Radon-222

[4] Around the same time, Ernest Rutherford and Robert B. Owens observed a similar (though shorter-lived) emission from thorium compounds.

[5] German physicist Friedrich Ernst Dorn extensively studied these emanations in the early 1900s and attributed them to a new gaseous element, radon.

Radon-222 is especially dangerous because its longer half-life allows it to permeate soil and rocks, where it is produced in trace quantities from decays of uranium-238, and concentrate in buildings and uranium mines.

This contrasts with the other natural isotopes that decay far more quickly (half-lives less than 1 minute) and thus do not contribute significantly to radiation exposure.

[9] At higher concentrations, gaseous 222Rn may be inhaled and decay before exhalation, which leads to a buildup of its daughters 218Po and 214Po in the lungs, whose high-energy alpha and gamma radiation damages cells.

The decay chain of uranium-238, known as the uranium series or radium series, of which radon-222 is a member.