Great site, John.
The following is from William D. O'Neil, posted to the NBR list. I didn't ask him if I could post this here but I don't think there will be a problem passing it along.
It deals with radiation levels. It is a little more detailed and should show, while the situation is not normal. the press coverage is lacking in detail.
It may be helpful to say a few words about
radiation dose measurements. The standard unit of
measurement is the sievert (abbreviated Sv).
There is a pretty good article athttp://en.wikipedia.org/wiki/Sievert
. The basic
point to remember is that a measurement in
sieverts tells us how much total dose would be
absorbed per unit of mass by a human in that
position. For instance, a dose of 1 Sv is enough
to make you sick, although you would probably
recover without gross ill effects.
(For the benefit of those who were introduced to
radiation more than twenty or thirty years ago is
may be helpful to note that 1 Sv = 100 rem.)
Most of the reporting about radiation in
connection with Fukushima is in terms of the
_rate_ of dosage accumulation, usually reported
in millisieverts per hour, mSv/h, meaning
thousandths of a sievert per hour. Some is in
microsieverts per hour, meaning millionths of a
sievert per hour. (The standard symbol for
microsievert involves the Greek letter mu, µ,
which may not always reproduce well in email, so
I use mmSv, for milli-millisievert.) The peak
rate so far reported at the plant perimeter is
about 12 mSv/h. It's alarming in the sense that
exposure to this for four days would make you
quite ill. But that was a brief spike and it
seems clear that so far no one outside the plant
perimeter has been exposed enough to be a really major source of worry.
There is word now that the exposure limit for the
workers struggling to contain the damage in the
plant has been raised to 250 mSv. How serious is
this? It is certainly very high relative to the
typical radiation burden from natural background
sources such as cosmic rays and naturally
occurring radionuclides -- roughly 100 years
worth for people living in most places. But there
are places on earth where rates of 250 mSv per
year occur naturally and have throughout human
history, without any notable ill effects on the
local population. So an exposure of 250 mSv may
not be too bad, especially since I'm sure that
they will take all the protective steps. One
problem is that it does not need much margin in
case a person comes down with some unrelated
disease that requires high-dose radiation
treatment. Certainly, anyone who is exposed at
such a level should never be allowed to work near a nuclear plant again.