Document jBBnqoedwNGzvqkJXD2J7BqL5

670 JEROME R. COX, JR. TABLE 7 (continued) 3. Infectious diseases, such as mumps and meningitis, that usually occur during oSS hood. 9L 4. Heavy dosage of certain drugs, particularly quinine, but individual susceptihilffi varies widely. * 5. Mechanical damage from a blow on the head or an explosion. 6. Presbycusis, a permanent andjjrogressive degeneration of the hair cells and/or nf| endings that normally accompanies advancing age. 7. Exposure to intense noise (see Section III.P). B. Lesions of the auditory nerve, usually caused by tumors. III. Disorders of the central nervous system. ;|| ..J A. Organic changes that accompany: 1. Advancing age, including effects of arteriosclerosis. . 2. A skull fracture, penetration wound, brain tumors, multiple sclerosis, and cerpf palsy. jp B. N1.oPnsoyrgchaongicendiicstduerbaafnnecsess,:a kind of hearing loss that cannot be shown to involyeif organic disorder and often may yield to psychiatric treatment. -H 2. Malingering, the willful feigning of a hearing loss for gain, usually monetary;!' '? Any such listing will be complicated by certain boundary-line disorders? cases of so-called "mixed" hearing impairment that cannot properly be pjf entirely in one group. The table is not a complete catalogue, but most impatif1 important in adults are included. Disorders that may be present in the nefjj infant are omitted because they are of little importance in the working populff For example, congenital malformation of any part of the auditory apparatu? be present at birth. Such disorders do not complicate industrial hearing coiif tion programs. sjfl The diagnosis and specification of the cause of the disorder become,- ir eral, more difficult in the groups headed "Receiving Apparatus" and'?* Nervous System." For many of these disorders the characteristics are n6ti| and the cause is incompletely known. 4 P. CHARACTERISTICS OF HEARING LOSS RESULTING FROM Nf Early losses in hearing sensitivity resulting from most industrial ndi| posures ordinarily are not recognized by the individual and can be .detee|M bypme^fone'audibinetryrTliis'iTbecausrtlie'initia'rhearihg'lbss'isfii^fTM^ frequency range between 3000 and 6000 c.p.s. and does not noticeably hearing for speech. As the impairment increases because of continued expos intense noise, losses begin to appear at 2000, 1000, and finally 500 c.p.sv'wp| _he.aringjk>aaiat-:theae-frequencie8-is sufficiently, great, a reduction-ln-thevh^M speech atWJc'--onver--s1a.1..t.io.I .n.--a..l..i.n--.--t.--e.--.-n-.--.-s-.--i.t.ie..s..is noted. Hearing loss beyond the "range of normal" in an individual exp,os| may represent a permanent hearing loss, but a certain amount of hear tivity is often recovered when the individual is removed from the noise NOISE AND THE CONSERVATION OF HEARING 671 KfMost of the improvement in the threshold will occur within a short time IBhbly less than a week) after removal from the noise. Imere is good evidence to indicate that these shifts in the threshold of hearing Bifiost obvious during the first year or so of noise exposure. A person who has ]3$iki in a noisy environment for long usually will show considerable improve- |in his hearing following removal from the noise. Those with considerable mmioss resulting from many years of noise exposure are likely to show only pltumprovement. ||g|iere are two major pathways that conduct sound to the inner ear, the normal jvayand a bone pathway that circumvents the ear canal, the drum membrane, ft' ossicles. Hearing by "bone conduction" through the skull is much less iiff than hearing in the normal manner by "air conduction." A comparison ..Jtfhone- and air-conduction thresholds will often yield important diagnostic Station about the condition of the conducting apparatus. For instance, a licit loss by air conduction coupled with no hearing loss by bone conduction Sites a malfunctioning of either the external or middle ear, but not the inner Eifthe other hand, a person with a significant noise-induced hearing impartvill have a hearing loss by bone conduction as well as by air conduction. RELATIONS OF HEARING IMPAIRMENT TO NOISE EXPOSURE 1' Jolonged exposure to intense noise may cause a hearing impairment. The nj.data relating hearing loss to noise exposure are described in detail in a ' ^p-the-American Standards'Association.20 Although insufficient data are S|le on exposure to noise that is either extremely intense, produced by im|rs intermittent in nature, it is possible in other instances to make an. esti1e!pf..the average of the pure-tone hearing losses that a group of workers may ^expected to incur. yet there are insufficient data to describe properly the distribution of inl^mufiljjnoise-induced hearing losses about the average :for,aiSgiyem nqise,.exp.o- Jgyreyer, the susceptibility to noise clearly varies -widely,.from, individual ||||r&idual. Some severe noise exposures can cause either,a negligible hearing "ipgejit or almost complete, deafness.in. anjndfvidual,, depending .up,on whether fgPy susceptible or very resistant ta ||giiy?h^informationri:.elatm'gta"^ fjpiments that it may produce were complete, it still would be impossible, P|'additional information, to estimate the practical seriousness^of a rnoise jHfejsThe missing information is the relation between a.man's pii^tone'iie^r- p^and--his,4igabillty_.r.esulting-from the hearing' loss. The mimbtaLoi-pfinpln-- jfcSyjiclaim compensation for their hearing disability and the magnitude of gjSims will depend upon which of the several methods of computation are Relations,of Hearing Loss to Noise Exposure, Z24-X2, American Standards Aflso- York, 1954. , i i1 :! 5I : S. -V , t^S-T^A A msM&mmKmm