How might increasing the energy efficiency of a home might lead to increased radon levels indoors?

Improving energy efficiency of a home generally means reducing the exchange of outdoor air (ventilation) with indoor air that can be contaminated with radon gas.
Explanation:
The presence of radon gas is not detectable by humans. It is odorless, colorless, tasteless, and radioactive. It can cause lung cancer over time and quantity breathed in. It enters buildings through cracks and voids in their foundations from the soil surrounding them.
The energy efficiency of buildings has been steadily increasing as we try to conserve energy and reduce costs. By so doing we also reduce the airflow between inside and outside, especially during the colder months of the year.
This practice will result in radon gas concentration buildup to values that could become unsafe for human consumption.
There are a number of methods to reduce the threat of radon gas in a house, but first there needs to be a measurement of the level of gas in the building. If the threat is high, the radon gas must be removed.