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Radiation Exposure: From Bananas to Nuclear Reactors

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Have you ever considered bananas radioactive? If not then you may want to read on.

Few may think of radiation therapy as a natural remedy, akin to the likes of garlic, ginseng, and chicken soup. However, radioactivity levels in the primordial soup were high – humans have evolved alongside radiation for eons. Still today, radiation in low levels permeates our lives. Before considering its medical applications, it is useful to have a general sense of the various amounts of the radiation doses commonly around us every day.

First, radiation dose is most often characterized by one of two units of measure: the Gray, or the Sievert. These are units of measure no more complicated than feet, pounds, or watts. “Gray” is a unit of absorbed radiation dose, and is the common unit of measure in the therapeutic setting. One Gray equals one joule per kilogram. Of note, the term Gray is used for both singular and plural (like “deer” – one deer, two deer, three deer, not three “deers”… you get the idea). Other common terms for radiation in the therapeutic setting are the centigray (cGy), equal to 1/100th of one Gray, and its older sibling, the “Rad.” Rad and centigray are equivalent. In therapeutic radiation oncology, the operative dose range for killing cancer is 2 to 200 Gy – often split up into multiple treatments referred to as “fractions.”

The Sievert is a measure of absorbed equivalent dose – for the most common type of radiation, which is photon irradiation (rays of pure high energy), the Sievert is equivalent to the Gray. The Sievert is the common unit of measure when background and non-therapeutic radiation doses are measured and discussed.

Following the tsunami disaster and resultant nuclear tragedy in Japan, much has been in the news about radiation exposure, with a lot of media attention and concern as to what are safe levels of radiation. A colleague of mine recently pointed out a very helpful illustration of natural radiation dose exposures, therapeutic doses and doses associated with prior nuclear accidents: http://xkcd.com/radiation. Please take a look – the chart highlights a few radiation suprises.

The first surprise is that bananas are indeed radioactive. It is actually the potassium which is radioactive (potassium’s symbol is “K”). As it turns out, the most abundant forms of potassium, K39 and K41, are stable isotopes (not radioactive). However, K40, an isotope present in approximately 0.012% of total potassium concentration, is radioactive. Thus in every banana, and throughout our bodies, 0.01% of the potassium is radioactive, and generative beta rays (small negatively charged particles like electrons) with maximum energy of 1.33 MeV and gammay rays (high energy photons generated from within the potassium nucleus) of 1.46 MeV. On a yearly basis, the dose from potassium in the body is on the order of 390 uSv (microSieverts).

The second surprise may be that the typical dose received from living within fifty miles of a nuclear power plant for a year is just slightly less than eating one banana, and both are less than annual exposure living within 50 miles of a coal power plant. Many may be surprised that a coal plant typically releases more in terms of radioactive substances than a nuclear plant, but indeed, the combustion of coal is far less than a clean process. Keep in mind too that that most of our electricity in the United States is generated from coal.

Continuing through the chart, the three mile island accident led to a maximum external dose on the order of 1 mSv, which is in between a quarter and third of the normal yearly background dose. Although we do not know a minimum threshold below which radiation is completely safe, we generally think about safe levels of radiation falling within a few multiples of our yearly background radiation exposure (which is between 3 and 4 mSv) – as shown by the chart depicting the maximum yearly dose permitted for US radiation workers of 50 mSv. In mid-March 2011, daily readings at various sites up to 50km away from Fukushima were in the range of total annual background exposure – hence the US and Japanese government’s warnings for evacuation to beyond this periphery. For workers close to and within the reactors, I suspect that the radiation doses were much higher.

The maximum total body doses at which radiation can be immediately harmful or life threating are in the range of 0.4 to 8 Sv. In this range, the ability of the bone marrow to make infection fighting cells is compromised. Keep in mind that these doses are single exposures delivered to the whole body. We routinely use doses much higher than this as anti-cancer treatment – the key difference is that the targets are not nearly the whole body (and in the case the whole body needs to be treated, the treatment is fractionated).

Finally on the chart, one can appreciate the relative magnitudge of the Chernobyl accident – when a brief exposure imparted doses on the order of 50 Sv, which is immediately fatal due to cerebrovascular toxicity. Indeed, this accident was much worse than that of Three Mile Island or Fukushima.

Altogether, I hope that this chart and discussion help convey an understanding of how we live in the midst of low levels of radiation.


6 Responses to Radiation Exposure: From Bananas to Nuclear Reactors

  • ts says:

    So, stay away from banana splits and bananas with the morning cereal?

  • Dr West
    Dr West says:

    No, they can be therapeutic, but you have to regulate the dose properly.

  • ssflxl says:

    Dr. Loiselle,

    Thanks for your perspective on radiation in general. I always wonder if the dose you give for cancer treatment is weight based?

    ssflxl

  • Jazz says:

    Wow, who knew? Very interesting post, Dr. Loiselle. Not to be cheeky, but one can almost parlay this into cocktail party conversation (ah, the good old days…), albeit of the serious kind. I practically sunbathed in the shadow of the San Onofre Nuclear Power Plant for many years and didn’t think about it at all. The folly of youth.

    Jazz

  • Jazz says:

    Oh, and I have a bunch of antique dinnerware (from the 40’s and 50’s) made with uranium oxide glaze…wonder how much radiation I’ve ingested just from using it?

    Jazz

  • Dr Loiselle says:

    Wow, I don’t know anything about uranium oxide glaze.

    However, in regard to whether radiation treatment is weight based – the answer is more yes than no. More precisely, it is weight normalized. Radiation is dosed in units of energy per mass (Joules per Kg). Thus, in order to reach the same dose, targets that are larger or deeper require more energy in order to reach a certain prescription dose.

    Of note, in practical terms, it is more an issue of thickness than it is weight, though patients that weigh more tend to have more body thickness around tumors or lymph node regions at risk.
    CL

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