02/02/2021
It’s all about context
People tend to panic about unfamiliar and dangerous sounding components in food and medicine. Even worse is when you find out there’s something you’ve trusted forever and, perhaps, it was a glowing beacon of disaster all along.
Today’s Moment of Science… the radioactive banana.
You may recall what an isotope is from high school chemistry, but let’s take that journey together. If you look at the periodic table, you see a few numbers listed for each element. Atomic number reflects the number of protons (positively charged particles) in an atom. Then there’s atomic mass, which is the combined mass of protons and neutrons (neutrally charged particles). This gets a little trickier, because though the number of protons in an element is static, it can have a variable number of neutrons. So the number you see on the periodic table is for an average sampling of the stuff. For example, if you stumble upon a potassium atom in the wild, most of the time it has nineteen protons and twenty neutrons.
Then, every hundredth of a percent of the time or so, you get an as***le potassium atom with twenty-one neutrons.
This is called potassium-40, and that extra neutron makes it just a smidge radioactive.
Over a half-life of 1.3 billion years, potassium-40 undergoes a couple types of radioactive decay, forming either calcium-40 or argon-40. Because of the known steady rate of decay, measuring the amount of argon is a fairly reliable way to estimate the age of mineral formations containing potassium. This is known as potassium-argon dating.
But about those naturally delicious little pods of carbs, electrolytes, and radiation.
It’s not like potassium-40 is segregated from the less temperamental stuff out in nature, so your early pandemic banana bread attempts were indeed radioactive. Trucks transporting bananas are even known to set off false alarms for radiation monitors at ports of entry. So what does that mean for you as you’re nervously looking around for lead shielding to throw on your banana pudding?
We use the term ‘banana equivalent dose’ (BED) to describe the radiation dose you get from eating an average sized banana. Technically there’s some radiation from carbon-14 decay as well, but the lion’s share is from potassium-40. We measure the amount of ionizing radiation we absorb in Sieverts, and one banana is estimated to dole out 0.1 microsieverts (μSv) of radiation.
For context, you typically get a dose of about 10 μSv daily by just existing on this planet. A chest x-ray blasts you with about 20 μSv. A mammogram is significantly higher at 400 μSv, or 0.4 millisieverts (mSv).
The lowest annual dose linked to increased cancer risk is 100 mSv, and two whole sieverts all at once can goddamn kill you.
Your yearly natural dose of radiation from potassium is 390 μSv.
The point of having this measure isn’t to make you wary of bananas or potassium. You need potassium, and avoiding bananas won’t change the radioactivity level from the potassium you need to stay healthy. The point of the term is to help us understand our chemical world is complicated, many things that we need to stay healthy can also hurt us, and the dose makes the poison.
This has been your daily Moment of Science, asking you to mind your fiber intake.
Chart source: XKCD https://xkcd.com/radiation/
