The becquerel — why your body fires thousands of decays every second

Hold a sensitive scintillation counter to a ripe, yellow banana. Against intuition, the device does not stay silent — every few seconds it registers a quiet, steady click. Now press the same detector to your own chest. The rhythm surges, the clicks blur into a burst of crackles. Right now, this very second, with no contamination, failure, or disaster, your body is firing thousands of invisible particles that pass freely through the air around you.

We live in a constant, though imperceptible, nuclear hum that accompanies us from birth to death. This subtle dance of atomic decays is a natural feature of the matter that builds the universe. To stop fearing it, you have to look at the unit that counts it — the becquerel. It measures the pure dynamics of the nucleus, though, as we will see, its value alone does not answer the most important question: are we dealing with something safe, or with a real threat?

The becquerel — a metronome for the microworld

In the International System of Units (SI), the intensity of a radioactive source is described by the becquerel, symbol Bq. The definition is extremely simple: one becquerel is on average one nuclear transformation per second in a given amount of material (1 Bq = 1 s⁻¹).

It is worth distinguishing it right away from the hertz, which has the identical dimension (also 1/s). The hertz describes strictly periodic phenomena — a vibrating string, a processor's clock, an electromagnetic wave. The becquerel describes stochastic processes: decays are random and unsynchronized, and "on average one per second" is a statistic, not the regular ticking of a clock.

Because a single atom is unimaginably small, one decay per second is an activity almost imperceptible on a macroscopic scale. That is why in medicine, industry, and dosimetry we use multiples: kilobecquerels (kBq = 10³ Bq), megabecquerels (MBq = 10⁶ Bq), gigabecquerels (GBq = 10⁹ Bq), and even terabecquerels (TBq = 10¹²) and petabecquerels (PBq = 10¹⁵).

The name honors the French physicist Antoine Henri Becquerel, author of one of the most consequential accidental discoveries in the history of science. In February 1896 he was investigating whether uranium salts, which show natural phosphorescence, emit penetrating radiation after being exposed to sunlight. But clouds settled over Paris. Unable to expose his samples, Becquerel put photographic plates wrapped in black paper into a dark drawer together with the uranium salt crystals. When, a few days later, on March 1, he developed the plates — expecting at most a faint outline — he saw sharp, deep shadows of the crystals. Uranium was radiating continuously, without any external stimulation. The phenomenon, which Marie Skłodowska-Curie later named radioactivity, earned Becquerel the 1903 Nobel Prize in Physics, shared with the Curies.

The curie, a gram of radium, and Marie Curie's stubbornness

Before the becquerel became the standard, the basic unit of activity for half a century was the curie (Ci), introduced in 1910 at a congress in Brussels where the pioneers of the new science met — with Ernest Rutherford and Marie Skłodowska-Curie at the forefront. It was defined as the activity of radon in equilibrium with exactly one gram of pure radium-226. Modern measurements put that value at 3.7×10¹⁰ decays per second, that is 37 GBq — and that is exactly what our activity converter uses when you convert curies to becquerels.

Behind such a large unit lies a fascinating dispute. Part of the committee proposed that the curie correspond to a much smaller, more practical activity — on the order of ten nanograms of radium. Marie Skłodowska-Curie firmly opposed it. She judged that attaching such a microscopic value to the name of her tragically deceased husband, Pierre (killed in an accident in 1906), would diminish his memory. She insisted on a full gram of radium — an amount astronomical for the time. She got her way, and as a result medical physicists later had to work in millicuries (mCi) and microcuries (μCi) to describe sources safe for humans.

The Polish thread of this story is strong. Polonium, discovered by the Curies in July 1898, was named after Poland, which at the time did not exist on maps, being under partition — Marie believed that publicity around the discovery would draw the world's attention to her homeland's fate. The isolation of radium itself was a testament to persistence: for four years, in a leaky shed on rue Lhomond, she stirred by hand great vats of tons of waste from the Jáchymov mine to obtain barely one-tenth of a gram of pure radium chloride. In 1911 she personally prepared the first international radium standard — an ampoule with just under 22 milligrams of radium chloride, deposited at the International Bureau of Weights and Measures near Paris.

The four faces of radiation

In public discourse "radiation" and "dose" are often treated as synonyms — yet these are four different quantities worth separating:

• Activity (becquerel, Bq) — how fast the source itself decays. It says nothing about how much energy reaches the surroundings.
• Absorbed dose (gray, Gy) — the energy transferred to a kilogram of matter, 1 Gy = 1 J/kg. A purely physical quantity, covered in a separate piece on the gray.
• Equivalent and effective dose (sievert, Sv) — the real biological risk. It accounts for the fact that an alpha particle damages a living cell about twenty times more effectively than a gamma ray of the same energy; we develop this in the piece on the sievert.
• Exposure (formerly the roentgen, R) — the ability of radiation to ionize air.

This article is only about activity. The becquerel counts events at the source — the other units describe what radiation does along the way and in tissue.

The half-life paradox

One of the least intuitive aspects of nuclear physics is the link between activity and half-life (T½) — the time in which the number of atoms of an isotope drops by half. It is governed by the decay constant λ, inversely proportional to T½: λ = ln 2 / T½. The activity of a sample (A) is in turn the product of that constant and the number of unstable atoms N: A = λ·N = (ln 2 · N) / T½.

From this follows a surprising paradox: the shorter the half-life, the greater the activity of the same mass of an element. Long-lived atoms are slow and release radiation rarely; short-lived ones behave like an avalanche, dumping their energy in a fraction of the time. Take three samples of exactly one gram each:

The same gram of matter — from a few tens of thousands to over four quadrillion decays per second. The difference spans eleven orders of magnitude and depends solely on the rate of decay.

The scale of activity — from a single atom to a sterilizer

To get a feel for the orders of magnitude nuclear physicists work in, let us line up typical sources on one axis:

The span is colossal: fourteen orders of magnitude separate the banana from the sterilizer. And it is precisely this span that leads to two myths worth disarming.

Myth one: a big becquerel count always means death

Big numbers intimidate. "This object has an activity of thirty thousand becquerels" sounds ominous — and it is a deep misunderstanding. The number of becquerels tells you only how many decays happen per second. It says nothing about whether the released energy has any physical chance of harming you.

The best counterexample is the home smoke detector. Each contains a trace of americium-241 with an activity of about 33–37 kBq. Thirty-seven thousand decays per second — and yet as long as the detector hangs untouched on the ceiling, the dose the household receives is zero. Americium-241 decays by emitting alpha particles. They are heavy, strongly charged, and interact with matter so violently that they lose all their energy over a few centimeters of air. A thin foil inside the detector stops them, and even if they escaped, they could not pierce the hardened, dead outer skin.

Everything changes with internal contamination. If americium dust were inhaled or swallowed, there is no skin barrier inside the body: alpha particles would strike the unshielded cells of the lungs or digestive tract directly, and soluble americium also deposits in the bones, where it destroys the marrow. The danger of a source therefore depends not on the absolute number of becquerels, but on the geometry of exposure, the type of radiation, and whether the substance entered the body. This is exactly the distinction the sievert captures and the becquerel, by definition, cannot see.

Myth two: the human body is not radioactive

The belief that a person is "free" of radioactivity is common and completely wrong. In the body of an adult weighing 70 kg, between 4000 and 8000 atomic nuclei decay every second — from isotopes we take in with food, water, and air.

The lead actor is potassium-40 (⁴⁰K). Potassium is a key electrolyte, essential for conducting nerve impulses and contracting muscles, including the heart; an adult carries about 140 grams of it. A constant, cosmologically fixed fraction of potassium (0.0117%) exists as unstable potassium-40, formed even before the Solar System took shape. That means about 16 milligrams of radioactive ⁴⁰K in our tissues, with an activity on the order of 4000–5000 Bq. As it decays, it emits beta particles and penetrating gamma rays of 1.46 MeV that easily leave the body. The second source is carbon-14 (¹⁴C), giving about 3000–3700 beta decays per second; it forms in the atmosphere under cosmic radiation and reaches us through plants.

Here comes the "banana equivalent dose" (BED): bananas are rich in potassium, and one fruit (~150 g) has an activity of about 15 Bq. Many people fear that eating bananas raises the body's radioactivity. Not true — and the reason is homeostasis. The body keeps potassium at a strict equilibrium: the kidneys capture any excess almost immediately and excrete it in urine within hours. Eating a banana therefore does not raise your annual dose, because the extra 15 Bq quickly leaves the body. (BED is, incidentally, a concept from the world of sieverts, not becquerels — more on it in the piece on the sievert.)

A lesson from Chernobyl: why the limits are about caesium, not potassium

Natural potassium-40 is subject to homeostasis. Artificial isotopes released in the 1986 Chernobyl disaster behave entirely differently — above all caesium-137 and strontium-90. The body cannot tell them apart from their stable, natural counterparts and has no mechanism to protect against their accumulation:

• Caesium-137 (T½ ≈ 30 years) chemically resembles potassium, so after eating contaminated food it embeds itself in the muscles. It stays there for months (its biological half-life in adults is about 110 days), continuously irradiating the tissues.
• Strontium-90 (T½ ≈ 28.8 years) is a chemical twin of calcium and deposits in bones and teeth, from which it is very hard to remove. It can lodge there for years, destroying the marrow and raising the risk of leukemia.

In the spring of 1986 a radioactive cloud passed over Poland. In some regions grass contamination reached the order of 10⁵ Bq/kg, and fresh milk near Lublin — up to several thousand Bq/l. The authorities then gave Lugol's solution (stable potassium iodide) to over 18 million citizens, mainly children, to block the thyroid from absorbing radioactive iodine-131, and introduced the first intervention limits for food on the order of 1000 Bq/kg.

Today the European Union applies strict import limits (including Regulation 2020/1158). The permitted activity of caesium-137 in products from areas affected by the Chernobyl fallout is 370 Bq/kg for milk, dairy, and infant food, and 600 Bq/kg for other foodstuffs, including wild mushrooms and berries. These thresholds protect against the stochastic effects of radiation, for which science knows no safe threshold dose.

The becquerel counts, the sievert decides

The becquerel is a precise, useful tool — an objective counter that registers every beat of matter's nuclear heart. In itself, though, it carries no information about how that radiation will affect health. Thirty-seven thousand decays in a smoke detector and thousands of decays in your body are entirely harmless; the same number of becquerels in a soluble isotope inside the body can be dangerous.

An honest assessment of the threat requires moving from the physics of the source (becquerels) to the biology of the tissue (sieverts), taking into account the type of radiation and the sensitivity of organs. It is precisely the equivalent and effective dose that hold the key to radiological safety — and to them we devoted a separate article on the sievert. The becquerel tells you how loudly the nuclear pulse beats. Only the sievert tells you whether it is worth worrying about.

Further reading

• Andrzej Czerwiński, Blaski i cienie promieniotwórczości (WSiP, Warsaw 1995) — an accessible introduction to the basics of radiochemistry.
• Andrzej Czerwiński, Energia jądrowa i promieniotwórczość (Oficyna Edukacyjna, Warsaw 1998) — a classic textbook of nuclear reactions.
• Józef Hurwic, Maria Skłodowska-Curie i promieniotwórczość (Żak, Warsaw 1993) — a monograph on the beginnings of nuclear physics.
• Ewa Curie, Madame Curie (1937) — a warm biography of the Nobel laureate written by her younger daughter.