How many atoms are in the human body?

A 70 kg human body contains roughly 7 × 10²⁷ atoms. By count, about 62% are hydrogen, 24% oxygen and 12% carbon; by mass, oxygen dominates because of water. The atoms are not permanent: most are exchanged over years through eating, breathing and excretion, and the heavier elements were made in stars before the Sun formed.

Source: Freitas (1999), Nanomedicine, Vol. I, Table 3.1