut is a hazardous occupation, and you're entering
into it knowing that it's hazardous for a variety of reasons, including potentially
radiation, and there should be higher limits. And so they won this petition, and so astronaut
limits are set in a different way than they are for terrestrial limits. And it's rather
complicated the way they do it, but it basically has to do with allowing an astronaut to have
an increased risk, I think normally it's a 3 percent risk of long-term fatal cancer which
is calculated, and they try to derive to the best of their abilities through things like
results from radiation biology experiments, and all kinds of bodies of evidence, and then
they set the limits based on the astronaut's age at first exposure and then whether they're
a male or a female, because males and females have different responses to certain types
of radiation.