What are Safe Radiation Limits?

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The short answer is that there is no absolute level that can be certified as safe.  What is known is that the average annual dose that a person receives during the year from naturally occurring radiation, mostly from airborne radon with smaller contributions from cosmic rays and internal body radiation, is about 300 millirems at sea level.  In Denver, one mile above sea level, the annual background radiation is about 400 millirem.  The 300 millirems is roughly equivalent to 30 chest x-rays or one-third of a whole-body CT scan.  Radiation dosage is accumulative throughout one's life.  So if you are 50 years old and have received no radiation other than average background radiation, your life-time dose would be 50 x 300 mrems = 15,000 mrems or 15 rems total.  The fact that the cancer rate is lower in Denver than the average in the rest of the USA (Bulletin of the Atomic Scientist, Letters, Bertram Wolfe, President of the American Nuclear Society, San Jose, Ca, November 1986, p55) indicates that it is probably reasonable to assume that this level of radiation is relatively safe, that is, the number of deaths due to background radiation is likely small.


At the other extreme, based on data from the aftermath of the atomic bombs that were dropped on Nagasaki and Hiroshima, 600,000 millirems is 100% fatal.  Anyone receiving this dose of ionizing radiation will die of radiation sickness.  50% of the people exposed to a total dose of 450,000 millirems died.  You can determine your approximate annual exposure at this handy website: Radiation Dose Chart.


Each nation sets what the limits it considers safe for its citizens.  For the USA, dosage limits are as follows:
  Astronauts:  25,000 millirems per mission
  Occupational, adult:  5,000 millirems per year (reduced from 15,000 mrems/yr. in 
                                 1957, reduced from 25,000 mrems/yr. in 1950)
  Lifetime exposure:  1,000 mrem/yr. multiplied by ones age in years
  Occupational, minor:  500 mrem/yr.
  Fetus of pregnant worker:  500 mrems total dose; 
                   recommended:  maximum of 50 mrem/month. (limit created in 1994)


  Note:  All these levels are in addition to the background radiation one receives.


So how much additional radiation is a person living on the west coast of the USA likely to receive?  Most likely, one will receive about 300 microrems per year, roughly one-thousandth of the normal background radiation exposure coastal residents experience.  So fear not for the radiation Americans will receive from Japan; hope for the best for the Japanese living within 50 miles of the Fukushima nuclear plant.  The radiation levels are continuing to increase.  Most recently, traces of plutonium were found in soil samples around the plant site.  This can only occur when the shielding around the core has been breached.  Reports indicate that this nuclear situation is approaching the severity of Chernobyl.
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