the recent events in Japan and the nuclear reactor damage and
radiation entering the atmosphere there, the following information
may help to understand the units of measurement being discussed, and
how it may correlate to Geiger Counter readings such as those being
displayed around the country on the Radiation
of Measurement (Radiation)
1 rad =
0.01 gray (Gy) 1 rem =
0.01 sievert (Sv)
1 gray (Gy)
= 100 rad 1 sievert (Sv)
= 100 rem
and Gray are ‘absorbed dose’ units. Rem and Sievert are
‘equivalent dose’ units.
a Rem and a Sievert?
relate to biological damage done to human tissue and factor the
differences between types of radiation. A multiplication factor is
used that represents the ‘effective’ biological damage of a given
type of radiation. This is the main reason for these units – to
factor the differences in damage that is caused from one type of
radiation to the next.
example, the list above shows that a ‘rad’ or ‘gray’ unit of
‘Alpha’ energy that is absorbed by soft human tissue does 20
times more damage than a ‘rad’ or ‘gray’ of Gamma, X-ray or
Radiation with a Geiger Counter CPM
is CPM (also the ‘number’ used on the Radiation Network )?
per minute) is a measure of radioactivity, a unit of measurement for
a Geiger counter. Technically, “It is the number of atoms in a
given quantity of radioactive material that are detected to have
decayed in one minute.”
Geiger counters are calibrated to Cs137 (Cesium).
CPM on the meter (for Cs137) is about 1 mR/hr (milliRad per
hour). 120 CPM on the meter (for Cs137) is about 1 uSv/hr
(microSievert per hour).
Howmany CPM of radiation is bad?
depends on how long you are exposed at any given level. The Radiation
Network website, for example, uses a threshold warning level of 100
CPM, mainly because it is unusual to observe levels of 100 or higher
without something more going on in the area than just background
said that, how could one figure out the ‘badness’ of a given
level? How bad is bad? All we need to do is put in terms that makes
we must understand a few radiation facts and numbers regarding
dosage. There tend to be lots of conversions and it can be confusing,
but by plodding through the math, you can determine a better idea and
relationship of the Geiger counter numbers versus the risks to your
Radiation dosage is
a measure of the risk of biological harm that the tissues receive in
unit of absorbed radiation dose is the sievert (Sv). Since one
sievert is a large quantity, radiation doses normally encountered are
expressed in milliSievert (mSv) or microSievert (µSv) which are
one-thousandth or one millionth of a sievert. For example, one chest
X-ray will give about 0.2 mSv of radiation dose.
average, our annual radiation exposure due to all natural sources is
about 300 milliRem, which is equivalent to 3 milliSieverts (3 mSv).
Adding man-made sources (medical procedures, and others) the average
annual U.S. radiation dose is about 600 milliRem, which is equivalent
to 6 milliSieverts (6 mSv).
annual human exposure to radiation (U.S.) 600
milliRem (mRem) 6 milliSievert (mSv)
dose for increase cancer risk of 1 in a 1,000 1,250
milliRem (mRem) 12.5 milliSievert (mSv)
of radiation poisoning 300,000
milliRem (mRem) 3,000 milliSievert (mSv)
50% death from radiation 400,000
milliRem (mRem) 4,000 milliSievert (mSv)
do the Radiation Network CPM numbers mean with regards to health
the examples of radiation dose listed above, we can correlate how
long it would take to experience those effects based on a
counter CPM number.
let’s use the number 100, since this is the threshold that
has chosen. The Cs137 calibration factor listed above (120 CPM) was
converted to obtain the proper factored results listed below (0.83x).
Higher CPM numbers are also listed for relevancy.
compared with the avg. annual human exposure (U.S.) 207
(at 100 CPM) 42 (at 500 CPM) 14 (at 1,500 CPM) 2 (at 10,000
to receive dose for increase cancer risk of 1 in a 1,000 432
(at 100 CPM) 86 (at 500 CPM) 28 (at 1,500 CPM) 4 (at 10,000
for earliest onset of radiation sickness 25,937
(at 100 CPM) 5,187 (at 500 CPM) 1,729 (at 1,500 CPM) 259 (at
the radioactive fallout from Japan reaching here to the U.S., the
metered Geiger Counter CPM that we see on the Radiation Network can
be compared to the equivalent ‘what-IF’ scenarios listed
saying though that anything less would not be ‘bad’ for us, there
are lots of theories out there regarding long-term effects of various
types of radioactive ionized particles making it into the food chain,
food sources combined, expose a person to around 40 millirems per
year on average.
Many foods are naturally radioactive, and bananas
are particularly so, due to the radioactive potassium-40 they
contain. The equivalent dose for 365 bananas (one per day for a year)
is 3.6 millirems (36 μSv).
foods that have above-average levels are potatoes, kidney beans, nuts
(especially brazil nuts), and sunflower seeds.
to limit radiation exposure
1. Time (limit
2. Distance (intensity
decreases sharply according to the
3. Shielding (alpha:
nearly anything… a sheet of paper will stop it) (beta: wood,
water, plastic-acrylic, aluminum) (gamma: water, concrete,
Do not rely upon this information for life or health, it is only one
person’s estimation based on a several hours research and punching
calculator buttons. We have no affiliation with the Radiation
Network, who may or may not agree with these numbers.