Ionization smoke detectors use a small amount of radioactive material to detect smoke, specifically americium-241. Americium-241 is a man-made radioactive isotope that emits alpha particles, which ionize air molecules in the detectors sensing chamber. When smoke enters the sensing chamber, it interrupts the electrical current, triggering the smoke detectors alarm. The amount of radioactive material in smoke detectors is very small and poses no health risk when the detector is properly handled and not tampered with.