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H ow Mu ch R ad ia tion is Safe ?
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 34 urem/hr background radiation.
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
Flight from LA to NY
Dental X-ray
Chest X-ray
Mammogram
Background Radiation
Background radiation consists of three sources; Cosmic radiation from the sun 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.
Because of the randomness of radioactivity, background radiation can vary from minute to minute and
place to place. In my corner of the world I have a background radiation that triggers the counter 22-34
times a minute.
DTG-01
Common Radiation Exposure (General Population)
Dose(conventional) Dose (SI)
1.5 mrem
9 mrem
10 mrem
70 mrem
620 mrem/year
.015 mSv
.09 mSv
0.1 mSv
0.7 mSv
6.2 mSv/year
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