The human body is, in molecular terms, mostly water, mostly carbon-based organic compounds, mostly the same elements that compose every other multicellular organism on Earth. Among these elements, in concentrations that vary slightly by body size and muscle mass but average around 140 grams in a 70-kilogram adult, sits potassium — an alkali metal that is, among other things, essential for nerve transmission, muscle contraction (including the contractions of the heart), the maintenance of cell membrane potential, and the synthesis of most of the proteins the body manufactures. Approximately 0.0117 percent of all the potassium atoms in the human body, in the universe, and in the chemistry of every potassium-containing compound everywhere, are atoms of the unstable isotope potassium-40. This isotope decays spontaneously, with no external trigger, on a statistical timetable governed by quantum mechanics, releasing a small burst of energy as it transforms into either calcium-40 or argon-40. Every adult human contains approximately 16 to 17 milligrams of potassium-40. Every adult human is, by virtue of containing it, faintly radioactive — and has been since the chemistry of their own body first began to assemble, in utero, from the materials of their mother’s bloodstream.
According to a comprehensive reference summary of the physics of potassium-40 and its presence in the human body, the specific decay rate inside an average adult sits somewhere between 3,850 and 4,400 becquerels — meaning between 3,850 and 4,400 individual atoms of potassium-40 inside the body undergo spontaneous radioactive decay every second of every day for the entire duration of the person’s life. Approximately 89 percent of these decays produce a high-energy electron (a beta particle) and an antineutrino, with the parent potassium-40 atom transforming into stable calcium-40. Approximately 10 percent produce a gamma ray with an energy of 1.46 megaelectronvolts, along with the transformation of the parent atom into argon-40. The remaining fraction of decays produce variations of the same processes. Each of these events is, in the strict physical sense, a tiny nuclear explosion taking place inside human tissue. They are happening right now. They have been happening since you were a single cell. They will continue happening until your body’s potassium chemistry stops being maintained, which happens approximately at the moment of death.
Where the atoms came from
The potassium-40 atoms currently sitting inside your muscles were not manufactured by your body. The body does not have any biochemical pathway capable of synthesising new potassium atoms from other elements. The potassium-40 in your body today was absorbed from the food you ate over the past several months — primarily from leafy greens, bananas, potatoes, beans, and the broader range of potassium-rich foods that human diets include — which in turn extracted the potassium from soil through plant root systems, which in turn obtained the potassium from the weathering of rocks. The potassium atoms in those rocks have been present on Earth since the planet formed approximately 4.54 billion years ago. The potassium atoms in those rocks were created, in turn, by nuclear processes inside dying stars that exploded as supernovae before the solar system itself existed.
As reported by a Mirion overview of naturally-occurring radioactive material in the human body and the broader environment, the potassium-40 atoms specifically were produced by the same supernova nucleosynthesis processes that produced the heavier elements composing the rocky planets, the iron at the centre of the Earth, and the trace minerals essential to biology. The half-life of potassium-40 is 1.248 billion years, meaning that approximately half of the potassium-40 present in the solar system at the moment of its formation has already decayed away in the roughly 4.6 billion years since. The 16 milligrams of potassium-40 currently sitting in your body is a sample of the surviving fraction. Each individual atom is, in a literal sense, a fragment of a star that died before the planet you live on existed. Approximately 4,400 of those atoms are completing their final transformation right now, this second, somewhere inside the tissue of your body.
Why this is, in practice, harmless
The number 4,400 explosions per second sounds dramatic, particularly when described in the language of nuclear physics. The practical biological impact is, by every available measure, negligible. The annual radiation dose delivered by the body’s internal potassium-40 to the body itself is approximately 0.2 millisieverts — roughly the equivalent dose of two standard chest X-rays per year, and a small fraction of the typical 2.4 to 3 millisieverts of background radiation that every human receives annually from cosmic rays, terrestrial gamma radiation, radon gas, and other natural sources. The energy of each individual potassium-40 decay event is small. The body’s natural cellular repair mechanisms — which evolved over hundreds of millions of years in a constantly low-radiation environment — easily handle the damage produced by 4,400 decays per second.
Per an overview of the radiation-physics characteristics of potassium-40 in human tissue, the practical implication is that there is no realistic biological pathway by which the body could reduce its own potassium-40 content. Potassium-40 is chemically identical to stable potassium-39 (which composes 93.3 percent of natural potassium) and stable potassium-41 (the remaining 6.7 percent, minus the K-40 fraction). The body’s kidneys cannot distinguish radioactive potassium from stable potassium; they regulate total potassium concentration via homeostatic feedback, excreting excess and conserving deficit, but they treat all three isotopes identically. The ratio of K-40 to total potassium in the body is therefore essentially constant at the same 0.0117 percent that characterises potassium everywhere else. There is no diet, no medication, no behaviour available to any human being that could meaningfully reduce the internal radioactivity. The radiation is, in the most literal possible sense, part of being made of the same atoms as the rest of the planet.
The other internal radionuclides
Potassium-40 is the dominant contributor to internal radioactivity in the human body, but it is not the only one. As detailed in a comprehensive reference on the broader question of natural radioactivity in biological tissue, the human body also contains approximately 3,700 becquerels of carbon-14 — the same radioactive carbon isotope used in archaeological carbon dating — which is incorporated into essentially every organic molecule the body synthesises, from amino acids to fatty acids to the DNA in every cell. Carbon-14 has a half-life of 5,730 years, substantially shorter than potassium-40’s 1.25 billion years, and is continuously replenished in the biosphere by the action of cosmic rays striking nitrogen atoms in the upper atmosphere. Trace amounts of rubidium-87, lead-210, polonium-210, tritium, and a small list of other naturally-occurring radionuclides bring the total internal radioactivity of an average adult human body to approximately 7,000 to 8,000 becquerels — meaning that 7,000 to 8,000 individual atoms inside your body are undergoing spontaneous radioactive decay every second, every day, for as long as you are alive.
The result is that human biology, despite the everyday impression of solidity and stability, is actually a continuously transforming chemical process operating against a low-level background of constant internal nuclear activity. Atoms of potassium in your muscles are turning into atoms of calcium and argon. Atoms of carbon in your DNA are turning into atoms of nitrogen. The transformations are happening too slowly to be perceived, too gently to cause noticeable damage, and too thoroughly distributed across the body to be concentrated anywhere worrying. They are, however, real. They have been real since the chemistry of the human species first emerged on this planet, and they will remain real for as long as there are humans to host them. You are, by every available physical measure, a low-grade radiation source. You always have been. So is everyone you have ever known.
