The average person receives a dose of about 360 millirem of radiation per year from natural sources of radioactivity as well as from typical medical exposures.
Another thirty to forty millirems a year comes from within our own bodies. Potassium contains a fraction of radioactive atoms, and these are constantly decaying in our bodies and releasing nuclear particles. As a result, people are among the most radioactive objects in our environment.
Just by interacting with other humans for eight hours a day gives us a dose of about 0.1 millirem in a year.
A person who smokes two packs of cigarettes a day receives a radiation dose of about 1,300 millirem per year. This is because polonium, a radioactive element, is part of the smoke and when inhaled, it gets trapped in the lungs.