Radon
Radon is an odorless, tasteless, and colorless gas formed by the radioactive breakdown of uranium in soil, rock, and water. It can penetrate any type of building and can build-up to high levels. Radon gas decays into radioactive particles that can get trapped in your lungs when you breathe. As these particles breakdown further, they release small bursts of energy.
Radon can damage lung tissue and lead to lung cancer over the course of a lifetime.
Unfortunately, the only way to know the radon concentration in your home is to test!
For more info on radon:
Home Buyer's and Seller's Guide to Radon

