Radon scarcely justifies the alarm.
It is odorless, colorless, tasteless, a noble gas in the periodic table of the elements, the byproduct of the decay of uranium. Since uranium is widespread, the gas escapes constantly into the atmosphere. It is moderately soluble, so well water passing through uranium-bearing rocks emits radon upon contact with the air. They in turn decay, releasing alpha particles that can penetrate lung cells, damaging DNA and other cellular components. Humans have the capacity to repair damaged DNA and do so regularly.