For a long time, noise has been known to cause sleeplessness and irritability.The effect of noise on people is borne out by experiments on rats, which show that those exposed to certain high frequency sounds had up to 20 times the normal amount of adrenalin in their blood.Now other studies are linking it to a wide range of mental and physical disturbances.In Los Angeles, researchers have found that people living near airports have a higher rate of mental illness than people living eight kilometres away.