The Banana Dosage Scale
The banana scale is a way to put the radiation doses received from medical procedures into perspective, without needing knowledge of the units radiation is measured in. Click on the video to learn about the banana scale used for the ionising radiation parts of the website.
The banana scale on the right shows the amount of radiation you would receive from a specific treatment in bananas.
The green displays the daily background dose an average person receives. The blue is the radiation dose you would receive from a CT brain scan. The pink is the lowest (yearly) dose linked to an increased risk of cancer. Remember this scale is not linear! The end of the pink bar is 100 times bigger than the end of the blue bar!
How does it work?
Due to bananas having 0.5g of the radioactive isotope potassium-40, when ingested they give off around 0.1microSv.[1]
This is calculated by finding the dose given when potassium-40 is ingested by an average adult, which comes to about 5.01nSv/Bq, given we can then multiply this by the radioactivity of potassium (31 Bq/g) and then multiply that by the amount of the isotope in a banana (0.5g).
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The banana scale is then just a simple division:
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Where D is the dose received (from the treatment or a banana), although care should be taken with the units of the radiation.
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However, the banana scale is more of a simple way to portray radiation, in fact eating more bananas would not mean you had a cumulative radiation dose (ie, it doesn’t scale linearly) because the human body would remove excess potassium due to homeostatic controls.[2]
Nevertheless it is still a good way to represent the amount of radiation received from different procedures.
References:
[1] EPA: Dose Conversion Factors For Inhalation, Submersion and Ingestion. Available at https://www.epa.gov/sites/production/files/2015-05/documents/520-1-88-020.pdf (Page 156, table 2.2) [Accessed 01/02/17]
[2] EPA: Cancer Risk Coefficients for Environmental Exposure to Radionuclies. Available at https://www.epa.gov/sites/production/files/2015-05/documents/402-r-99-001.pdf (Page 16) [Accessed 01/02/17]
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