Your House is a Chimney!
Many times radon gas is actually sucked into the house through what is called the stack, or chimney effect. To understand this effect you must understand how a fireplace or chimney works. The concept is actually pretty simple, when a fire is lit in the fireplace it starts to burn. As the fire buns it creates heat and this heat rises up the chimney into the air. As the heat rises it takes the concentrated smokey air with it, which then gets replaced by fresh oxygen rich air that make the fire burn more intensely, creating higher heat, and feeds the fire more and more oxygen rich air.
Heat Rises, So does Radon
This same concept applies to your home. In the winter, your home is heated by your heating system. The natural tendencies of the heated air is to rise to the highest level of you home. If your basement is heated, and you live in a two story home, that heat is striving to reach the top floor, attic, and ultimately leave your home thought the roof, soffit and attic venting. This rising heated air has to be replaced from somewhere. So what happens is that your basement is actually place into a state of slight vacuum. This slight vacuum wants to naturally become equalized and this is done by finding weaknesses in your basement to allow air in. These places are cracks, holes and joints of in your basement floor. Some of these are difficult to see with the naked eye. If you could create a perfect seal in your basement then radon would not be a problem for you!