From the Minnesota Department of Health website:

Radon is a naturally occurring radioactive gas that enters buildings from the surrounding soil. It is colorless, odorless and tasteless. Radon is also the main source of ionizing radiation that most of us are exposed to. It is believed to be the second leading cause of lung cancer in the United States — second only to smoking.

Radon is a preventable health threat to many Minnesotans. MDH estimates that one in three (1/3) existing Minnesota homes have radon levels that may pose a large health risk over many years of exposure. Fortunately, most radon problems in existing homes can be fixed. However, when building a new home, it is more cost effective to prevent a radon problem by building to keep radon out.

It is relatively inexpensive to fit a new house with an anti-radon system, but much more expensive for existing houses. Additions and remodelings are a great time to consider adding such a system, when it can be coordinated with the new work.