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About 99% of the mass of the human body comes from just six elements: oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus. Another five make up most of the remaining 1%, and a couple of dozen more are present in tiny but essential amounts.
The big six
Approximate percentages by mass for an adult body. Values vary between people and sources.
| Element | % of body mass | Main roles |
|---|---|---|
| Oxygen | ~65% | Water; almost every biological molecule; respiration |
| Carbon | ~18% | The backbone of proteins, fats, carbohydrates and DNA |
| Hydrogen | ~10% | Water and all organic molecules |
| Nitrogen | ~3% | Proteins (amino acids) and DNA/RNA bases |
| Calcium | ~1.5% | Bones and teeth; muscle contraction; nerve signals |
| Phosphorus | ~1% | Bones; DNA backbone; ATP, the cell’s energy currency |
Why so much oxygen? Because you’re roughly 60% water, and oxygen makes up about 89% of water’s mass.
Mostly oxygen by mass, mostly hydrogen by atoms
Here’s a nice twist. By number of atoms, hydrogen wins by a mile — roughly 60% of all the atoms in your body are hydrogen. Oxygen is about 25%, carbon about 12%. Hydrogen atoms are so light that, despite their numbers, they add up to only a tenth of your mass. (The same distinction between mass percent and atom percent appears in percent composition.)
The next five (about 0.85% together)
| Element | Approximate % by mass | Roles |
|---|---|---|
| Potassium | ~0.4% | Nerve impulses; fluid balance inside cells |
| Sulfur | ~0.3% | Some amino acids (cysteine, methionine); keratin in hair and nails |
| Sodium | ~0.2% | Nerve impulses; fluid balance outside cells |
| Chlorine | ~0.2% | As chloride: fluid balance; stomach acid (HCl) |
| Magnesium | ~0.05% | Hundreds of enzymes; bones; muscle and nerve function |
Sodium, potassium and chloride are the body’s main electrolytes. Nerve cells work by pumping sodium and potassium ions across their membranes — see what is an ion.
Trace elements: tiny amounts, big jobs
These are present in amounts from grams down to milligrams, but you can’t live without them:
- Iron (~4 g in total) — in haemoglobin, which carries oxygen in your blood. Iron deficiency causes anaemia.
- Zinc — needed by more than 300 enzymes and for immune function.
- Copper — enzymes involved in energy production and iron metabolism.
- Iodine — in thyroid hormones, which regulate metabolism. Iodine deficiency causes goitre, which is why many countries add iodine to table salt.
- Selenium — antioxidant enzymes.
- Manganese — enzymes involved in metabolism and bone formation.
- Cobalt — at the centre of vitamin B₁₂.
- Molybdenum — a handful of enzymes.
- Fluorine — as fluoride, strengthens tooth enamel.
- Chromium — has been linked to insulin function, though its essential role is debated.
Some other elements are present in the body without any known useful function — including traces of lead, mercury and arsenic picked up from the environment. Whether an element is harmful depends heavily on the amount: selenium, for instance, is essential in micrograms and toxic in larger doses.
Why these elements?
Life is built mainly from light elements that are common in the universe and form strong, versatile covalent bonds. Carbon in particular can form four bonds and long chains, making it uniquely suited to building the huge variety of molecules life needs. Oxygen, hydrogen and nitrogen are among the most abundant elements in the cosmos. See the most abundant elements in the universe — nearly every atom in you, apart from hydrogen, was made in a star.
Quick answers
What is the most common element in the human body? Oxygen by mass (~65%), hydrogen by number of atoms (~60%).
How much of the body is metal? By mass, only a few percent — mostly calcium, potassium, sodium and magnesium.
How much iron is in a person? Around 3–5 grams in a typical adult — about enough to make a small nail.
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