High radon levels in basement — Den Haag, Zuid-Holland

Radon is an odorless, colorless radioactive gas that seeps into homes through cracks and gaps in the foundation where the building contacts the soil. It is the second leading cause of lung cancer after smoking, responsible for roughly 21,000 deaths per year in the United States alone. The EPA recommends taking action when indoor radon levels reach or exceed 4 pCi/L (picocuries per liter). A short-term test kit (2–7 days) provides a screening result, but a long-term test (90+ days) gives a more accurate annual average. The standard mitigation method is sub-slab depressurization: a pipe drilled through the basement slab connects to a fan that draws radon-laden air from beneath the foundation and vents it above the roofline. A foundation specialist or radon mitigation contractor installs the system, seals visible cracks in the slab and walls, and re-tests to confirm levels have dropped below 4 pCi/L. The installation typically costs $800–$2,500 depending on home size and foundation type.