studyof occupational deafness* · smaller losses in the other regions of the auditory spectrum,...

Brit. J. industr. Med., 1953, 10, 41.

A FIELD STUDY OF OCCUPATIONAL DEAFNESS*BY

COLIN M. JOHNSTON(RECEIVED FOR PUBLICATION FEBRUARY 20, 1952)

Fosbroke (1830-31) described deafness in black-smiths since when there have been many investi-gations of deafness caused by noise in industry.But there are still many questions of practicalimportance to the industrial medical officer as yetunanswered. He needs to know how loud a noisemust be to cause deafnesss, how to measure noise,and the methods of protecting the workman'shearing where the noise is very loud. Thesequestions cannot be answered simply as there aremany factors to be considered, the more importantof which are the duration of exp6sure to noise, itsloudness when it reaches the ear, and the suscepti-bility of the ear to damage (Perlman, 1945).

If a group of men have worked in a loud butsteady noise for the same number of years theeffects on the hearing of the men will vary widely.Thus it is not possible to give a precise level ofnoise above which hearing will be impaired. Bunch(1942) and MacLaren and Chaney (1947) agree thatnoise of an average overall value of 70 dbt above astandard reference intensity of 0 0002 dyne/sq. cm.is innocuous and that a noise of 100 db or moregreatly increases the incidence of impaired hearing.The assessment of the hearing loss produced by

acoustic trauma requires a method which willexclude deafness due to other causes. Larsen(1939), Fowler (1928), Bunch (1937), Perlman(1945), Nussdorfer (1942), Maclaren and Chaney(1947), and Weiss (1947) agree that a circumscribedloss of hearing in the high tones, 3,000-6,000 c.p.s.,the so-called " C5 dip," is to a certain extent typicalof traumatic deafness. It is known, however, thatthis circumscribed hearing loss may be due to other

*Based on a report to the Medical Research Council's Committeeon the Medical and Surgical Problems of Deafness.

tDecibel (db) is the unit of intensity of sound. In Great Britain itis usually applied to the measurement of pure tones only. For themeasurement of the loudness of noise the phon may be employed.For a " noise " of a frequency of 1,000 c.p.s. (cycles per second) thedecibel and phon are identical. For noise, which consists of a mixtureof pure tones of different frequencies, a subjective comparison ismade with a pure tone of 1,000 c.p.s. Hence the phon is not a physicalunit of energy but an approximate expression of the loudness of anoise as heard by the observer. In the American literature the decibelis also used to describe the loudness of noise.

causes. As Causse and Falconnet (1947) state, itis simply the expression of a special fragility in thisregion of the auditory spectrum. Nevertheless itis probable that a well marked loss of hearing inthe 3,000-6,000 c.p.s. frequency range with markedlysmaller losses in the other regions of the auditoryspectrum, particularly in the region above 6,000c.p.s., is pathognomonic of traumatic deafness.Weiss (1947) described the development of theC5 dip and MacLaren and Chaney (1947) classifiedthe severity of their cases according to the develop-ment of the " dip ".

Present InvestigationThe investigation was made between June, 1948,

and May, 1949, in three factories in the Birminghamarea (Messrs. Edwin Danks Ltd. of Oldbury, JohnGarrington and Sons Ltd., of Bromsgrove, andGuest Keen and Nettlefold Ltd., of Birmingham).The manufacturing processes carried out in thesefactories gave rise to many types of noise, inter-mittent or continuous, and with various proportionsof low and high frequencies. The NationalPhysical Laboratory gave their cooperation andcarried out a full series of noise level measurements.

Boiler Making.-For chipping, caulking, andriveting a portable compressed air hammer in whichvarious tools can be mounted is used. The hammersweigh from 7 to 15 lb. and they deliver up to 600blows per minute. The light hammers give a rapidsuccession of light blows and are used for caulking-the operation in which the edges of a rivet or aplate are turned in to make a steam-tight joint.The medium and heavy hammers are mainly usedfor riveting or chipping.When riveting, the early stages of forming the

head will be relatively quiet when the hammertouches only the heat-softened rivet, but in thelater stages when the edge of the tool touches thepart being riveted hard metal meets hard metal andmore noise is produced. On certain types of work,particularly shipbuilding, several hammers are

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BRITISH JOURNAL OF INDUSTRIAL MEDICINE

often in use simultaneously and the noise is con-ducted by metal throughout the fabric. Althoughthe peak noise levels are no higher in shipbuilding,the workman is more continuously subjected tonoise of a traumatizing intensity than his fellow inthe boiler shop. Of the two methods of joiningtogether steel sheets, welding, which necessitatesthe chipping of excess metal, is just as noisy asriveting. In pneumatic riveting the riveter isassisted by a " holder-up " or " backer-up " whoinserts the rivet and exerts counter pressure witha heavy tool or another pneumatic hammer whilstthe head is being formed. When riveting boilers the" holder-up " works inside the boiler and is there-fore exposed to more noise than the riveter.

Plate Fitting.-In this work the plates and end ofa boiler are assembled for riveting or welding.Minor adjustments are made to the curve of a partby hammering with a heavy sledge hammer. Withcertain jobs the noise levels from hard hammeringmay be intense, but the workman is exposed forshorter periods to more intense noise than thechipper or riveter.

Other processes include welding, template making,shearing, bending, drilling. By themselves theseoperations make relatively little noise but thosewho do this work are subjected to the noise ofpneumatic tools and hard hammering, usually at adistance of several yards but occasionally in closeproximity.

Drop Forge Industry.-Red hot steel ingots areforged by stamping hammers in dies. The lowerhalf of the die rests on a firm foundation and theupper half is fixed to the lower surface of a weightof up to 2 tons. The weight is raised to a heightof two to six feet, according to the strength of blowrequired and allowed to drop by gravity; with thesteam hammer it is aided by steam pressure. Theingot is roughly shaped in one part of the die byseveral light blows and is then placed on anotherpart of the die where harder blows are given toform the final shape. The last blow is the hardestand emits a metallic ring when the two halves of thedie come in contact. There is also a continual roarproduced by oil-fired furnaces used for heating theingots and a high-pitched squeal or various clankingnoises of the hammers. The work is carried out inlong sheds where from seven to 15 hammers ofvarious sizes are in continuous use. The noise, theheat of the furnaces and ingots, the continuallifting of heavy forgings, together with the dusty,smoke-laden atmosphere, make the trade, althoughwell paid, extremely arduous. The majority of the

workmen seek less strenuous employment after theage of 40.

Screw Manufacture.-In the shop studied, steelscrews for wood work were being made on auto-matic machines. The blank for the screw is madefrom wire of appropriate thickness on " heading'"machines. The " turning" section trims the headof the blank and cuts the "nick " or slot for thescrew driver. Finally the thread or " worm " ofthe screw is cut on an automatic lathe. The noisein all sections is of a fairly constant intensity, beinghighest in the heading section and lowest in theturning section. Many of the operatives in theturning section, however, work close to the headingmachines.

Method of Examination of SubjectsAt two factories disused underground air raid

shelters were fitted with double doors. Theseshelters were more than 50 yards from the nearestworkshop and, apart from occasional and infrequentrumbling noises of passing traffic, ambient noisewas at a low level. At another factory the workerswere examined in a quiet room in the factorymedical department situated 100 yards awayfrom the nearest workshop.

All the subjects came straight from their workand after a preliminary explanation of the purposeof the investigation and the recording of theiranswers to a set questionnaire, underwent a shortexamination of the ears, nose, and throat. Cerumen,if present, was removed and the hearing was thentested by tuning fork (C512), speech, watch tick,and pure tone audiometry. For speech tests thesubject sat with the ear being tested towards theobserver, blocking the opposite ear by pressure onthe tragus. Mixed letters and numbers were readin a soft (residual air) Whisper and a conversationalvoice.The following list of mixed numbers and letters

were read in groups of three:X2C 4AZ SA5 U8G1SN M7D L85 7BN83Y 09F 3H6 X9ELP6 3KI V4Q 82C

The maximum distance at which two or moregroups of three numbers or letters could be correctlyrepeated was recorded for each ear separately.Masking was employed where necessary.Pure tone audiometry was made with a " maico

D5" audiometer by air conduction only, using asingle ear piece. This latter test was started within20 to 30 minutes of the subject leaving his place ofwork. The hearing was tested at frequencies of

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OCCUPATIONAL DEAFNESS

64, 128, 256, 512, 1,024, 2,048, 2,896, 4,096, 5,792,8,192 and 11,584 c.p.s. (The maximum calibratedoutput of this instrument is 100 db at all frequencies,except 128 and 11,584 c.p.s. where it is 70 db, andat 256, 5,792 and 8,192 c.p.s. where it is 80 db.)

Results of Hearing TestsTwo hundred and nineteen workers were examined,

but in 28 bilateral ear disease of an infective ordegenerative origin not due to acoustic traumawas found, leaving 191 for consideration.

Figs. 1 to 7 show the relation between the durationof work in the trades studied and the hearing lossof the better ear to speech at conversational level.They show that after any given period of exposureto noise the workers in each occupational groupshow wide variations in the degree of damage totheir hearing. Although a high proportion of theworkers in each of the factories visited was examined,(all the chippers, riveters, and heading machinistswere examined) the numbers in each group aresmall and precise conclusions are not possible.The results shown in Figs. 1 to 7 are summarized

in Tables 1 and 2. From Table 1 it will be seen thatthe number of men with more than 20 years'exposure who were unable to hear a conversational

TABLE 1NO. OF MEN WITH HEARING LOSS AFTER MORE THAN20 YEARS' EXPOSURE IN DIFFERENT TRADES RELATED

TO INTENSITY OF NOISE

Occupation Noise Level Notof No.- of Men withOccupation (phons) Men Hearing Loss_ I~~~~~*|II

Chippers and riveters More than 130 12 9 11Stampers .. .. 117-130 11 7 9Platers .. .. 130 12 6 7Headers .. .. 108-112 14 4 5Welders, etc. .. 90-114 12 2 4Wormers .. .. 105-108 13 1 1Turners .. .. 100-101 24 0 1

*1 = unable to hear conversational voice at 3 feet.I1 = , ,, ,, ,, ,,9 ,, 10 ,,

TABLE 2NO. OF MEN WITH HEARING LOSS AFTER LESS THAN20 YEARS' EXPOSURE IN DIFFERENT TRADES RELATED

TO INTENSITY OF NOISE

Occupation Noise Level Notof No. of Men withOccupation Noi(phons) MNoof Hearing Loss

I II

Chippers and riveters More than 130 2 0 0Stampers .. .. 117-130 37 4 7Platers .. .. 130 8 1 2Headers .. .. 108-112 6 0 2Welders, etc. .. 90-114 18 0 0Wormers .. .. 105-108 13 0 1Turners .. .. 100-101 9 0 0

02

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0u 6

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0

io 20 30 40DURATION OF EXPOSURE IN YEARS

FIG. l-Chippers and riveters.

50

0*I.

Limit of seduI1 hgring

*I

*

,r 10 20 30 50

DURATION OF EXPOSURE IN YEARSFIG. 2.-Stampers.

°r

I2IJ34

4J6

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>K0:

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>12-

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Limit of useft hearing

0~ ~

10 20 30 40 50

DURATION OF EXPOSURE IN YEARS

FIG. 3.-Platers.

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I~~~: .~~~~~~~~~~~~~~~~Limijt ofL of usdul hearing

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ond 0 10 20 30 40more DURATION OF EXPOSURE IN YEARS

FIG. 4.-Headers.

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Limit of useful hearingI

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DURATION OF EXPOSURE IN YEARSFIG. 5.-Welders, template makers, and macliine operators.

°r

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0 - MALESX -FEMALES

Limit of useful' hearing

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DURATION OF EXPOSURE IN YEARS

Fio. 7.-Turners.

50

voice at 3 feet and at 10 feet bears a definite relation-ship to the intensity of the noise in the trade, butTable 2 shows that the same relationship is notapparent for the men with less than 20 years'exposure.

Table 3 summarizes the audiometric results. Inat least 70% the maximum loss occurred at a single

TABLE 3FREQUENCY OF MAXIMUM LOSS OF HEARING AT

VARIOUS FREQUENCIES IN 190* WORKERS

No. of PercentageMen of Total

2896 c.p.s. .53 282896 and 4096 c.p.s. .32 174096c.p.s.. 61 324096 and 5792 c.p.s. .14 85792 c.p.s. .. 18 92896 to 5792 c.p.s. .. 8 4Complete loss above 2048 c.p.s. 4 2

*Excluding one case with a normal audiogram.

2

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Limit of useful hearing

0 - MALESX FEMALES

andm

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cnd morer_Twa x;0 10 20 30 40 50

DURATION OF EXPOSURE IN YEARSFiG. 6.-Wormers.

frequency. In the remainder the maximum losswas equal at two or more frequencies, e.g. a flat-bottomed " C5 dip." The maximum loss mostoften occurred to tones at a frequency of 4,096c.p.s. (34%), but a maximum loss to tones at afrequency of 2,896 c.p.s. was almost as common(28%). If, in addition, the audiograms with a" C5 dip" at 5,792 c.p.s. and those with a flat-bottomed " C5 dip " are considered, the meanvalue for the position of the dip lies at less than4,096 c.p.s.The development of the " C5 dip" is demon-

strated by examination of the series of audiogramsof which Figs. 8 to 13 are a selection. The " C5dip" at first appears as a steep-sided notch ofusually one or occasionally up to two octaves in

44

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Q~~~~~40-

° 60

0-.0 RIGHT EARx80 _--- LEFT EAR

64 128 256 512 1024 2048 4096 8192FREQUENCY IN CYCLES PER SECOND

FIG. 8.-Stamper, age 29. Duration of exposure, three months.Whispered voice heard at 10 ft. distance from both ears.

200

t 40 5

e~~~~~~~~~~~~II60

0-0 RIGHT EAR bx80- _--- LEFT EAR


FIG. 9.-Stamper, age 22. Duration of exposure, one and a halfyears. Whispered voice heard at 10 ft. distance from both ears.

width (Fig. 8). With longer exposure to noise theapex of the dip descends (Fig. 9), and in most casesthe sides remain steep until the hearing loss at theapex of the dip lies between 60 and 80 db (Fig. 10).Up to this stage the hearing at other frequencies isnot affected except in the case of the stampers,where some loss occurs at frequencies below 2,048c.p.s. The hearing at 8,192 c.p.s. is within normallimits for the age of the individual. At a laterstage the sides of the " C5 dip " become less steepdue to losses at frequencies immediately above andbelow, with some additional loss at a frequency of11,584 c.p.s. (Fig. 11). The widening of the mouthof the dip, together with a continued increase in itsdepth, continues (Figs. 11 and 12) until total loss ofhearing to tones above a frequency of 1,024 or2,048 c.p.s. occurs (Fig. 13). In the few audio-grams obtained showing this extreme degree ofdeafness there was an equal loss of hearing atfrequencies of 64, 128, and 256 c.p.s.


FIG. 10.-Stamper, age 36. Duration of exposure, 12 years.W-- - - Rt 7Whispered voice heard at Et =4-ft. distance.


FIG. 11.-Stamper, age 27. Duration of exposure, eight years.

Whispered voice heard at L-t= I S ft. distance. Conversational

voice heard at more than 10 ft. distance from both ears.


FiG. 12.-Chipper, age 60. Duration of exposure, 43 years.

Whispered voice heard at Rt= on25 ft. distance. Conver-Rt 1sational voice heard at -E=2ft. distance.

D

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O--O RIGHTTEA@80so 0--LEFT EAR

10064 128 256 512 1024 2048 4096 8192

FREQUENCY IN CYCLES PER SECONDFIG. 13.-Chipper, age 60. Duration of exposure, 46 years.

Whispered voice not heard. Conversational voice heard atRt ContactLt 0-25-ft. distance.

Relation of External and Middle Ear Conditions toOccupational Deafness

Cerumen.-Meatal occlusion of long standing wasfound in one subject only. His audiogram for theright ear (Fig. 14) mad-6- after the removal of waxshowed little loss of hearing, whereas the left earin which there was no wax showed loss in the C.region.

024

Q40

° 60

B0


FIG. 14.-Welder, age 31. Duration of exposure, 16 years. Meatalocclusion of long standing in right ear. Audiogram madeafter removal of wax.

Chronic Adhesive Otitis Media.-Out of a totalot 438 ears in 219 subjects, 19 ears in 17 subjectswere the site of active suppuration, and 58 ears in38 subjects showed scarring or a dry perforation ofthe tympanic membrane. The audiograms of thecases with unilateral active or healed otitis mediashowed with four exceptions a hearing loss in the2,896 to 5,792 c.p.s. frequency range equal in bothears. The hearing for the low tones was either the

same in the both ears or diminished by the presenceof otitis media. The exceptions just mentioned,two cases of unilateral active otitis media and twocases of healed otitis media, showed greater hearinglosses in the 2,896 to 5,792 c.p.s. range in theunaffected ears of from 20 to 40 db.

Peyser (1943) found that the hearing in earsaffected by active otitis media was more readilyfatigued by experimental exposure to pure tones ofhigh intensity, but Theilgaard (1951) did not con-firm this finding and suggested that a conductiondeafness due to middle ear disease offered protectionto the organ of hearing of workers in noisy sur-roundings. It was not possible in this study toshow an increased susceptibility to damage bytraumatic noise of an ear affected by disease but thefour exceptions mentioned show that in a smallproportion of cases otitis media may afford someprotection to the cochlea in the 2,896 to 5,792frequency range. No opinion is expressed as to aprotective effect on the cochlea at the speechfrequencies 512 to 2,048 c.p.s. The loss of hearingto these tones is usually more rapid from otitismedia than from acoustic trauma although the finalresult may be less severe.

Recovery of Hearing after Leaving WorkForty-five per cent. of all the subjects (86 out of

191) stated that their hearing improved afterleaving their work. Some noticed the improvementwithin half-an-hour to four hours. In others theimprovement in hearing was less marked and onlynoticeable over a period of a few days or severalmonths. The subjective improvement in hearingfollowing absence from work was generally moremarked during the earlier months or years of aworker's exposure to noise. To obtain an objectiveassessment of the actual recovery which may haveoccurred, a few workers were re-examined after aperiod away from work. The re-examination wasmade at the start of the work period and before thesubject had entered the workshop. Table 4 sum-marizes the findings.Apart from two cases (Nos. 4 and 12) there was

no significant* improvement after 12 days' restfrom noise at either the speech frequencies or inthe " C5 dip ". Experimental work indicates thattemporary deafness from fatigue of the end organdue to loud tones is rapidly lost, mostly within a fewminutes but to a lesser extent in a few hours. Asalready stated, the first audiograms were madeafter the subjects had been in quiet surroundingsfor 20 to 30 minutes. The relatively slight changes*A gain or loss of 10 decibels is considered to be within the margir of

error in pure tone audiometry.

"'~~t-% ,.. ..*'s

00RIGHT EAR~~~~~~~~~F_4~~~~~~~~~~~~LEFEA

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TABLE 4RECOVERY OF HEARING AFTER ABSENCE FROM WORK

Average Loss (db) at 2896, 4098, Average Loss (db) at 512, 1024, andDuration of and 5792 c.p.s. 2048 c.p.s. Duration of

Case No. Exposure in ,Rls.tYears On First Re-test after Gain On First Re-test after Gain RestTest Rest Test Rest

1 1 28 27 + 1 16 12 -r4 62 hours2 32 83 83 0 40 40 0 623 12 45 45 0 19 15 +4 624 14 68 62 +6 33 19 14 12 days5 1 1 36 36 0 2 5 3 12,6 8 36 32 +4 20 20 0 127 35 71 74 -3 18 19 -1 2,.8 33 40 -7 8 14 -6 12,.9 2 35 27 +8 14 9 + 5 1210 l~ 42 39 -3 13 1 3 0 1211 ~~~~~~2629 -3 1 3 1 1 ±2 12

12 27 55 35 +20 17 8 + 9 12,,

in the majority of the second set of audiogramswould appear to confirm this experimental findingand indicate that little restoration of hearingcould be expected if the worker were to change to aquiet occupation.

Adaptation of Hearing to NoiseIt is a common experience for visitors conducted

round a noisy factory to have difficulty inhearing speech. The high level of ambient noiseeffectively masks the speaker's voice. Yet it iswell known among factory managers (who conductsuch visitors) that those who spend their workinghours in noise can readily hear each other withoutshouting. It is probable that an ability to lip readplays a part. Thirty-seven per cent. (71 out of191) of the subjects exaimined in this investigationwere conscious of lip reading during working hours.It was noticeable that in well lit factories where theworkers can see each other over the machines, as inthe screw factory, 56° (45 out of 79) of theoperatives admitted to lip reading at work. Wherethe standard of illumination was low and theworkers could not, by the nature of the work andthe machinery, see each other's faces, as amongchippers, platers, and stampers, only 200% (26 outof 127) admitted to lip reading. There must be,however, another mechanism whereby the subjectcan hear speech through the noise. Proof of thiswas obtained during a visit to a cotton weavingshed containing approximately 1,000 looms inmotion. While the ambient noise was not asintense as, say, the heading section of a screw mill,the visitor experienced some difficulty in hearingspeech, which was only intelligible when spoken ina raised voice close to the ear. A musical enter-tainment programme was being relayed during thevisit and amplified through loud speakers in theshed. The visitor, by standing close to a loudspeaker, was just able to hear the bass accompani-

ment of the musical composition. Ihe air playedby the treble instruments was effectively masked bythe noise of the looms. Yet a middle aged femaleweaver could easily recognize the composition andstated she could hear the words of a song whensung by a soprano voice.

This adaptation of hearing to speech in noise isto some extent lost during periods of rest. Ihirty-six per cent. (69 out of 191) of the subjects slatedthat after two days at the weekend or after theannual 12 days' holiday, they found difficulty inhearing speech for the first half to four hours afterbeginning work again.Most of the workmen who had been severely

deafened by occupational noise experienced thephenomenon of paracusis willisiana* in the factorybut not in other noisy surroundings, such as trainsor omnibuses. While it is probable that theycould hear speech better in the factory because of arise in the intensity of the speaker's voice, theability to hear speech through the noise of theirwork (also shown by less deafened but seasonedworkmen) may well have some effect. It is, there-fore, suggested that the phenomenon of adaptationof hearing to speech in noise is of central ratherthan cochlear origin.

Other Subjective Effects of NoiseTinnitus.-This was a common complaint. While

most workers were free from it at the time ofexamination, 40%' of all subjects (78 out of 191)could remember suffering from it during the earlyyears of their work in a noisy trade. High pitchedand ringing, it was always slight in intensity, nevercaused loss of sleep, and was rarely loud enoughto be heard while at work.

Vertigo.-Vertigo induced by noise was com-plained of by only two subjects, both platers, with

*The ability of a deaf person to hear speech easily in noisy, but notin quiet, surroundings.

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normal ears. It occurred when they were inside asmall boiler which was being hammered on theoutside by heavy sledge hammers by four men.Nausea did not result and the vertigo passed offwithin a few minutes of emerging from the boiler.Considerable pain was experienced in the ears fromthe noise, a rare complaint amongst workers innoisy occupations.

Poor Concentration, Headache, or Irritability.-None of these ascribed to occupational noise wasoften complained of and was more usually due todissatisfaction with the nature of the work or otherextraneous factors. It was well recognized amongthe foremen that if a workman objected to the noiseof the work, he would soon change to a quieterjob. A high proportion of new hands left suchnoisy work from this cause, but if the worker couldpersist at the work for a week or two, acclimatizationusually took place. The subject was then able toignore the noise and found it neither irritating norunpleasant. In fact many stampers found thenoise of their trade soothing.

Awareness of Risk of Occupational Deafness.-With few exceptions, workers in the boiler-makingand stamping trades were aware of the high incidenceof occupational deafness, but were not apparentlyperturbed by the prospect. The insidious onset ofdeafness which resulted in a man being unaware ofthe degree of his hearing loss and the relatively highproportion of. old hands with serviceable hearingengenders a spirit of laissez faire. The wearing ofprotective devices in the ear was observed only inthe boiler-making trade. Even here in the noisiestsection (the chippers) only four out of 14 menhabitually wore wool plugs for the express purposeof protection from noise. In the same group noless than three men merely inserted the plugs toprevent the ingress of dust. Out of the remaining50 boiler-makers who were examined, only sixhabitually wore wool plugs.

Prevention of Occupational DeafnessThe ideal remedy for the prevention of occupa-

tional deafness would be to eliminate injuriousnoise at its source. Improved designs of machinesand their mountings have already done much tohelp. Technical advances in the method of manu-facture may diminish noise but the reverse is oftenthe case. For example, the electrical welding ofsteel plates, which is to a limited extent supplantingthe lapped riveted joint, necessitates the extremelynoisy process of chipping. The reduction of noiseat the source has thus only limited possibilities.

The reduction of reflection and reverberation ofnoise by sound proofing surfaces and erectingbaffle walls have in certain instances been effective,but again scope is limited.There remains a third alternative, the acoustic

insulation of the worker's ear by wearing aprotective device. Two main types have beendescribed, the meatal plug and the ear " muff ".The former may consist of dry cotton wool (com-monly used but of little value), cotton wool impreg-nated with vaseline or " plasticine ", or rubber.Vaseline impregnated cotton wool is stated byFowler (1940) to produce a relatively constantattenuation of 20 db for frequencies below 1,000c.p.s. and 30 db for frequencies above 1,000 c.p.s.Of the numerous types of rubber ear plugs, theV-51R ear warden provides an acoustic insulationof 25 to 30 db or more at the high end of the audio-metric frequency spectrum (Taylor, 1947). Anexample of the second type of ear protector is theRiuedi defender (Riuedi and Furrer, 1946). Designedprimarily for use by gun crews, it consists of ametal box with a rubber flange on the edge fittingclosely to the head and covering the whole of theexternal ear. The outer wall of the box is perforatedby several fine holes. The size of the holes and thecapacity of the box are calculated to the desiredresonance, frequency, and damping. The dampingreduces the steepness of the front of the soundpressure wave and also the maximum pressureamplitude operating on the ear drum. The hearingloss obtained when the defender is worn is insigni-ficant at the middle and lower frequencies but above2,000 c.p.s. amounts to 25 to 30 db. An experi-mental test of the Riledi defender on several subjectswith normal hearing under the noise conditions of aboiler shop showed that considerable aural fatiguefrom noise occurred when the defender was worn.Both types of protective device have their dis-

advantages. The meatal plug, while giving a highdegree of protection, is unsuitable in cases of otitismedia or otitis externa. It may irritate the meatalskin, particularly in hot and dusty trades. Repeatedinsertion will drive plugs of wax further into themeatus. It may allow noise to reach the ear unlesscorrectly adapted to the shape of the meatus,particularly where the meatal canal is oval in cross-section. It is liable to cause discomfort when wornregularly for long periods. Owing to its smallsize a rubber plug is easily lost and must be regularlycleansed to avoid the risk of infection. The Ruedidefender has none of these disadvantages exceptthat in very hot occupations excessive moisture willaccumulate inside the box and may exacerbate apre-existing dermatitis. In the boiler-making and

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ship-building trades, where the wearer often has towork in a confined space, some modification of thehead band would be necessary to allow for thewearing of a protective cap.The Riuedi ear defender was designed to allow

the hearing of speech. Although necessary for agunner, a workman in a noisy factory only occasion-ally needs to hear spoken instructions. When thefine holes of the defender are occluded by a layer of" plasticine " a considerable improvement in thesound occlusion results and little aural fatigue iscaused on exposure to loud noise. In view of thepreviously mentioned advantage of the box type ofdefender over the meatal plug it would seemdesirable that further experiments should be carriedout with ear " muffs " of various materials.

ConclusionsMinimum Values for Harmful Noise.-Table 1

shows that a larger proportion (26 out of 49) of themen employed in chipping, riveting, stamping,plating, and heading became deafened to speechat more than 3 feet after 20 years' exposure, whereasof those engaged in worming and turning, only oneout of 13 of the former and none out of 24 of thelatter are deafened to a like degree. In theheading section four out of 14 men were so deafened.Although the size of the samples in each of these

groups is small it would appear that the risk ofdamage to hearing is considerably greater in theheading section than in the worming section. Thenoise level measurements in the heading, worming,and turning sections vary from 100 to 112 phons.It would appear therefore that noises of an intensityof 100 to 112 phons represent the border linebetween that which is harmless to hearing and thatwhich is harmful to an appreciable proportion ofsubjects.

It is therefore suggested that it is only when theoverall noise level exceeds the region of 105 to 108phons that serious damage to hearing is likely tooccur in an appreciable proportion of the populationexposed. Noise less intense than 105 phons willprobably cause damage only to the hearing of themost susceptible subjects.

Prevention of Occupational Deafness.-Mentionhas already been made of the limited possibilitiesof the reduction of noise at its source or by reflectionor reverberation. Where reduction to harmlesslevels cannot be effected by these means, the ears ofthe individual worker must be protected. Further-more, the degree of protection must be related to theintensity of the noise level in which he works andthe susceptibility of the particular individual.

When the intensity of the noise is more than 105to 108 phons, protective devices should be worn byall workers. As pointed out by Perlman (1945) adevice which gives an insulation of 30 db whenworn in an ambient noise of 120 db wili probablyreduce the noise reaching the cochlea by 30 db,e.g. to 90 db. An additional protection of, say,15 db will be required for safety. Hence a protec-tive device with an insulation value of 30 db willbe adequate for the worker in a noise of 110 db butwill fail to give adequate protection at a highernoise level of 130 db.The average workman is notoriously careless in

the use of safety devices, particularly where theseare provided to prevent uncommon accidents ordisease of delayed onset. Thus MacLaren andChaney (1947) found that of a group of workmenprovided with ear defenders, only one third usedthem regularly, one third used them occasionally,and one third discarded them soon after issue.Education of the workman on the risks involvedand official recognition by legislation woulddoubtless effect improvement.Any form of ear defender worn by the worker

whose hearing is very susceptible may be inadequateto prevent loss of hearing if he works in veryintense noise. Furthermore, such a susceptibleindividual working in a source of noise of less than105 phons (but probably of more than 90 phons)will, if he is to. continue in such work, requireprotection. To discover such susceptible indivi-duals in any occupational group at risk beforepermanent loss of hearing results, it will be necessaryto devise a reliable pre-emplovment test for sus-ceptibility. Failing this, such individuals shouldbe weeded out in the earliest stages of their deafnessbefore permanent loss of hearing results. Theil-gaard (1949) suggests that previously describedtests are likely to be unreliable, so, for the present,reliance must be placed on the latter method.Grove (1949) presents the views of the Sub-

Committee on Industrial Noise of the Committeeon Conservation of Hearing set up by the AmericanAcademy of Ophthalmology and Otolaryngology.He recommends that every workman engaged in ajob where the noise level exceeds 90 db should havea pre-employment audiometric examination and bere-tested after one week and subsequently at statedintervals. Any workman who complains of tinnitusafter working in a noisy environment should havehis hearing re-tested. If on any of these re-tests itis found that an average loss of 10 db at 3,000 to6,000 c.p.s. has occurred, he should be forced towear a protective device or be transferred to a lessnoisy job.

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SummaryA report is presented on an investigation into the

hearing of 219 men and women employed in severalnoisy engineering trades. Twenty-eight workerswere found to be suffering from bilateral ear diseaseor deafness not due to noise. Of the remaining191, all except one man were found to have hearinglosses due to noise which could be demonstratedeither by speech or pure tone audiometric tests.The tests were carried out at the factories duringworking hours.

After 20 or more years' exposure to the noiseof their work, nine out of 12 chippers and riveters,seven out of 11 stampers (drop forgers), six out of12 boiler plate fitters, and four out of 14 headingmachine operators who were examined were

unable to hear speech at more than 3 feet distancein either ear.

Reference is made to noise level measurementsobserved by the Physics Department of the NationalPhysical Laboratory in the factories where thehearing tests were carried out. Figures from theirreport suggest that only when the overall noiselevel exceeds 105 to 108 phons is serious damagelikely to be caused to the hearing of an appreciableproportion of the workmen. Noise less intensethan 105 phons will probably cause damage to the

hearing of only a small group of individuals mostsusceptible to injury.

Observations are also made on the recovery ofhearing after a period of rest from work, thephenomen of adaptation of hearing for speech inambient noise, and other subjective effects of noise.

I am indebted to Mr. Terence Cawthorne for hisadvice throughout the investigation; to Dr. DonaldStewart for advice and for arranging the field trials;to Dr. W. Jeaffreson Lloyd of Guest Keen and NettlefoldsLtd; and to the Directors and Staff of Messrs. EdwinDanks Ltd, John Garrington and Sons Ltd, and GuestKeen and Nettlefolds Ltd.

REFERENCESBunch, C. C. (1937). Laryngoscope, St. Louis, 47, 615.

(1942). J. Amer. med. Ass., 118, 588.Causse, R., and Falconnet, P. (1947). Ann. Oto-laryng., Paris, 64, 436.Fosbroke, J. (1830-31). Lancet, 1, 533, 645, 740, 777, 823.Fowler, E. P., Sr. (1928). Arch. Otolaryng., Chicago, 8, 151.Fowler, E. P., Jr. (1940). Acta oto-laryng., Stockh., 28, 283.Grove, W. E. (1949). Industr. Med., 18, 25.Larsen, B. (1939). Acta oto-laryng., Stockh., Suppl. No. 36.MacLaren, W. R., and Chaney, A. L. (1947). Industr. Med., 16, 109.Nussdorfer, R. (1942). Oto-rino-laring. ital., 12, 431.Perlman, H. B. (1945). Ann. Surg., 122, 1086.Peyser, A. (1943). Acta oto-laryng., Stockh., 31, 351.Ruedi, L., and Furrer, W. (1946). J. acoust. Soc. An,er., 18, 409.Taylor, H. M. (1947). Laryngoscope, St. Louis, 57, 137.Theilgaard, E. (1949). Acta oto-laryng., Stockh., 37, 347.

(1951). Investigation in Auditory Fatigue in Individuals withNormal Hearing and in Noise Workers (Weavers). Thesis.Munksgaard, Copenhagen.

Weiss, J. A. (1947). Ahn. Otol., St. Louis, 56, 175.

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studyof occupational deafness* · smaller losses in the other regions of the auditory spectrum,...

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