What happen if all the carbon-14 atoms in a person body decays at once? What happens if all the carbon-14 atoms in a persons body decays at once? Would they die or will they be unaffected?
 A: Carbon-14 has a relative abundance to Carbon-12 of about 0.0000000001% (one part per trillion). The average human body has about 15 kg of Carbon give or take, meaning about 1.5 micrograms of Carbon-14. That's 107 nanomoles of carbon, or about 6.444×10^16 atoms (1/3000th a grain of sand). When a Carbon-14 atom decays into a Nitrogen-14 atom via beta decay, it loses 0.001% of its mass in the form of an electron and kinetic energy (156.5 keV of kinetic energy to be precise). The total amount of kinetic energy released into the body if all 6.444×10^16 atoms in it were to decay would be 1.6 kJ.
It's hard to say exactly how much damage that would do to the body, but it would definitely do something. That's about the energy your body releases in 15 seconds, so the damage that would be done entirely depends on how quickly the decay happens. If its over a few seconds, likely nothing would happen. If its instantaneous, that would be like a very sudden hotflash that could damage some tissue, though even that wouldn't kill you. The dangerous thing you'd have to worry about is cancer. To give perspective, only about 20 atoms of carbon would decay in a given cell, and only a 20% chance that in a given cell one of those would be in a DNA molecule. About 1 in every 5 of your cells will likely have a single base pair destroyed, meaning one letter of your genetic code. While these happen all the time, if this scenario occured over one second, it would be about 200,000 times higher than the normal rate of mutation per second. That wouldn't guarantee cancer of course, but I wouldn't volunteer for this to happen to me any time soon.
A: There is about 1 radioactive $^{14}$C atom for every $10^{12}$ $^{12}$C atoms. With a half-life of 5730 years, this means there are usually about 0.2 decays per gramme of carbon per second.
Carbon is about 18% of your body mass, so an 80 kg adult would have about 14 kg of carbon and $7\times 10^{14}$ $^{14}$C atoms. If these all decayed in say one second, the dose rate would be $1.9\times 10^4$ Ci
To estimate the effects, we could assume all the beta particles are absorbed and that each has an energy of about 0.1 MeV. This gives an absorbed energy of just 11 Joules of energy. Totally negligible in energetic terms.
In terms of absorbed radiation the quantity is about 11 J/80 kg = 0.14 Grays and for beta particles, roughly the same number of Sieverts, i.e. 140 mSv.
This is roughly the same radiation dose you would get from $\sim$20 CT scans or about the same as a few decades worth of exposure to ambient radiation in the environment and is enough to raise your long-term cancer risk slightly (a few per cent) but likely not enough to cause acute radiation poisoning.
A: More important than the radiation dose is that the Carbon-14 would decay to Nitrogen-14 and random proteins, cellular structures, RNA and DNA would no longer function. There might be secondary effects, such as The Bends, a.k.a decompression sickness, from nitrogen in the blood, or other diseases. I would hesitate to predict anything but a painful lingering death.
