The Roentgen is a unit of radiation exposure. The Rem (Roentgen Equivalent Man)
... Background radiation levels are typically around 300 millirem per year. ...
millirem is 1/1000 of REM. Same as millimeter is 1/1000. meter
(for basics of these prefixes, kilo, milli, see
rem is the older unit of dose, today
the SI unit for dose (Sieverts) is used (1 Rem = .01 Sieverts).
Here are SI units of related quantities
there is activity of source, intensity at a given point, absorbed dose and effect
here is a less technical description of the risks:
The risk from radiation exposure can be reduced by shortening the time
of exposure, getting farther away from the source, and shielding or
blocking the source. It is known that whole-body radiation doses of
more than 10,000 millirem over a short period of time can cause a
slight increase in a person's risk of developing some types of cancer
years after exposure. The risk that radiation-induced cancer will
develop during the person's lifetime is estimated by the National
Academy of Sciences to be about 1/20 of one percent for every 1,000
millirem of short-term exposure greater than 10,000 millirem. In other
words, if you were to receive a short term exposure of 11,000 millirem
(11 rem), your estimated risk of developing some type of cancer would
increase by 0.05 percent.
By comparison, the dose from one chest x-ray is approximately 10
millirem. Regulatory agencies in both the U.S. and Canada have set
limits for worker exposure which are considerably higher than the
level of natural background radiation. In the U.S., the annual dose
limit for radiological workers is 5,000 millirem (the limit for a
pregnant worker is 500), while the dose to the public from nuclear
industries is limited to 100.
Average American exposure
What radiation dose does the typical person receive? Several sources
of radiation and the dose that each source gives to the average
American each year are listed in Table 1.
and here is piechart for sources:
Amounts We Receive
Most Americans receive about 360 mrem per year from all sources of
radiation, including radon and medical exposure. About 40 mrem per
year comes from the natural radioactivity in our own bodies. Most of
our natural background radiation comes from cosmic radiation from
outer space and from radioactive materials in the earth's rocks and
Normal Radiation Exposure - efect of elevation
At sea-level, the average radiation level is approximately 0.03
microsieverts per hour. As the altitude increases, the radiation
exposure increases exponentially. Mexico City, 2240 m above sea-level,
is exposed to about 0.09 microsieverts per hour; La Paz (in Boliva,
South America) - the highest city in the world - has radiation of
about 0.23 microsieverts per hour.
and here are answers to some FAQs on measuring radiation:
Civil Defense Ultimate CD
Over 300 PDF files, close to 20,000 pages, totalling over 475
megabytes, with Civil Defense documents, Military Field Manuals, NBC
Prep, Medical, and Survival topics. Classic books from many authors in
text format and a small selection of religious books for personal
research and audio files included.