Many areas of the US have high background radon level in the ground. When radon gets into a home it can increase the resident’s cancer risk. How does radon get into a home?
The most common way is through cracks in basement floors, walls, and sump pump sumps. In the winter, if a furnace or boiler is in the basement, the chimney can act as a depressurization device since combustion air is vented to the outdoors. If the basement is tight, and there is no source of combustion air, the heating system (and water heater too) can depressurize the basement. If there is radon in the soil gas below the house, this depressurization will increase radon infiltration through cracks and sumps.
Another infiltration route is through groundwater. In areas with elevated radon in rock formations, and in homes using on-site wells for water, the water carries radon into the shower where it vaporizes to gaseous radon.