Carbon-14 content of the body is based on the fact that one 14C atom exists in nature for every 1,000,000,000,000 12C atoms in living material.
Do we have carbon-14 in our body?
The carbon-14 atoms that cosmic rays create combine with oxygen to form carbon dioxide, which plants absorb naturally and incorporate into plant fibers by photosynthesis. At this moment, your body has a certain percentage of carbon-14 atoms in it, and all living plants and animals have the same percentage.
Does carbon-14 decay in living things?
Radiocarbon decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food. Once the organism dies, however, it ceases to absorb carbon-14, so that the amount of the radiocarbon in its tissues steadily decreases.