Tests indicate that the earth has still not reached equilibrium. There is more C-14 in the atmosphere now than there was 40 years ago. This also means that plants and animals that lived in the past had less C-14 in them than do plants and animals today. Just this one fact totally upsets data obtained by C-14 dating. Plants breathe CO2 and make it part of their tissue.
Animals eat the plants and make it part of their tissues.
A very small percentage of the carbon plants take in is radioactive C-14. When a plant or animal dies it stops taking in air and food so it should not be able to get any new C-14. The C-14 in the plant or animal will begin to decay back to normal nitrogen. The older an object is, the less carbon-14 it contains. One gram of carbon from living plant material causes a Geiger counter to click 16 times per minute as the C-14 decays.