Changes in hearing acuity in relation to long-term exposure to industrial noise were studied in Federal penitentiaries. The workers were employed in textile mills, wood products and sheet metal products manufacturing, brush, shoe, and clothing factories, and printing. Over-all noise levels ranged from approximately 75 to 100 decibels. It was found that if a steady type of noise exposure is severe enough to produce eventually a marked adverse effect on hearing in the speech range, a definite elevation or deterioration of the hearing level in the test frequencies of 3,000, 4,000, and 6,000 cps will usually appear within a few months after exposure begins. With increased duration of exposure, less of such elevation of hearing level will disappear after noise exposure is discontinued.
This scholarly monograph supports recommendations on the desirability of instituting measures for hearing conservation when the work environment includes regular prolonged exposure to steadystate, continuous-spectrum noise reaching