Radioactivity of the human body
A typical human body contains approximately 40 grams of potassium. About 1/1000 of this is potassium-40, a radioactive isotope. This means that you contain 0.04 grams = 40 milligrams of a radioactive cancer-producing isotope in your body. This is not an artificial radioactivity, but it is left over from the formation of potassium in the supernova that gave birth to our solar system. The half life of potassium-40 is 1.26 billion years. The solar system was created about 5 billion years ago. That is not enough time for all of the radioactive potassium to decay, and that is why there is so much in our bodies.
Atoms of radiopotassium in your body explode approximately 12,000 times every second. Your body is radioactive. Only 11% of the radioactive decays yield a gamma ray, so there are 1,300 self-inflicted gammas per second from your own body. (This is not easy to see in a geiger counter, because gammas are hard to detect.) This results in an exposure of approximely 0.00018 Sieverts over a 50 year period. If the linear hypothesis is correct, this is responsible on average for 7 millionths of a cancers per individual. Since 0.2 of us die from cancer, this self-inflicted radiation is responsible for 1/28000 of our cancers.
The results are more astonishing if you consider a large population. There are about 250 million people in the United States. Multipy 250 million by 7 millionths of a cancer per person, and you find that 250 x 7 = 1750 people will die of cancer over the next 50 years, induced by their own radioactivity. That averages to 35 per year.
Of course, we can't get away from our own radioactivity. But think about double beds. If we spend a lot of time very close to someone else, we are exposed to their radioactivity as well as our own. Suppose we spend 1/3 of a day in close proximity (e.g. double beds). Then we expect to increase the potassium-induced cancers by about 1/3, or about 600 people. So if everyone in the US slept in double beds, the number of additional cancers expected in the next 50 years is 600.
That may sound silly. But remember, those are 600 people who otherwise would not die of cancer. And that's just in the US. If we consider world-wide statistics, the numbers will be 20 times larger: 12000. That is the number I would quote if I wanted to scare you. (Actually, it is the number I put on the homepage to get you to read this section.) If I wanted to make you feel the risk was negligible, I would tell you that the chance of you getting cancer from sleeping in a double bed are 0.000002. (That number is 0.000007 divided by 3.)
Do you think we should do something about this? We could install shields between people that protect them from gamma rays. Or outlaw double beds? At what point is it silly to worry? What level of "additional cancer" would you consider to be negligible? Would you accept a different level of risk for yourself than you would allow for the entire US? A risk of 0.000007 sounds negligible for me, but 600 preventable cancers in the US sounds substantial.