Human Radioactivity

Radiation doses are expressed in SI units as milli Sievert (mSv) effective doses. This unit takes into account the type, the intensity and duration of radiation, the amount and type of body tissues irradiated, and the different radiation sensitivity of the irradiated tissues. The average natural background dose rate in many countries is 1–5 mSv a year. On average, medical exposures contribute about another 0.5–0.7 mSv a year. The current recommended limit for occupational exposure in many countries is about 20mSv effective dose per year averaged over five consecutive years.

The typical human adult body has an inherent internal radiation dose from its natural amounts of radioactive elements, including its major contribution of about 40 milligrams of radioactive potassium as the isotope K-40, which has a half-life of about 1.3 Giga years. This isotope is not the result of artificial radioactivity but remains from the formation of potassium in the supernova that gave birth to our Solar System about 5 billion years ago. There has not been enough time for all of the radioactive potassium to decay, so that is why there is so much in our bodies. Eileen wonders whether this inherent K-40 radioactive source is exposing our bodies to more than the recommended limit? Is the limit exceeded when several people gather together in a small circle?

Answer

At one time in the history of radiation safety, before extensive and long-term measurements, the recommended radiation limit was much less than the limit today. During those times, in the mid-1900s, two people in close proximity would have been emitting enough gamma radiation to exceed the recommended limit.

We can estimate the exposure amount and compare its value to the recommended limit today. There are approximately 105 decays of K-40 isotopes per second in your body, but the decay chart tells us that only about 11 percent yield a gamma ray, producing about 1100 self-inflicted gamma rays per second, amounting to about 0.36 mSv per year, well below the recommended limit today. Even a group of 10 people closely packed would not provide a radiation exposure more than 3.6 mSv per year. So we are not radiation dangers to ourselves nor to our friends!