Radiation exists everywhere in the environment and has been around since before the Earth formed. We and all other living things are constantly exposed to radiation from both natural and human-made sources – from the natural decay or breaking down of radioactive elements in the Earth’s crust, from the air we breathe, from the sun and outer space, and from inside our bodies from food and water.
Radiation is measured in a variety of ways. One of the most important is the absorbed dose – the amount of radiation that deposits energy in our bodies. The International Standard unit for radiation is the millisievert (mSv). The average dose per person on Earth is 2.4 mSv1 and 1.8 mSv2 for the average Canadian. People are naturally exposed to lower or higher doses than these averages, depending on where they live and their lifestyles. For people who work with nuclear materials, an average dose of 20 mSv per year to a maximum of 50 mSv per year, and no more than 100 mSV in five consecutive years, are allowed under international standards and Canadian regulations.3
An average annual dose for a member of the public greater than 20 mSv is likely due to radiation from medical procedures. When prescribing a medical procedure involving radiation, medical professionals take into account that the potential health benefits from doing the procedure outweigh the risk from the radiation dose that the patient receives.
Since a millisievert is pretty abstract, another way to think about radiation dose is in comparison to common sources of radiation exposure. For example, one chest X-ray delivers a dose of 0.1 mSv – so the radiation dose for the average Canadian is the same as 18 chest X-rays. Another example of a dose from a common natural source of radiation is a ‘banana equivalent dose,’ the dose that a person could receive from eating one banana, or about 0.0001 mSv. Bananas are high in potassium, some of which will be the radioisotope potassium-40. However, since our bodies regulate the amount of potassium present (both radioactive and non-radioactive) there is a limit to how ‘equivalent’ the banana equivalent dose really is.
How is radiation measured? The international unit for measuring radiation exposure is the sievert (Sv). One Sv is equal to 1000 mSv. As well, 1Sv = 100 rems and 1 mSv = 100 mrems.
What if my total is greater than 20 mSv? If you received an annual dose higher than 20 mSv chances are that the dose is due to medical procedures. When prescribing a medical procedure that uses radiation, a medical professional takes into account that the potential health benefits of doing the procedure usually far outweigh the health risk that could come from the radiation. For more information, see the Patient Safety section of RadiologyInfo.org or speak to your physician on your next visit.
Primary sources for this information are National Council on Radiation Protection and Measurements Reports: #92 Public Radiation Exposure from Nuclear Power Generation in the United States (1987); #93 Ionizing Radiation Exposure of the Population of the United States (1987); #94 Exposure of the Population in the United States and Canada from Natural Background Radiation (1987); #95 Radiation Exposure of the U.S. population from Consumer Products and Miscellaneous Sources, (1987); #100 Exposure of the U.S. Population from Diagnostic Medical Radiation (1989); and #160 Ionizing Radiation Exposure of the Population of the United States (2009).
- United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) Report to the General Assembly (2000).
- R.L. Grasty and J.R. LaMarre (2004). Radiation Protection Dosimetry, Vol. 108, 3, 215-226, DOI: 10.1093/rpd/nch0222. Some values updated 2014, Grasty and Dalzell, pers. comm.
- Radiation Protection Regulations, section 13. http://laws-lois.justice.gc.ca/eng/regulations/SOR-2000-203/page-4.html#docCont .
- Canadian Nuclear Safety Commission (2013). Natural background radiation.
Adapted with permission from the American Nuclear Society ("ANS") Radiation Dose Chart.
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