Where Do Our Atoms Go When We Die?

For the people we have lost: see their life through the eyes of the universe.

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When someone we love dies, a quiet question often follows the grief: where did they go? Physics cannot answer every version of that question. But it can answer one of them precisely, and the answer is more beautiful than most people expect. Every atom that made them is still here. None are destroyed. Over hours, years and centuries, they return to the air, the water and the soil, and become part of the living world again.

This is where those atoms go, and how we know.

A Person Is About 7 Billion Billion Billion Atoms

An adult body of about 70 kilograms contains roughly 7 × 10²⁷ atoms. Nearly all of them belong to a handful of elements. Counted by number, about 63% are hydrogen, 24% oxygen and 12% carbon, with nitrogen, calcium and phosphorus making up most of the rest.

Atoms are astonishingly durable. Chemistry, whether it is digestion, decay or fire, only rearranges them into new molecules. Turning one element into another takes nuclear reactions of the kind found in stars and reactors. So when a life ends, the atoms that made it simply begin their next journey.

That journey started long before death. A person takes in and loses about 2.5 litres of water a day from a body holding around 40 litres, so most of the water in us is replaced within a couple of months. Atoms have always been passing through us. Death changes the direction of the flow, not the fact of it.

The Water Goes First

About 60% of an adult body is water, and water is the fastest traveller. Whether through cremation or burial, it returns to the air and the ground and rejoins the water cycle. A molecule of water vapour stays in the atmosphere for about nine days on average before falling as rain or snow somewhere else on Earth.

So within weeks, water that was part of someone you loved has fallen as rain, run into rivers and been drawn up by roots.

After Cremation

Cremation typically works at around 760 to 980 °C (1,400 to 1,800 °F). At those temperatures, almost all of the body's carbon, hydrogen, oxygen and nitrogen combine with oxygen and leave as gases, mostly carbon dioxide and water vapour. Within a few hours, nearly all the atoms of a person are in the sky.

What remains is the mineral part of the bones, mainly calcium phosphate. A study of 100 adult cremations found the remains averaged about 2.4 kilograms, a little over 3% of a typical body weight.

The carbon dioxide doesn't stay in one place. Winds mix the air of the whole planet in about a year. Here is a rough calculation you can check. A person holds around 7 × 10²⁶ carbon atoms. The atmosphere holds about 900 billion tonnes of carbon. Once a person's carbon has spread evenly through the air, an ordinary breath, which contains around five billion billion molecules of carbon dioxide, carries something like tens of thousands of carbon atoms that were once part of them. That is true of every breath, taken by anyone, anywhere on Earth.

A carbon dioxide molecule typically stays in the air for about four years before a plant takes it in or the ocean absorbs it. So within a few years, the carbon of a cremated person becomes part of leaves and grasses, grain and fruit, plankton and coral, and from there part of the animals and people who eat them.

After Burial

Burial is slower and quieter. Microbes and soil life break the body's molecules down, releasing water, carbon dioxide and nutrients into the ground. Scientists who study this describe the soil around a decomposing body as a "cadaver decomposition island": a concentrated island of fertility, rich in nitrogen and other nutrients, where soil life flourishes. Roots find it. The atoms become plants, and plants feed everything else.

How quickly this happens depends on the soil, the climate and the burial itself. Embalming and sealed caskets slow it a great deal, while a natural burial in a simple shroud lets it happen sooner. Bones can last for centuries or much longer, slowly giving their calcium and phosphorus back to the earth.

We Are Already Mixed Together

None of this is unusual. It is how the living world has always worked. In his book Caesar's Last Breath, the science writer Sam Kean works through a famous calculation: because air mixes so thoroughly, the breath you just took very likely contains at least one molecule from the last breath Julius Caesar exhaled in 44 BC. The same logic applies to the atoms of everyone who has ever lived. We are built, in part, from atoms that were once part of other people, animals and plants.

The carbon of someone you lost may now be in a tree on another continent. Some of their water has fallen as rain over an ocean. A few of their atoms are very likely in you.

A Few Leave Earth Altogether

Earth is slowly leaking. High in the atmosphere, sunlight splits water vapour, and the lightest atoms, hydrogen, can escape into space. Earth loses roughly 3 kilograms of hydrogen every second this way. Over the centuries, a tiny fraction of a person's hydrogen makes that journey too, and leaves the planet entirely.

That hydrogen is the oldest part of us, made in the first minutes after the Big Bang. Some of it eventually goes back out into the universe it has been part of for 13.8 billion years.

In Billions of Years

The Sun is about halfway through its life. In roughly five billion years it will swell into a red giant, and later shed its outer layers into space. Whatever happens to Earth then, the atoms of this planet, including every atom that was ever part of a person, will still exist. Some may be swept into the gas clouds that form new stars and new worlds, just as the atoms in us were once part of stars that died before the Sun was born.

What This Means

Physics doesn't tell us whether a person's memories or self continue. That question lies beyond what atoms can answer. What it does tell us is that nothing they were made of is lost. It is in the rain, the trees, the air and the people who loved them. As the physicist's eulogy puts it, not a bit of them is gone. They are just less orderly.

Sources

Davidson Institute of Science Education, Weizmann Institute of Science: the elements of the human body counted by number of atoms (about 63% hydrogen, 24% oxygen, 12% carbon).

Freitas, R.A. (1999), Nanomedicine, Volume I: an estimate of about 7 × 10²⁷ atoms in a 70 kg human body.

van der Ent, R.J. & Tuinenburg, O.A. (2017), Hydrology and Earth System Sciences: "The residence time of water vapour in the atmosphere revisited." A global average of about nine days.

Cremation Association of North America: typical cremation temperatures of 1,400 to 1,800 °F.

Warren, M.W. & Maples, W.R. (1997), Journal of Forensic Sciences: "The anthropometry of contemporary commercial cremation." Remains from 100 adults averaged about 2.4 kg.

Jacob, D.J. (1999), Introduction to Atmospheric Chemistry, Princeton University Press: how quickly air mixes around the globe, including exchange between the hemispheres in about a year.

Friedlingstein, P. et al. (2023), Earth System Science Data: "Global Carbon Budget 2023." The amount of carbon held in the atmosphere.

IPCC (1990), First Assessment Report: an atmospheric turnover time for carbon dioxide molecules of about four years.

Carter, D.O., Yellowlees, D. & Tibbett, M. (2007), Naturwissenschaften: "Cadaver decomposition in terrestrial ecosystems." The cadaver decomposition island and its effect on soil life.

Kean, S. (2017), Caesar's Last Breath: the calculation of how much of Caesar's last breath is in each of ours.

Catling, D.C. & Zahnle, K.J. (2009), Scientific American: "The Planetary Air Leak." Earth's present loss of about 3 kg of hydrogen to space each second.