How does radon primarily enter buildings?

Radon primarily enters buildings through cracks in floors and walls due to its nature as a gas that emanates from the subsoil. The gas is produced from the natural decay of uranium found in soil and rock, and it typically moves up through the ground. When radon gas reaches the surface, it can seep into buildings through any openings, but the most significant entry points are often the cracks and gaps in the foundation, walls, and floors where contact with the ground is present.

This entry occurs because a building's construction creates a negative pressure in its lower levels, especially when there is ventilation, heating systems, or exhaust fans operating. This negative pressure can draw radon from the ground into the structure more effectively through these cracks, making them particularly susceptible entry points for radon gas. Other options listed, such as entry through rooftops or windows and doors, are far less significant in comparison, as they do not typically provide a direct path for ground-source gases to infiltrate the indoor environment. Therefore, understanding that cracks in floors and walls serve as the primary conduits for radon entry is key to assessing radon exposure risk in buildings.