Elements that emit ionising radiation are called radioactive.     Radioactive elements are naturally found in the environment. They emit background radiation.     The behaviour of radioactive elements cannot be changed by chemical or physical processes.      Background radiation comes from two main sources: Natural sources: the Sun, rocks, food, outer space (cosmic rays). Artificial sources: hospital waste, medical uses (X-rays), nuclear power stations, fallout from        nuclear explosions and nuclear accidents.      Radioactive radon gas is given out from some rocks, like granite.        Trapped radon gas in houses can cause cancer.