Radon

The air pressure in the ground around most houses is often greater than the air pressure inside the house. Thus, air tends to move from the disturbed zone, resulting from its construction, into the house through openings in the house's foundation.
You can get an idea as to how concerned you should be about radon in your house by learning about the geology of the site and its radon potential.
Radon is a gas produced by the radioactive decay of the element radium.
If your home has a radon level of 4 picocuries per liter (pCi/L) of air or more, you should take steps to fix your home and reduce the radon level.
Each state has a radon program and contact, from which you can obtain information. While a
Radon is a radionuclide. Radionuclides are radioactive isotopes  (unstable forms of elements).