ONE WILD WORLD — NEWSLETTER

Issue #019  |  Useless But Fascinating  |  May 2026

Right now, roughly 5,000 atoms inside your body are decaying every single second. You emit gamma rays. You contain measurable amounts of uranium, thorium, and a radioactive form of potassium that physicists use as a calibration reference. Eat a banana and you become measurably more radioactive for several hours. Cuddle a partner overnight and you irradiate each other. None of this is dangerous. All of it is true.

BY THE NUMBERS

Most people picture radiation as something that happens elsewhere — at Chernobyl, in nuclear submarines, in airport scanners. The truth is rather more intimate. Every human body contains a small, slow, harmless reactor. It is running right now, and it has been running for as long as you have been alive.

The fuel is mostly potassium. Potassium is essential for muscle contraction, nerve signalling, and heart function. The average adult body contains roughly 140 grams of it. Of that 140 grams, a tiny fraction — 0.012%, or about 17 milligrams — is a naturally occurring radioactive isotope called potassium-40. It has been here since the Earth formed. It is in every banana, every avocado, every bag of salt substitute, every cell in your body.

Potassium-40 has a half-life of 1.25 billion years, which sounds reassuringly stable. But within those 17 milligrams, there are an enormous number of atoms — and a small percentage of them tick over every second. The result: roughly 4,400 potassium-40 atoms inside you decay per second. Each decay releases a tiny burst of energy, sometimes as a beta particle (a fast electron), sometimes as a gamma ray that can pass straight through your body and out the other side.

Add in carbon-14 (another natural radioisotope, the same one used in carbon dating), trace amounts of uranium and thorium absorbed from food and water, and a sprinkling of radium and polonium that lodges in your bones, and the total comes to somewhere around 5,000 to 7,000 decays per second. Every second of your life.

Every banana contains about 450 milligrams of potassium — including, by the same fixed natural ratio, a tiny bit of potassium-40. Eat a banana and you have just absorbed a measurable dose of radiation. The dose is around 0.1 microsieverts: about one ten-thousandth of what you would get from a single chest X-ray, and several thousand times less than the annual dose your own body produces from the inside.

This is so reliable that radiation physicists, half-jokingly, use the "Banana Equivalent Dose" as an informal unit. A transatlantic flight is about 400 bananas. A chest X-ray is around 200 bananas. Living within 80 kilometres of a nuclear power plant for an entire year, according to US regulatory data, is roughly the same dose as one banana. The unit is not used in serious health-physics papers, but it has become a useful way of showing that very small radiation doses are everywhere, and that the public's intuitive fear of any radiation at all is wildly miscalibrated.

Bananas don't actually accumulate in you, by the way. Your body maintains potassium at a fixed concentration. Every banana's worth of extra potassium-40 is excreted within hours. You glow slightly brighter for an afternoon, then return to baseline.

If bananas were genuinely radioactive in any meaningful sense, every grocery store in the world would be a Superfund site.

This is where the story turns properly strange. Because you are a continuous low-level emitter of gamma radiation, anyone who stands next to you for long enough is being irradiated by you. And vice versa.

The dose is, of course, miniscule. Health physicists have done the maths. Sleeping next to another adult for eight hours a night, every night for a year, exposes you to an additional dose of roughly 0.5 microsieverts annually — about five extra bananas' worth. Sitting next to a stranger on a long-haul flight gives you a similar background nudge, mostly drowned out by the much larger dose from cosmic rays at 35,000 feet.

The body that produces the most measurable signal in this kind of exposure is, perhaps unsurprisingly, the largest one. A 100-kilogram person emits roughly twice the gamma flux of a 50-kilogram person, simply because they contain twice as much potassium-40. Anyone who has ever felt mildly unsettled by a much larger seatmate on the bus is being slightly irradiated more than usual. This is a deeply unhelpful fact and you should not bring it up at parties.

The total annual radiation dose from a typical human body's own internal radioactivity is around 0.39 millisieverts per year, or roughly 13% of the average global background dose of 3 millisieverts. Background radiation comes from cosmic rays, from radon gas seeping out of the ground, from soil and rock, and from the food and water you consume. The internal contribution is the part you carry around with you wherever you go.

To put 3 millisieverts in perspective: it is roughly the dose you would receive from one CT scan, or from working full-time as an airline pilot for about a year, or from living in a particularly rocky part of Cornwall. The acute radiation sickness threshold is around 1,000 millisieverts in a single dose — about 330 years' worth of background radiation absorbed at once. The lethal dose without medical treatment is roughly double that. Your body's own internal dose is so far below any health-relevant threshold that it has, almost certainly, no measurable effect on you whatsoever.

And it has been like this for as long as multicellular life has existed. Every animal that has ever lived has spent its entire life inside a slow drizzle of internal radiation. Evolution has had several billion years to deal with it, and the cellular machinery for repairing radiation damage is astonishingly good. The damage rate from your potassium-40 is dwarfed many times over by the damage rate from ordinary cellular metabolism — your own mitochondria do far more genetic harm to you every day than your radioactive potassium ever will.

You contain the same isotopes the Earth was forged with. You carry them around for eighty years. They give you the energy to think and move. And then they go back into the ground, exactly as they came.

There is one last detail worth knowing. Almost every radioactive atom inside you — every potassium-40, every uranium nucleus, every thorium trace — was created in the violent death of a star, billions of years before the Sun was born. Heavy radioactive elements are forged in supernovae and in collisions between neutron stars. They drift through space for a few hundred million years, get incorporated into the dust cloud that becomes a new solar system, and end up in the rocks of a young planet.

Eventually, they get pulled up by plants, eaten by animals, and absorbed into a primate that is reading a newsletter on its phone. The decays you are emitting right now are, in a very literal sense, the last whispers of stars that died before there was a Sun. They will keep ticking for billions of years after you are gone.

You are not just made of stardust. You are still slightly burning.

5,000 decays a second. A milligram of star-forged potassium in every cell. A banana that nudges your dose for an afternoon. A partner who slightly irradiates you in the night. A body that is, in the most literal sense, a quiet, harmless, ancient nuclear reactor.

You're not just radioactive. You're radioactive on purpose, by design, and you wouldn't be alive without it.

SOURCES

US Environmental Protection Agency — "Radiation Sources and Doses", epa.gov/radiation

Oak Ridge Associated Universities — "Radiation and Risk" educational materials, orau.org

Centers for Disease Control and Prevention — "Radiation in Healthcare and Everyday Life", cdc.gov/nceh/radiation

Health Physics Society — "Radiation Doses in Perspective" (position paper, updated 2022)

American Nuclear Society — "Banana Equivalent Dose" educational handout, ans.org

International Commission on Radiological Protection — Publication 119 (2012), reference values for human radioactive content

United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) — Sources and Effects of Ionizing Radiation, 2020/2021 Report