Radon is a noble gas which is colourless, odourless and flavourless, does not bind and escapes via cracks and gaps from the earth into the breathing air. Radon exists only as radioactive isotopes. Radon-222 is the isotope with the longest half-life. It is a natural decay product of the uranium-thorium series which exists all over the earth and contributes considerably to the natural environmental radioactivity. Through further decay further radioactive decay products are generated which enter the lung via the breathing air and emit there, among others, alpha radiation. This can damage the cells of the lung. These damages can favour the generation of cancer.