The timing belt is what synchronizes the cam and crankshafts in your engine and keeps the entire car running smoothly. For everything to work properly, the timing belt, has to perfectly sync the valves with the motion of the pistons. To prevent the timing belt from failing, it should be replaced every 50 to 70,000 miles. Ask your service advisor what your recommended interval is for a timing belt replacement.If the timing belt slips or breaks, the split second synchronization of engine parts is lost and the damage could be catastrophic.A broken timing belt can cause the pistons to collide with the valves,potentially bending or breaking them, as well as the piston, cylinder and possibly the crank-shaft.From a safety standpoint, there is slight safety risk, because if the car breaks down and you are not prepared, it could be very dangerous. There is no environmental impact of timing belt replacement.