Doctors have long known that sustained exposure to loud noise can permanently damage hearing. Now there is evidence, based on animal experiments, that if the victim has high blood pressure, the degree of hearing loss can be worse. Moreover, if such a victim eats a high-fat diet, the loss can be worse still.

The findings were based on experiments in which various groups of rats were exposed to random intervals of loud "white noise," a mixture of many pitches, from 7 am to 3 pm daily for six months. The noise was rated at 95 decibels, somewhat higher than the 90 decibels that the federal Occupational Safety and Health Administration (OSHA) allows in the work place during an eight-hour day.

Periodically the rats' hearing was tested with electrodes that detect a type of nerve signal that is known to be triggered when the rat hears a sound.

Rats of a variety that spontaneously develops high blood pressure proved unable to hear a sound until it was much louder than that which a rat of normal blood pressure could hear. The hearing loss occurred even though the noise did not seem to worsen the rats' hypertension.

Another group with high blood pressure was also fed a high fat diet, which is known to cause hardening of the arteries and to lessen blood flow to the ear. This group showed the most hearing loss. The research was done by Harold C. Pillsbury, chairman of otolaryngology at the University of North Carolina in Chapel Hill.