Since radium 226 can be found in low concentrations in almost all rock and soil, it is not a question of “if”, but “how much” radon you really have. Radon is generated in rock and soil and it escapes into the atmosphere through cracks and spaces between the rocks and soil. This results in an average outdoor radon concentration of about 0.4 pCi/L (picocuries per liter of air). At these concentrations there is no real danger. However, if radon is allowed to seep into homes and buildings through cracks and holes in the foundations and walls, the concentration can build up to much higher levels, at which point research suggests there is a reason for concern.
The average indoor radon concentration is about 1.3 pCi/l. Radon concentrations in a home can very depending on several factors including; house design, soil conditions, local geology, and the weather, such as high or low atmospheric pressures, and warm or cold climate. For example, indoor radon levels increase substantially during the winter months. As indoor temperatures increase relative to the outside temperatures, a thermal effect occurs. The rising warm air within a building is displaced by cold denser outside air, some of which seeps in through the foundation cracks, vents, and holes from the underlying soils.
Furthermore, exhaust fans inside the house can create a lower (negative) pressure inside the home relative to the surrounding soil and air, and radon can actually be drawn into the building.