Reports in the media of the nuclear plant disasters in Japan often cite the amount of radioactivity released in units called millisieverts. Not knowing what a millisievert is, I resorted to Wikipedia for the needed clarification. We reproduce some of their explanation here as a public service.
The sievert (symbol: Sv) is the SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to the physical aspects, which are characterised by the absorbed dose, measured in gray. It is named after Rolf Sievert, a Swedish medical physicist renowned for work on radiation dosage measurement and research into the biological effects of radiation.
As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Sv). When an SI unit is spelled out in English, it should always begin with a lower case letter (sievert), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.
For those used to older units such as the rem, (a unit for the equivalent dose), one sievert = 100 rem; 1 rem = 0.01 Sv = 10 mSv.
- Dental radiography: 0.005 mSv
- Average dose to people living within 16 km of Three Mile Island accident: 0.08 mSv during the accident
- Mammogram: 3 mSv
- Brain CT scan: 0.8–5 mSv
- Chest CT scan: 6–18 mSv
- Gastrointestinal series X-ray investigation: 14 mSv
- Maximum acceptable dose for the public from any man-made facility: 1 mSv/year
- Dose from living near a nuclear power station: 0.0001–0.01 mSv/year
- Dose from living near a coal-fired power station: 0.0003 mSv/year
- Dose from sleeping next to a human for 8 hours every night: 0.02 mSv/year
- Dose from cosmic radiation at sea level: 0.24 mSv/year
- Dose from terrestrial radiation: 0.28 mSv/year
- Dose from natural radiation in the human body: 0.40 mSv/year
The most frequent use of radiation units in public media is to mislead
people about radiation risks. The universally cited linear extrapolation
of health risks, from high exposures to low, has never been based on
measurements. It's hard to see how it ever could be. What we can say with
confidence is that it's almost certainly not linear.
Worse, full-body exposure is meaningless when radionuclides concentrate in
specific body tissues -- dust in lungs, iodine in the thyroid gland, metals
in bones. Then, your total radiation exposure may be indistinguishable
from nominal background, yet reliably destroy and mutate cells near the
contaminant over a long period. John Wayne was much more probably killed
by the picograms of fallout dust he breathed in and retained than the few
days' irradiation he got on-set downwind of that H-bomb test.
The curie is an interesting unit. It make releases of long half-life
radionuclides seem less harmful than short half-life materials. You may
release overwhelmingly more cesium than iodine, but the same number of
curies each. The level of iodine exposure may be considered small, given
that it will decay to nothing in a few weeks, but the much more abundant
cesium, when an immeasurable bit of it finds its way into your bones, will
continue radiating long after your death.
Examples of such misleading usage are easy to find, such as in the
frequently repeated Three Mile Island statistics. Average gamma radiation
dosage in a radius around TMI extrapolated from a few high-ground samplings
means nothing for the people in the valley basin downstream bathed in heavy
gaseous and suspended radionuclides for days.
As scientists we are congenitally disposed against misleading anybody.
It's all too easy, though, to repeat published numbers and seem to be
saying -- and even to believe ourselves -- what the numbers do not really
say at all.
An interesting discussion of specifics may be found in the postings at
http://www.tinyrevolution.com/mt/archives/2011_03.html , and in the month
Posted by: Nathan Myers | April 24, 2011 at 02:59 AM