than the other two frequencies. The threshold of hearing not very In handicap, or low fence was set at 20 decibels (by today's audiometric reference standard), and the ceiling of handicap was set at about 95 decibels. Since the procedure for calculating compensation was more complicated than it was for the Fletcher Point-Eight Rule, the AMA rule was popular and doctors preferred to use the older method. 1959, the American Academy of Ophthalmology and Otolaryngology (AAOO) adopted a new formula. According to one of the members who had drafted the 1959 formula, "it was essential to simplify the more accurate but complicated Fowler-Sabine scale in order to gain acceptance. Otologists just wouldn't use the complicated table...."4 The 1959 AA00 formula dropped the 4000 Hz frequency in favor of the simple average of 500, 1000 and 2000 Hz, and set the low fence at 26 decibels, but little experimental evidence was cited in support of that decision. Apparently because of the increasingly large body of evidence on the importance of audiometric frequencies above 2000 Hz, the AA00 is currently revising its formula, but the final draft has not yet been produced. For the adoption of its formula the AA00 made certain assumptions. In the words of the Subcommittee on Noise: "... hearing impairment should be evaluated in terms of the ability to hear everyday speech under everyday conditions.... The ability to hear sentences and repeat them correctly in a quiet environment is taken as satisfactory evidence of correct hearing for everyday speech." The Subcommiteee determined that if the average hearing level at 500, conditions. 1000 and 2000 Hz were 25 decibels or less, no impairment existed in the ability to hear everyday speech under everyday If indeed most communications took place in a quiet environment, the individual with a mild, noise-induced hearing loss might not experience much difficulty. However, today's urbanized, mechanized environment is not always quiet. In fact it is rarely as quiet as the sound-treated rooms in which speech audiometry is usually conducted. Therefore, the assumption of a quiet environment on which the AAOO formula was based, is not valid for everyday listening conditions. Over the past two decades a number of studies have been conducted in order to investigate the ability of hearingimpaired people to understand speech in various noise backgrounds. 5-12 The majority of these studies have demonstrated the importance of good hearing at 2000 Hz and above for understanding speech in less than ideal conditions. Some investigators have also studied the effects of various other distortions besides noise, such as rapid or muffled speech and atypical accent, on the ability of hearingimpaired people to understand speech. 5,13,14 One of these investigators estimates that speech is distorted as much as 50% of the time.14 These studies also demonstrated the importance of high-frequency hearing. Indivi The Environmental Protection Agency recently conducted a study at the Aerospace Medical Research Laboratory, Wright-Patterson Air Force Base. The purpose of the study was to investigate the adequacy of the 26-decibel low fence averaged over the frequencies 500, 1000 and 2000 Hz. duals with hearing losses that were at that level or less severe were asked to listen to speech materials presented in quiet and in various levels of background noise. The results showed that individuals whose hearing levels were better than the 26-decibel fence at 500, 1000 and 2000 Hz still had considerable difficulty understanding speech if they had greater losses in the high frequencies. When compared to a control group whose hearing was normal at all frequencies, individuals who would be considered "not handicapped" by the AA00 definition had significantly more difficulty understanding speech, even in the quiet condition. The EPA also examined a number of frequency combinations in order to see which combination correlated best with the ability to discriminate speech sounds. For the hearing-loss population under study, the combinations that included frequencies above 2000 Hz showed the highest correlations, and the combination of 500, 1000 and 2000 Hz showed the lowest correlation. Those that correlated highly include the average of 1000, 2000 and 3000, which is currently in use by the OWCP, and the average of 1000, 2000 and 4000 Hz which has been recommended by EPA to OSHA for use in the audiometric thresholds. An appropriate low fence or point of beginning hearing handicap, may be somewhat more difficult to pinpoint. It must be remembered that even people with normal hearing begin to have difficulty understanding speech when conditions become sufficiently noisy. Also, it seems that a certain amount of hearing loss can occur before significant differences appear in the ability to discriminate speech sounds, between normal-hearing and hearing-impaired groups. The point where the hearing-impaired individual is no longer able to "get by" the way his normal-hearing counterpart does has been called the "critical hearing loss" by one researcher.' The magnitude of this critical hearing loss will vary somewhat according to which audiometric frequencies are used to assess it. Our present estimate is that it occurs between 11 and 26 decibels for the averaged frequencies 1000, 2000 and 3000 Hz and between 15 and 30 decibels for the averaged frequencies of 1000, 2000 and 4000 Hz. If we assume that the critical level lies midway between these two points, then the low fence currently in use by the OWCP might be slightly too high. Until the issue of an appropriate fence is studied further, the 26-decibel fence at 1000, 2000 and 3000 Hz is not an unreasonable choice for compensation purposes. A question that often arises as a result of the lowfence discussion is whether or not adjustments should be made for presbycusis, or old age hearing loss. The fact that employers are reluctant to compensate for hearing losses that are caused by aging alone is understandable. However, it is fairly unlikely that aging alone could account for many hearing losses that exceed the present 26-decibel fence. According to the 1962 U.S. Public Health Survey, the median hearing loss for 60 year-olds for the average of 1000, 2000 and 3000 Hz is 15 decibels for women and 21 decibels for men. The median loss for 60 year-olds for the average of 1000, 2000 and 4000 Hz is 16 decibels for women and 25 decibels for men. Since the population was not screened for Occupational noise exposure, it can be assumed that the hearing levels are higher than they would be if only hearing loss from aging were considered. This is evident in the rather large difference between the thresholds of men and women, where there is no known biological difference in hearing ability. There is also a possibility that the women's thresholds may have been influenced to some extent by noise exposure, but it is unlikely that the median levels would have been much affected. For this reason, EPA has in the past selected the Public Health Survey data for women as the best representation of a contemporary non-noise exposed population. New Public Health Survey data are scheduled to be released by the National Institutes of Health during 1978. |