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What is radiation?

Radiation also exists naturally.

So long as humans live on Earth, we will always be exposed to small amounts of radiation.
For people living at high elevations, there is an increase in exposure to cosmic radiation (radiation atop Mt. Fuji is five times higher than that at sea level).
When flying in an airplane roundtrip from Japan to New York City, passengers are exposed to about 0.1 mSv (millisievert) of radiation. The atmosphere surrounding Earth does have somewhat of a blocking effect on cosmic radiation.
There's even radiation
in our bodies!
The radioactive element most prevalent in the human body is potassium-40 (40K).
There is a total of about 130 grams of potassium in the body, with the radioactive form of potassium comprising 0.012% of that total.
In other words, in our bodies each second 4,000 potassium nuclei decay, constantly emitting beta radiation.
Apparently, most geothermal heat results from 40K (half life of 1.2 billion years)--and uranium also plays a role in generating this heat.
Naturally occurring radiation
Naturally occurring radiation
Radon is a product of radium-226 (a form of natural uranium). Large quantities of radon are emitted from buildings and homes.