# Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

## Bananas

The Committed Effective Dose Equivalent for K-40 is 5.02*10^-9 Sv/Bq or 1.86*10^-2 rem per microcurie ingested. (For the uninitiated, recall that 1 Sv = 100 rem.)

Recall from my previous post that a banana contains about 10.92 Bq of radioactive K-40.

Run the numbers, and you find that the CEDE from eating one single banana is about 0.005 millirem.

These numbers can vary all over the shop, depending what your source is for the Potassium concentration, and the banana mass, but one thing is clear.

Living within 50 miles of a nuclear power plant in the US exposes you to an average dose of 0.09 millirem per annum, or 1.64 Banana Units.

Annual dose from living within 50 miles of a coal fired power plant: 0.03 millirem, or 5.45 Banana Units.

This “Banana Dose” is perhaps very useful in attempting to explain infinitesmal doses to members of the public.
(Interestingly, some of the anti-nuclear people just HATE this supposedly deceptive concept.)

Written by Luke Weston

July 25, 2007 at 3:26 pm

### 5 Responses

1. Isn’t 18 x 0.005 = 0.09 ? So that’s 18 bananas not 1.64.
and
Isn’t 6 x 0.005 = 0.03 ? That would be 6 bananas.
Neither is very much so i’m not disputing that.

Marc k

March 14, 2011 at 11:45 pm

2. Yes, the numbers are off. Also, note that the number for the nuke plant should be 0.009, not 0.09.

Gondring

March 15, 2011 at 3:16 am

3. I just posted a correction to this notion at http://theenergycollective.com/greenskeptic/53820/banana-your-pocket-or-are-you-radioactive?nocache=1#comment-11322 (where it’s currently awaiting moderation), and I’ve shared it elsewhere, too. The “banana equivalent dose” is wrong, as is the above coal-plant-exposure calculation.

It’s not exactly the math that’s wrong, it’s the physics. The calculation (or one of them) would be perfectly correct if the banana included that tiny amount of K-40, without being mixed with all the stable K, in terrestrial equilibrium. The concentration is identical to the mix in our bodies and all the rest of our food, and the resulting dose is zero, especially as our total K body burden is held constant.

The math for the radionuclides (uranium and daughters) in coal may be correct, but it’s dwarfed by the DECREASE in our radiation exposure from the coal-fired plant’s enormous emissions of 100% STABLE CO2, incorporating pure (stable) C-12, which changes the ratio of C-14/C-12 in our bodies.

Norman Rubin

March 18, 2011 at 9:15 pm

4. The whole problem with mickey mouse analysis is that you do not understand enough biology to know that your assumptions are totally wrong.

The body maintains a closely controlled level of potassium. Unless the individual is lacking in K he would not fix any of the K in the hypothetical banana.

If he was lacking a bit and so did absorb some he would not preferentially absorb the 0.0017% of K-40

While it is interesting to know how much (little) K-40 there is an a banana. the statement that you would “ingest” this amount of radioactive K-40 is simply WRONG .

bananarama

March 19, 2011 at 12:52 am

• Don’t like bananas? Then nuts!

I don’t mean you’re nuts, but rather that nuts can be used as a measure of equivalent dose too.

In terms of activity, 1 NEA (nut equivalent activity) is equal to 4 pCi.

In terms of dose, 1 NEM (nut equivalent man) is equal to 4 microrem.

That is, each brazil nut that you consume gives you 4 microrem of dose from exposure to Ra-224, which is not in biological homeostasis in the human body.

As mentioned above, living within 50 miles of a nuclear power plant gives you an exposure in a year that is less than what you get from eating 23 brazil nuts, or two nuts each month.

Brian Mays

March 19, 2011 at 3:27 pm