Radon gas occurs naturally in the soil. It enters buildings through sumps, drains, cracks, utility access holes and foundations. Soil gas has such a high radon content that if only 0.1% of the air infiltrating a building were from the soil, the indoor environment could become significantly contaminated. Concentrations in buildings where there is a source of radon depend on ventilation rates. Ventilation is often used to remediate radon contamination in buildings.