Half life of carbon dating
The half life of carbon-14 is about 5,700 years, so if we measure the proportion of C-14 in a sample and discover it's half a part per trillion, i.e.half the original level, we know the sample is around one half life or 5,700 years old.In these areas it can represent a significant health hazard.Concentrations over 500 times higher than the world average have been found inside buildings in Scandinavia, the United States, Iran, and the Czech Republic.
This radiation interacts with atoms in the atmosphere to create an air shower of secondary radiation, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons.
Most of these sources have been decreasing, due to radioactive decay since the formation of the earth, because there is no significant source of replacement.
Because of this, the present activity on Earth from uranium-238 is only half as much as it originally was because of its 4.5-billion-year half-life.
Radon and its isotopes, parent radionuclides, and decay products all contribute to an average inhaled dose of 1.26 m Sv/a.
Radon is unevenly distributed and variable with weather, such that much higher doses occur in certain areas of the world.