Cosmic rays interacting with atmospheric Nitrogen-14 produce C-14. The production rate seems to be nearly constant at the present time. The present carbon-14 to ordinary carbon (C-14/C) ratio is 1/1.2 trillion, with efficient atmospheric mixing. (Ten years after the first bomb tests, the northern and southern atmospheric hemispheres had mixed.) The C-14 produced rapidly turns into CO2 (with radioactive C-14), which is incorporated into foods by plants, which are eaten by animals.
2010 Arthur V. Chadwick, Ph.D.