The millisievert and milligray as measures of radiation
dose and exposure
In
the SI system, a millisievert (mSv) is defined
as "the average accumulated
background radiation dose to an individual for 1 year,
exclusive of radon, in the United States." 1 mSv is the dose produced by exposure to 1 milligray (mG)
of radiation.
In the historical system of dosimetry, exposure to 1 roentgen (R) of X-rays results in absorption of 1 rad [radiation-absorbed dose],
which had the effect of
1 rem [roentgen-equivalent (in) man].
The unit equivalences between the systems are given in the
following table. Note that
SI units are 1% of historical units, i.e., 100
mrem = 1 mSv.
SI units
|
Historical dosimetry
|
1 Gray
|
100 (R)oentgen
|
1 Sievert
|
100 rad => 100 rem
|
10 mGy
|
1 R
|
10 mSv
|
1 rad
=> 1 rem
|
The whole-body exposure threshold for acute hematopoietic syndrome
or "radiation
sickness" is 500
mGy. A dose of ~3,000
mGy produces an acute
gastrointestinal syndrome that can be fatal without
major medical intervention, and a dose of ~ 5,000 mGy is considered
the human LD 50 / 30,
that is, the lethal dose
for 50% of the population in 30 days, even
with treatment. These are acute thresholds: the same
dose fractionated over a series of exposures or over a longer
time may produce less injury, as the body has a chance to
repair damage between exposures. Annual limits for
radiation workers and the general public take this into
account.
"Radiation doses that exceed a
minimum (threshold) level can cause undesirable effects such as
depression of the blood
cell-forming process (threshold dose
= 500
mSv, 50 rem) or cataracts (threshold dose =
5,000 mSv, 500 rem)*. The scope and severity of these
effects increases as the dose increases above the corresponding
threshold. Radiation also can cause an increase in the
incidence, but not the severity, of malignant disease (e.g.,
cancer). For this type of effect, it is the probability of
occurrence that increases with dose rather than the severity.
For radiation protection purposes it is assumed that any
dose above zero can increase the risk of radiation-induced
cancer (i.e., that
there is no threshold)**.
Epidemiologic studies have found that the estimated lifetime
risk of dying from cancer is greater by about 0.004% per mSv
(0.04% per rem) of radiation dose to the whole body (NRC,
1990)."
*Cataracts may result from localized,
chronic exposure to high LET radiation at
close range, for example where a laboratory worker is repeatedly
exposed to 32P alpha radiation at eye level
during DNA-labeling experiments. Such radiation is easily
blocked by protective eye wear and plexiglass shielding.
**The LNT (Linear
No Threshold) model is a straight line through
zero, whose slope is based on empirical data that 100mSv increases
the probability of an individual's death from cancer by 0.4%.
The LNT is most applicable to industrial workers
subject to higher doses or radiation than the public.
Linearity for doses < 1 mSv has been challenged,
and it is suggested that psychological and societal effects
for populations exposed to such low doses may be more serious
than the radiation itself..
Quoted text from An
Evaluation of Radiation Exposure Guidance for Military
Operations: Interim Report (1997). J. Christopher
Johnson and Susan Thaul, Editors. National Academy of Sciences.
ISBN 0-309-05895-3.