Six elements make up almost all of your body's weight
Oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus account for about 99 per cent of a human body's mass. Five more, potassium, sulfur, sodium, chlorine and magnesium, supply roughly 0.85 per cent. Everything else, a scattering of trace elements, adds up to less than 10 grams, lighter than the body's store of magnesium alone.
A typical 70 kilogram adult holds approximately 7×1027 atoms and at least detectable amounts of 60 elements. About 29 of those are thought to do something useful for health. Of the 94 elements that occur naturally, 76 have been found in people; for the remaining 18, nobody knows how many turn up.
Water explains the odd split between weight and atom count. The average adult is around 53 per cent water, though a large study found 48 per cent for women and 58 for men, give or take several points. Because oxygen atoms are heavy and hydrogen atoms light, oxygen dominates the body by mass while hydrogen outnumbers every other element atom for atom. Individual recipes also shift with build: people carrying more fat have proportionally more carbon and less of most other elements.
Not every trace element earns its place. Caesium and titanium seem to be harmless hitchhikers with no job, while cadmium, mercury and lead are toxic depending on the dose. Aluminium, the third most common element in Earth's crust, does nothing useful in living cells and can harm in large amounts. Fluorine's only known human role is hardening tooth enamel, and one study suggests bromine is needed to build a type of collagen. Silicon, boron, nickel and vanadium are probably required too, but in minute doses.
Counting cells gives yet another picture. Microbes living on and inside a healthy person are roughly as numerous as human cells, though they make up far less of the body's mass. Some are essential to health, and those that neither help nor harm are called commensals.
Source: Composition of the human body
