Everyone is a source
You cannot avoid potassium 40. It makes up 0.0117 % of all natural potassium, the three isotopes are chemically identical, and potassium is essential to every cell you have.
A 70 kilogram adult carries about 140 grams of potassium, mostly inside cells. Of that, roughly 16 milligrams is potassium 40 — around 4,400 decays every second.
That is the largest single source of radiation inside a human body, ahead of carbon 14. The dose works out at about 0.17 millisievert a year against a natural background of 2 to 3.
⚠️ It is enough to be a practical nuisance. Sensitive radiation detectors have to be shielded from the potassium in the surrounding concrete, and an operator standing nearby registers on the instrument.
The banana problem
The banana equivalent dose is a popular way of explaining radiation: one banana is about 0.1 microsievert, a chest X-ray is 700 bananas, and so on.
It is a good illustration and a bad fact.
The body holds its potassium concentration within narrow limits. Eat more and the kidneys excrete the excess within hours. Your internal potassium 40 is set by your body mass, not by your diet.
⚠️ So eating a hundred bananas does not give you a hundred banana doses. It gives you the dose you already had, plus some fruit.
The comparison is still useful for scale — it makes an abstract microsievert concrete. It just should not be read as a mechanism.
The argon in the air
Potassium 40 decays two ways. About 89 % of the time it emits a beta particle and becomes calcium 40. The other 11 % capture an electron and become argon 40.
That minority branch filled the atmosphere.
Argon is 0.93 % of the air, which makes it by far the most abundant noble gas on Earth — a thousand times more common than neon, though neon is far more abundant in the universe. Almost all of it is argon 40, produced by potassium decay in crustal rocks over 4.5 billion years and leaked into the atmosphere.
⚠️ The isotopic signature gives it away. In the Sun, argon 36 dominates. On Earth, argon 40 is 99.6 % of the total. The difference is the record of four and a half billion years of potassium decaying underfoot.
Counting a person
Because potassium 40 emits a gamma ray in 11 % of its decays, and because that ray has a distinctive energy of 1.46 megaelectronvolts, a person can be measured from outside.
A whole-body counter is a shielded room with a detector, usually built from pre-war steel — steel smelted after 1945 carries traces of fallout that would swamp the signal. A subject sits inside for a few minutes and the instrument counts the gamma rays leaving their body.
The potassium 40 line is the reference peak. Since the body holds potassium in proportion to lean tissue, the count gives a measure of muscle mass, and the technique was used in physiology before body composition scanners existed.
⚠️ The same instrument detects anything else a person is carrying. Whole-body counters were how workers at Chernobyl and Fukushima were screened for internal caesium — measured against the potassium peak that everyone has.
Dating the rocks at Olduvai
The same decay is one of geology’s most important clocks.
When a mineral crystallises from magma, argon escapes — it is a gas and does not fit the crystal lattice. From that moment argon 40 produced by potassium decay is trapped. Measuring the two gives the time since the rock cooled.
The range runs from about 100,000 years to the age of the solar system, which covers most of geological history.
It is the method that dated the volcanic ash layers at Olduvai Gorge, and so established the age of the hominin remains found between them. Before it, palaeoanthropology had relative dates — this layer is older than that one — but no numbers.
⚠️ The method fails if the rock was reheated, because trapped argon escapes and the clock partly resets. That weakness became a strength: it lets geologists date later thermal events rather than only the original formation.