Concentration in the food chain
The example above shows how natural radioactivity can enter the food chain to affect human beings. Gaseous radon will transform into lead 210 after a few days, atoms of which will then rain down onto the surface of the Earth after being fixed on dust. The atoms found in water or in natural sediments will have plenty of time (their half-lives are of 22.3 years) to insinuate themselves into the bodies of living organisms. However, the concentrations of these heavy natural radioisotopes that build up in the body are entirely harmless.
IN2P3