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The middle of your eye is made of carbon from the air of the year you were born

Between 1955 and 1963, nuclear bombs tested in the open air nearly doubled the carbon-14 in the atmosphere. Plants took it up through photosynthesis, and it reached people through food. Almost all the proteins in the core of the eye's lens form around birth and are never replaced, so they keep the level of those years. As a radiation dose, everything left from the tests was already over 400 times below natural radiation by the 1990s. To scientists it is a date written into the body.

Carbon-14 in the air, compared with the natural level (northern hemisphere)

Birth years (an example: change them to yours)

In class, add the teacher's year too.

A few points on the curve: 1955, +1.6%; 1964, +84.6%; 1975, +38.3%; 1990, +14.9%; 2012, +3.0%; 2024, −1.2%.

How the bomb got into the sky

Carbon-14 is made naturally high in the atmosphere, from nitrogen struck by cosmic rays. Thermonuclear explosions made it too, enough to nearly double the amount in the air of the northern hemisphere.

Between 1945 and 1980 there were 543 nuclear tests in the atmosphere; the busiest years were 1961 and 1962. The largest bomb, of 50 megatons, was set off by the Soviet Union in 1961. On 5 August 1963, in Moscow, the United States, Britain and the Soviet Union signed the treaty banning tests in the air, in space and under water. It came into force on 10 October. Underground tests remained allowed, and France and China did not sign.

In August 1963 the air of the northern hemisphere held 94% more carbon-14 than the natural level: nearly twice as much as before the bombs. The curve has fallen ever since, although France kept testing in the air until 1974 and China until 1980. In the first decades, bomb carbon moved from the air into the ocean, plants and soils faster than the remaining tests added it; later, carbon from fossil fuels became the main cause of the fall.

Radioactive decay is far too slow to explain the fall: half of any carbon-14 is gone only after about 5,700 years.

As radiation, the trace is small. By the reckoning of UNSCEAR, the UN committee on the effects of radiation, all the radioactive substances from atmospheric tests, not only carbon-14, gave each person on average 0.11 millisieverts a year in 1963, and less than 0.006 in the 1990s; of that smaller dose, carbon-14 gave 30%. Natural radiation gives 2.4 millisieverts a year on average.

Which year each part of you is made of

Plants take their carbon from the air, and we take ours from plants and from the animals that eat them. What the body builds follows the carbon-14 level of the air, with a delay through food: in blood, hair and nails, measured delays run from zero to three years. Red blood cells last about 115 days; the proteins in the lens core, a lifetime. In between, every tissue has its own pace.

Each row below shows, on the same axis of years, when the material a part of you is made of today was built, and how much carbon-14 it would hold on average, calculated from the published rates.

  • The core of the eye lens The proteins at the centre of the lens are made around birth and never replaced. Niels Lynnerup and colleagues (2008) estimated the birth years of 13 people in Copenhagen from them, with an average uncertainty of about a year and a half.
  • Neurons of the cerebral cortex They are as old as you. Neocortical neurons are made around birth; no new ones have been detected in adults (Bhardwaj and colleagues, 2006).
  • The core of the Achilles tendon In the healthy people studied (28, in Denmark), its collagen had been made in the first 17 years of life and was essentially not renewed afterwards (Heinemeier and colleagues, 2013).
  • Heart muscle cells Replaced very slowly: by the model of Olaf Bergmann and colleagues (2009), about 1% a year at 25 and 0.45% at 75, so fewer than half over a whole life. Another team reported much higher rates in 2012, but the journal retracted the paper in 2014; a carbon-14 study published in 2015 by the same Karolinska team found under 1% a year in adults.
  • Fat cells Their number is set during childhood and adolescence. In adults, about a tenth are replaced every year, at every age (Spalding and colleagues, 2008). The fat inside them changes faster: over a cell's roughly ten-year life, it is renewed six times (Arner and colleagues, 2011).
  • Liver cells Their average age is under three years (Heinke and colleagues, 2022).
  • Red blood cells They live about 115 days on average (Mock and colleagues, 2011), so they carry the carbon of the last few months' food.

The same shows elsewhere in the body. The enamel of each tooth is made at a different age in childhood, and forensic scientists have estimated birth years from it with an average error of 1.6 years. Collagen in knee cartilage is essentially not replaced once the skeleton has matured. The reserve of egg cells forms before birth.

The rows for the heart, fat and liver are our calculations from the published rates described above; the studies give averages for groups of people, not the value for any one person. The monthly series used here starts in 1941, so birth years start there too.

Does the adult brain make new neurons? Here the scientists disagree

In 2013 Kirsty Spalding and Jonas Frisén's team at the Karolinska Institute measured carbon-14 in the DNA of neurons in the hippocampus, the part of the brain tied to memory, in people who died at different ages. Their conclusion: a third of the neurons are renewed, at 1.75% a year, about 700 new neurons a day in each hippocampus, declining modestly with age.

In 2018 Shawn Sorrells and colleagues at the University of California, San Francisco found no young neurons in the hippocampus of adults aged 18 to 77. In 2022 a study by Franjic and colleagues found no genetic signature of new neurons in humans, though it did in mice, pigs and macaques. Other teams report the opposite: Boldrini in 2018, Moreno-Jiménez in 2019 and Zhou in 2022 found immature neurons in adults, and in 2025 Dumitru and colleagues showed, in Science, dividing progenitor cells in the adult hippocampus.

The kinds of evidence differ: carbon-14 in DNA, markers of young neurons, active genes, dividing cells. The 2025 study found dividing progenitors; how many mature neurons they produce, if any, is still open. The figure of 700 a day comes from one lab and one model. For the person chosen above, the two possibilities give:

The bomb clock is losing its precision

Around 2020 the air measured at Jungfraujoch, in the Swiss Alps, fell below the natural level of carbon-14, and by 2024 it had reached −1.2%. The bomb carbon has gone into the ocean and into plants. In its place comes carbon from coal, oil and gas, so old that it holds no carbon-14 at all.

Heather Graven of Imperial College London worked out in 2015 what comes next. After about 2030, a raised level of carbon-14 will no longer show that something is recent. If emissions keep rising, by 2050 a fresh plant could hold as much carbon-14 as a sample from the year 1050. If they fall fast, the air would stay near the natural level through 2100, and the change from one year to the next could become too small to measure within two or three decades. So the clock the bombs started loses its precision within the lifetime of today's students; radiocarbon dating of old objects goes on.

In Romania, carbon-14 is measured with the RoAMS accelerator at Măgurele, running since 2012 for radiocarbon dating and also used for other isotopes.

In class: three questions for one lesson

  1. Physics. Enter the teacher's year and a student's. Which dot is higher, and where does each fall relative to the 1963–1964 peak? Can radioactive decay explain the fall of the curve after 1963? (No: the half-life is about 5,700 years.)
  2. Biology. Pick each person in turn. In the model, the grandparent and the student eat the same food today. Why do their red blood cells hold the same carbon-14, while their lenses do not? Which organ could give the age of an unknown person?
  3. History. Find the 1963 treaty on the curve. France and China did not sign and kept testing bombs in the air. Why did the curve fall anyway?

The years in the boxes stay in the page's address, so a copied link opens the same family on another screen.

Anyone born in 1963 or 1964 has a lens core made from the carbon of the air with the most carbon-14 since measurements began. If you know someone born then, look at them a little more closely next time.

More on the site

Sources and method

The curve from 1941 to 2018 is the monthly northern-hemisphere zone 1 series published by Quan Hua and colleagues in 2022. We use it for Romania, at 45° north, inferring the zone from nearby stations in it (Jungfraujoch, 46.5°N); before the mid-1950s, when direct air sampling began, the series rests on tree rings. From 2019 to 2024 we use the ICOS network's annual means at Jungfraujoch, as given in table 2 of the study by Thomas Laemmel and colleagues, not yet peer-reviewed; the authors' own series runs up to 3.6‰ lower. For 2025 and 2026 we keep the 2024 value.

The percentages are Δ14C divided by ten: the departure from the reference level used in radiocarbon dating, which corresponds to natural pre-industrial air. In 1950, before the bombs, the air was already 2.6% below it, because of fossil-fuel burning, mostly coal at the time.

For each body part, the value is the average of the curve over the years its material was made: at birth for the lens and cortex, the first 17 years for the tendon, the last 115 days for blood, the last three years for the liver. For the heart we apply Bergmann's (2009) rates from age 20 and no replacement before; for fat, even growth to age 20 and then 10% replacement a year; for the hippocampus, the adult rate of 1.75% a year from age 20. These are our assumptions, not measurements. We do not add the delay through food.