Plants take in atmospheric carbon dioxide by photosynthesis, and are ingested by animals.So, every living thing is constantly exchanging carbon-14 with its environment as long as it lives. The carbon in its body will remain until it decomposes or fossilizes.
All radioactive atoms eventually decay into something more stable, and carbon-14 decays into nitrogen.
Not only that, we top up our carbon-14 levels every time we eat.
And plants top up their radioactive carbon every time they turn carbon dioxide to food during photosynthesis.
At high geomagnetic latitudes, the carbon-14 spreads evenly throughout the atmosphere and reacts with oxygen to form carbon dioxide.
Carbon dioxide also permeates the oceans, dissolving in the water.