The effectiveness of hearing protective devices.

The effectiveness of a hearing conservation program, and specifically the effectiveness of hearing protection devices (muffs in this case), was investigated over a five-year period, since the inception of the program at a major plant of the Ingersoll-Rand Company. The study concentrated on three groups (Class I, II, and III) of 213 employees whose noise level exposure did not change during the five-year study period. A hearing conservation criterion of 90dBA was utilized. One group consisted of office workers in which the level was well below this, a second group from a machine shop in which the level was slightly below the criterion, and a third group from foundry areas in which the levels were considerably above the 90dBA level. Personal ear protection was worn during the study period only by the last group. The mean differences in hearing levels were insignificant and were essentially the same in the three groups, at each frequency, and regardless of the baseline hearing thresholds. It is apparent that there were no significant changes in hearing threshodls during the study period among a non-noise exposed group; a mild to moderate noise-exposed group; and a group exposed to high levels of steady state noise, but wearing ear protectors. These results indicate that a hearing conservation program which includes audiometric testing and personal ear protection, utilizing a hearing conservation criterion of approximately 90dBA, does adequately protect the hearing of noise-exposed workers.