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.
SI
units
|
Historical
dosimetry
|
1 Gray
|
100
R
|
1 Sievert
|
100
rad => 100 rem
|
10
mGy
|
1 Roentgen
|
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.
"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)."
Note:
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
eyewear and
plexiglass shielding.
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.