Document 4QKrBRwYVje1rMaxQD2M7BMRQ

not noticed until losses at the speech fre quencies, 500, 1000 and 2000 Hz are 15 dB (ASA 1951) or 25 dB (ISO 1964) Substantial losses may occur at frequencies from 3000 to 6000 Hz without producing any subjective awareness of changes in heanng The pro duction of noise-induced hearing loss is a slow insidious process and years of exposure may elapse before anv significant noticeable loss of hearing occurs (Fig 45-5) Second, the deletenous effects of noiseexposure cannot be directly equated to the energy content of the noise That is, doubling the energy content m the noise does not pro duce twice the hearing loss It is logical that the larger the total energy content of the noise is the smaller will be the time of exposure re quired to produce the same amount of hear ing loss, but the exact relation between time and energy is not known Third, the total amount of heanng loss produced by noise-exposure depends on many more vanables than was thought ongmally Heanng loss varies with the type of exposure and its degree of intermittency, with the individual exposed, the total dura tion of the exposure, and with the degree of consistency of use of ear protection Noise-exposure and permanent heanng loss. This discussion has referred to heanng loss produced by noise-exposure, not by noise The concept of exposure, of the need for time of development of heanng loss is important Brief exposures to most industnal noises will produce no illness or damage to the ear Whenever the causal relations of heanng loss to noise are considered, one should consider each of the four factors bear ing on noise-exposure 1 The overall noise level to which the man was exposed 2 The frequency composition or spectrum of this noise, probably as expressed in octave band sound pressure levels 3 The duration and time distribution of noise dunng a typical "work-day " 4 The total duration of the noise-exposure dunng the "work-life " (A-scale measurements may be substituted for 1 and 2 ) To elucidate these last two factors consider the following Frequently, one is confronted by persons who work in high level noises, perhaps as high as 110 or 120 dB Before drawing any conclusions about the effects of exposure to these noises, one must inquire carefully about the length of time spent in the noise Brief exposures to these noise levels will not produce severe hearing loss and in most cases, not even significant heanng loss On the other hand, one also encounters persons who work m lower noise levels, perhaps 100 dB, and again exposure time looms important The possible effects of extended exposure to these lower levels must not be minimized In many instances persons who are exposed to the 100 dB noise have much longer total duration of exposure than do the men who work in excessively high noise levels If the noise level is approximately 100 dB and the exposure has been long, existing hearing loss may be directly related to the noise-exposure Some industnal noises will produce signifi cant hearing loss only if the exposure con tinues over 15 to 20 years Many operations that are associated with these industnal noises, however, are charactenzed by rapid job turnover The total duration of exposure for any man's ears may not be more than two or three years One must remember, how ever, that men who work in noisy jobs at one location will generally transfer to a noisy job in another location Exposure to the average industrial noise for two or three years usually produces only minor changes m heanng These previous statements are based on long expenence with continuous noises There is insufficient expenence about the auditory effects of impulse-type noises, such as blasts, nfle fire, or drop hammer sounds Hearing Conservation and Noise Exposure Two questions must be considered before setting the limit beyond which noise ex posures are judged to be severe enough to warrant organizing a heanng conservation program First, can a realistic heanng con servation program provide complete protec tion for all ears'* And second, what do we know about the effectiveness of the different 1527