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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Conversion of Radiation Units, a Follow up to the 15 March 2011 Post

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The media have begun using a variety of units to measure radiation: sieverts, becquerels, millirems, etc.  How many people know all these units and whether 10 sieverts/hour is more or less dangerous than 100 becquerels/hr.?  To help with the confusion, here is a conversion table that may be of use:

Here is a conversion table for various other units of measure:

Conversion Factors

To convert from	To	Multiply by
Curies (Ci)	becquerels (Bq)	3.7 x 1010
millicuries (mCi)	megabecquerels (MBq)	37
microcuries (µCi)	megabecquerels (MBq)	0.037
millirads (mrad)	milligrays (mGy)	0.01
millirems (mrem)	microsieverts (µSv)	10
milliroentgens (mR)	microcoulombs/kilogram (µC/kg)	0.258
becquerels (Bq)	curies (Ci)	2.7 x 10-11
megabecquerels (MBq)	millicuries (mCi)	0.027
megabecquerels (MBq)	microcuries (µCi)	27
milligrays (mGy)	millirads (mrad)	100
microsieverts (µSv)	millrems (mrem)	0.1
microcoulombs/kilogram (µC/kg)	milliroentgens (mR)	3.88

From: http://orise.orau.gov/reacts/guide/measure.htm#Conversions

Radiation Measurements

	Radioactivity	Absorbed Dose	Dose Equivalent	Exposure
Common Units	curie (Ci)	rad	rem	roentgen (R)
SI Units	becquerel (Bq)	gray (Gy)	sievert (Sv)	coulomb/kilogram (C/kg)

From: http://orise.orau.gov/reacts/guide/measure.htm#Conversions

So the answer to the question is neither.  A becquerel (Bq) is a measure of the radioactivity in counts per second of a given sample where one count represents the decay of one nucleus. It is an SI (Systeme Internationale) unit of measure related to the older, non-SI unit of Curies (Ci).  One Ci is equal to the activity of one gram of radium-226.  A sample must be normalized to the atomic mass of the isotope being measured.

The absorbed dose is what is actually important when determining if a radiation level is of concern or not.  But the radioactivity is the number that is most easily measured.  Different types of ionizing radiation (radiation that will change a neutral atom to an ion by removing an electron - similar to creating a free radical that can then damage cells and DNA and such) are absorbed at different levels.  Absorbed Dose is measured in joules per kilogram (grays [Gy]), where joules measure the energy absorbed from the radiation and kilograms is the measure of the mass of the absorbing medium (a person's tissue).  The effective dose is determined by multiplying a conversion factor,  a weighted average of absorption of different organs (heart, liver, skin, e.g.), times the amount of radiation exposure.  It is not a simple calculation nor is it exact.

So when the reporter talks about there being so many becquerels of radiation, he is reporting how many counts are being recorded.  It is an indication of how much radioactive material is present.  By also determining what isotopes of what elements are present, one can evaluate the relative risk to humans and other living things.

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Relative Risk of Power Generation Methods

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The nuclear disaster in Japan has focused the world's attention on the dangers of nuclear power generation.  Germany has temporarily shut down seven reactors, China has suspended approval of new facilities, and the usual hew and cry has arisen from the anti-nuclear energy crowd in the USA.  No one denies that when a nuclear reactor malfunctions that the risk of a large number of deaths is possible and that it is essential to assure that proper precautions be taken to prevent such disasters.  However, when assessing the dangers associated with nuclear power to determine if pursuing this form of power generation is worthwhile, one must compare the risks to those associated with other forms of power production such as coal, oil, natural gas, and hydroelectric.  One must consider not only the extent of a possible tragedy but also the chance of such a tragedy occurring to determine the full impact of using any particular source.  One must also consider the full life cycle of the power production rocess for each fuel type, fuel extraction, plant building, fuel transport and processing, power plant productive life time, decommissioning, and environmental impact of all of these phases.

According to a study in China, based on experiences in their study, coal-fired energy production has caused twelve times the number of deaths per annual GW as the nuclear energy chain.  By implementing known improvements that are needed, this ratio could be reduced to a 4X disadvantage for coal.  However, this does not include any nuclear accidents, since none have occurred in China to date.  This report also found that the radiation from coal, totaled over the entire product cycle, is nearly twice as high for coal as for nuclear power; again, this does not include estimates for nuclear accidents such as occurred at Three Mile Island, Chernobyl, or Fukushima.  As another means of comparison, a study by the Clean Air Task Force, attributes 13,200 deaths to coal plant emissions annually in the USA, this does not include deaths from any other portion of the energy production chain.

In 1975 China, a one-hundred-year flood event caused 30 dams built for hydroelectric power to fail, drowning at least 230,000 people.

The point of this information is to highlight the fact that no means of energy production is without risks.  Nuclear power generation seems riskier for two reasons: when there is an accident, it tends to be a disaster, killing and injuring a large number of people at one time and causing environmental damage over a large area; plus it is relatively new and there are many unknowns about it.  Nuclear disasters are blatantly obvious and the news media takes advantage of people's fear to raise their ratings and make the consequences seem even more dire than they already are.  Predictably people react in fear and want to stay with power sources they know.  Coal-powered plants are "silent" killers.  Except for the occasional mining disaster, one doesn't hear about the 13,000 annual deaths from breathing particulates emitted by coal-fired plants because they don't happen in bunches nor is it always obvious that coal is the culprit except to a doctor who performs an autopsy and sees the person's black lungs or measures radioactive isotopes charactistic of coal in a person's body.

Whether nuclear power should remain an option should be based on fact-based reasoning and decision making.  One must compare all costs associated with each means of power production from acquisition of the fuel until the plant is decomissioned and torn down or buried.  Not only the direct human impact, some of which has been highlighted here, but also the environmental impact to teh Earth as a whole and the other organisms that share this p[lanet with us.  Currently, fossil-fuel powered plants do not pay for the effect they have on the environment, there is little social cost assessed on carbon emission or the emission of other pollutants.  To make economically sound decisions for the long term, these costs must be included.  Making such an assessment is beyond the scope of today's post but will be the subject of one in the future.


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Some Radiation Information

Radiation Primer

Some revelant definitions and information:
A Sievert is a measure of radiation exposure, a millisievert is one-thousandth of a sievert, and a microsievert is one-millionth of a sievert.

Isotope: An atom with a particular neutron count in its nucleus. Whereas all the atoms of a given element will have the same number of protons present in the nucleus, the number of neutrons may vary. For example, hydrogen has one proton in its nucleus. It has three different isotopes: protium (most common form containing no neutrons), deuterium (one neutron), and tritium (two neutrons).

Iodine-131 and Cesium-137 are the most dangerous contaminants of concern so far. Iodine-131 is readily absorbed by the thyroid gland and can lead to thyroid cancer. By taking potassium iodide, one can saturate one's thyroid and effectively prevent the uptake. Since the half-life (the time it takes for half of the material to decay) of I-131 is only 8 days, this is an effective and practical method of reducing one's risk. In the case of Ce-137, however, the half-life is about 30 years and there is no effective means of reduction of risk other than physically preventing exposure. Cesium is incorporated in one's bones, primarily and can lead to bone cancer and leukemia.
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