The level of radon in the air of mines is now normally controlled by law. In a working mine, the radon level can be controlled by ventilation, sealing off old workings and controlling the water in the mine. The level in a mine can go up when a mine is abandoned; it can reach a level which can cause the skin to become red (a mild radiation burn). The radon levels in some of the mines can reach 400 to 700 kBq m.