There are a few scales that one can use to measure radiation. Depending upon
your application, one scale may be better than the others.

Roentgen: Is the measurement of energy produced by Gamma or X-Ray radiation in a
cubic centimeter of air. It is abbreviated with the capital "R". One
milliroentgen, abbreviated "mR" is one-thousandth of a roentgen. One
microroentgen, abbreviated “uR” is one-millionth of a roentgen.

absorption of all types of ionizing radiation (alpha, beta, gamma, neutrons,
etc) into any medium. One rad is equivalent to the absorption of 100 ergs of
energy per gram of absorbing tissue.

REM: Roentgen Equivalent Man is a measurement that correlates the dose of any
has the same biological effect, the dosage is multiplied by a "quality factor"
(Q). For example, a person receiving a dosage of gamma radiation will suffer
much less damage than a person receiving the same dosage from alpha particles,
by a factor of three. So alpha particles will cause three times more damage than
gamma rays. Therefore, alpha radiation has a quality factor of three. Following
is the Q factor for a few radiation types

 Radiation: Quality Factor (Q) Beta, Gamma and X-rays 1 Thermal Neutrons 3 Fast n, a, and protons 10 Heavy and recoil nuclei 20

The difference between the rad and rem is that the rad is a measurement of the
radiation absorbed by the material or tissue. The rem is a measurement of the
biological effect of that absorbed radiation.

For general purposes most physicists agree that the Roentgen, Rad and Rem
may be considered equivalent.

System International (SI) of Units

The System International of unit for radiation measurements is now the official
system of measurements. This system uses the “gray” (Gy) and “sivert” (Sv) for
absorbed dose and equivalent dose respectively.

The conversion from one system to another is simple:

 1 Sv = 100 rem 1 rem = .01 Sv 1 mSv = 100 mR (mrem) 1 mR = .01 mSv 1 Gy = 100 rad 1 rad = .01 Gy 1mGy = 100 mrad 1 mrad = .01 mGy

To get an idea of this visually try using this Radiation Dose Chart.
(Courtesy of XKCD.com)
*Sources

In the United States the U.S. Nuclear Regulatory Commission (NRC) determines
what radiation exposure level is considered safe. Occupational exposure for
worker is limited to 5000 mrem per year. For the general population, the
exposure is 500 mrem above background radiation in any one year. However for
long term, multi-year exposure, 100 mrem above background radiation is the limit
set per year.

Let’s extrapolate the 100 mrem number to an hourly radiation exposure rate.
There are 365 days/yr x 24 hr/day equals 8760 hours. Divide 100 mrem by 8760
hours equals .0114 mrem/hr or 11.4/hr microrem. This is an extremely low
radiation level. The background radiation in my lab hovers around 32 uR/hr. Am I
in trouble? No. Typically background radiation in the United States averages 300
mrem/yr, or 34 microrem/hr. The NRC specifications is for radiation above this

Notice that my lab readings are in microrad (uR/hr) and the exposure limit is
given in microrem (urem/hr). I do not know what type of radiation (a , b or y)
the geiger counter is reading in my lab at any particular instant, so I do not
know the Q factor of the radiation and therefore can not calculate the mrem.
However for general purposes I consider them the one and the same. Remember,
the digital geiger counters are calibrated using a Cs-137 radioactive source.
Therefore the highest accuracy in reading radiation levels will be from Cs-137
sources.

 Exposure Source Dose(conventional) Dose (SI) Flight from LA to NY 1.5 mrem .015 mSv Dental X-ray 9 mrem .09 mSv Chest X-ray 10 mrem 0.1 mSv Mammogram 70 mrem 0.7 mSv Background Radiation 620 mrem/year 6.2 mSv/year

and stars. Terrestrial radiation from low levels of uranium, thorium, and their
decay products in the soil, air and water. Internal radiation from radioactive
potassium-40, carbon-14, lead-210, and other isotopes found inside our bodies.

Our line of Digital Geiger Counters are extremely sensitive and will detect
radioactivity that may be present in food and other materials to cause
a increase in the background radiation one must first establish the

To establish background radioactivity level, set the geiger counter to
1 minute mode. Then record the next 5 (one minute) readings. Average the
area. Also record the (Counts Per Minute) CPM at the end of each minute
run. Record the CPM number at the last second, before the screen updates
for the next minute reading. The equivalent radiation level will stay on
the second display line of the LCD making that easier to record, until the
line is updated at the end of the next run.

The more readings you take, say 25, the more accurate will be your average
and lowest CPM count will establish your minimum and maximum CPM. These
numbers will establish a baseline so that you will be able to determine
if the background radiation has changed, or to detect trace amounts of

To run a test position the probe (or geiger counter) very close to the top surface
of the material you are testing, and run the counter in its 1 minute or 5 minute

U.S. Nuclear Regulatory Commission
http://www.nrc.gov/

CDC - Center for Disease Control maintains a radiation emergency web site: