I read that the average adult human body contains somewhere around 10^27 or 10^28 atoms, most of which are carbon, hydrogen, and oxygen. Among these atoms, there are some radioactive ones. I would assume that carbon-14 would be the most abundant of these radioactive isotopes. From this, I would guess that most atoms undergoing decay at any given moment are carbon-14 atoms, although I could be wrong, given that half lives of different radioactive isotopes vary so widely. The half life of carbon-14 is about 5730 years.
How frequently does an atom in the human body undergo radioactive decay? Would it be millions per second, or would it be more like one every few hours?
Carbon is 9.5% of the human body (wiki cite), so that’s around 10^26 to 10^27 atoms. Let’s assume that the amount of carbon-14 in the human body is roughly the same as the amount in the atmosphere (this seems like a fair assumption). The atmospheric concentration is about one part per trillion, or 10^-12, which means there’s 10^14 to 10^15 carbon-14 atoms in the body.
The half life of carbon-14 is 1.80810^11 seconds, so to get the number of decays per second, we take (10^14 to 10^15)/(1.80810^11) =500 to 5000 decays of carbon 14 per second.
So we know its at least that large, only looking at carbon 14.
Potassium-40 adds substantially to that total - apparently its the largest source of radioactive decay within the human body, at around 4500 decays per second via a similar calculation: cite
That brings us up to a total of somewhere between 5,000 to 10,000 decays per second, only counting potassium-40 and carbon-14, which are the two most significant contributors.