Natural radioisotopes present in the earth's crust produce another component of background radiation. Radon-222 is a chemically inert gas emanating from the soil and stone; it diffuses through the air and decays with the emission of an alpha particle. This particle appears in the cloud chamber as a dense, straight track several centimeters long, with no connecting track on either end. Radon concentration can be measured by recording the frequency of such events.