Behavioral Techniques in Audiology and Otology
Presbycusis and Noise-Induced Hearing
Loss*
Ulf Rosenhall; Kai Pedersen; Alvar Svanborg Departments of Audiology and Otology, Sahlgren’s Hospital [U.R., K. P.] and Department of Geriatric and Long-term Care Medicine, Vasa hospital [A. S.], University of Gothenburg, Sweden; and University of Illinois, Department of Medicine, Section of Geriatric Medicine [A. S.], Chicago, Illinois
ABSTRACT In a longitudinal and an age cohort comparing study the influence of aging and occupational noise exposure on hearing sensitivity was studied. The participants of the e studied at 70, 75, and 79 years of hoi Pen exposed to occupational noise age. had 10 to 15 dB poorer hearing in the high frequency range than nonexposed men. The difference in hearing acuity decreased with increasing age. The differences between exposed and nonexposed older persons was no P longer significant at age 79. In women there were-no* differences in hearing sensitivity between those exposed to noise and those not exposed to noise$Men not exposed to noise had 10 to 15 dB poorer hearingat 4 kHz compared with women of the same age also not exposed to noise.
THE DECLINE IN HEARING sensitivity caused by the aging processes at different levels of the auditory system is called presbycusis. It is, however, difficult to differentiate between hearing loss due to aging and hearing loss due to extraneous factors such as noise exposure, ototoxic drugs, infections, and other precursors.
Exposure to excessive noise is one of the more common etiological factors which can cause hearing loss especially in modern industrialized countries (Glorig & Nixon, 1960).Occupational noise is the most important source, but noise exposure related to military service or leisure time activities are also of importance. Men are more often exposed to harmful noise than women. In many industrialized countries, for example, the U.S.A. and Sweden, noise-induced hearing loss (NIHL) is a common reason for claiming occupational disability compensation. NIHL can be found in persons of differEar and Hearing, Vol. 11, No. 4, 1990
ent age groups; above all in adults but also in adolescents (Axelsson, Jerson, Lindberg, & Lindgren, 198 1). The influence of NIHL on the hearing capacity in old age is a topic of considerable interest. In the classical study regarding the hearing of members of the Mabaan tribe in southern Sudan, Rosen et a1 (1962) concluded that presbycusis is caused by exposure to noise during many decades. Members of another isolated tribe, the Todas in India, were also reported to have remarkably unaffected hearing in old age (Kapur & Patt, 1967). Rosen, Plester, El-Mofty, & Rosen (1964) coined the term “socio-acusis” emphasizing their opinion that presbycusis, so commonly seen in industrialized countries, has extraneous causes. According to this concept noise is the major etiological factor causing presbycusis. However, another factor, the marked population homogenity, could explain the stability of hearing in these isolated populations (Bergman, 1966). The aim of the present investigation was to study age-related changes in hearing in a representative sample of 70 year olds followed longitudinally and to evaluate the effect of protracted occupational noise exposure on the hearing at old age. The study also included a comparison of two 70 year old cohorts born with a 5 year interval. The study was camed out in Gothenburg, Sweden. Gothenburg has a population of 430,000 and is the center of an urban area of altogether about 700,000 inhabitants. It is an industrialized city with heavy mechanical industries, the largest being the Volvo automobile factory and a manufacturer of ball bearings. Earlier, three shipyards were found in Gothenburg. STUDY POPULATION AND METHODS
The longitudinal study of 70 year old people in Gothenburg (H 70) started in 1971-1972 (Rinder, Roupe, Steen, & Svanborg, 1975). The design procedures and representativity of the samples (3/10 of the 70 year olds) have been recently reviewed (Svan-
* The study was
made possible by grants from Hjalmar Svensson’s Foundation; Loo and Hans Osterman’s Foundation; the Swedish Ministry of Health and Social Affairs, Commission for Social Research; the Swedish Medical Research Council; the Gothenburg Medical Services Administration; the Gothenburg Administration of Social Services and Wilhelm and Martina Lundgren’s Foundation.
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0 196/0202/90/1104-0257$02.00/0 Ear and Hearing Copyright 0 1990 by The Williams & Wilkins Co. Prinred in U.S.A.
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Table 1. Number of participants, women and men, who answered the questionnaire regarding occupational noise at age 70 (F 06 cohort) and at age 75 (F 01 cohort) and who had been tested with pure-tone audiometry for cohort F 01 at 70, 75, and 79 years of ages and cohort F 06 at age 70. The numbers and percentages of individuals not exposed to noise, those exposed to noise for 1 to 15 years, and those exposed to noise for 15 years or more are given separately. Eleven men and 9 women belonging to the F 01 cohort were tested at age 70, answered the questionnaire at age 75, but were not tested again at that age.
Noise Exposure 1-14 Years
No Noise Exposure
Noise Exposure 215 Years
Total
Cohort
Age years
Men
Women
Men
Women
Men
Women
Men
Women
F 01
70 75 79
79 57% 7257% 39 56%
117 78% 10977% 73 82%
14 10% 1411% 10 14%
18 12% 1713% 7 8%
44 32% 4032% 21 30%
15 10% 1511% 9 10%
137 126 70
150 141 89
F 06
70
58 46%
145 85%
12 10%
13 8%
56 44%
13 8%
126
171
~~
~
Table 2. Correlation coefficients (r) and significance levels (s) between exposure to occupational noise expressed as a continuous variable and hearing acuity at the frequencies 0.25, 0.5, 1, 2, 4, and 8 kHz for men belonging to the F 01 cohort (ages 70, 75, and 79 years) and the F 06 cohort (age 70 years).
F 01 75 Years
70 Years
F 06
79 Years
70 Years
Freq. kHz
L
R
L
R
L
R
L
R
r s
r s
r s
r s
r s
r s
r s
r s
0.25 0.5 1 2 4 8
0.20' 0.19' 0.28.4 0.22' 0.21' 0.16 NS
0.16 NS 0.19* 0.30**' 0.280.29*** 0.20*
0.22' 0.27" 0.38**' 0.30*" 0.25*' 0.20'
0.19' 0.24' 0.28' 0.28" 0.26' 0.10 NS
0.17 NS 0.23 NS 0.17 NS 0.16 NS 0.21 NS 0.20 NS
0.00 NS 0.04 NS 0.06 NS 0.13 NS 0.20 NS 0.15 NS
0.14 NS 0.18** 0.19 NS 0.24 NS 0.27'* 0.10 NS
0.07 NS 0.09 NS 0.11 0.24" 0.28** 0.23**
* p < 0.05. " ' p < 0.07. " * p < 0.007. L, Left ear; R, right ear; NS, not significant.
~~~
~
Table 3. Pure-tone threshold values, left ear (medians and loth, 25th, 75th, and 90th percentiles) for men belongingto the F 01 cohort, at ages 70, 75, and 79 years. N-N, Subjects not exposed to noise; N, subjects exposed to noise for 15 years or more. The audiometrical thresholds are measured in 5 dB steps and not as a continuous variable. The medians and percentiles are therefore estimated from 5 dB intervals with the midpoint of each interval at respective test level.
0.25 kHz Age, years
70
75
79
258
0.5 kHz
1 kHz
2 kHz
4 kHz
8 kHz
Percentile
N-N
N
N-N
N
N-N
N
N-N
N
N-N
N
N-N
N
10 25 50 75 90 10 25 50 75 90 10 25 50 75 90
7.4 10.2 15.5 21.4 28.9 8.4 11.4 16.4 23.9 31.0 14.0 19.8 27.9 35.2 41.0
9.7 13.0 18.9 27.5 41.5 10.5 15.4 22.5 32.5 52.5 18.9 22.7 27.1 44.4 62.0
5.3 9.2 15.0 21.8 30.1 5.1 9.6 14.5 21.3 32.3 12.4 16.6 23.6 30.5 39.3
7.8 12.5 17.5 31.3 49.0 8.8 14.6 21.3 42.5 60.0 15.1 19.1 26.7 42.1 67.0
6.1 9.2 13.3 20.8 30.6 6.5 10.7 17.1 25.8 36.0 12.4 17.3 25.0 32.9 43.0
5.5 13.1 20.0 42.5 59.0 10.8 20.0 30.5 53.8 67.5 10.1 15.6 28.8 49.4 64.8
8.7 15.3 23.4 35.3 52.7 9.0 19.2 33.9 48.3 62.0 23.6 32.1 43.3 53.8 61.1
8.9 16.3 32.5 54.5 71.8 15.0 30.8 53.8 67.5 75.8 13.0 35.6 50.0 71.9 77.3
22.4 42.1 53.8 63.6 71.6 29.7 46.3 58.1 69.2 77.3 42.3 54.7 65.6 72.1 82.8
31.0 48.8 65.6 76.7 82.2 40.0 57.5 70.0 80.5 92.5 50.8 64.0 71.0 76.8 96.8
33.6 53.3 64.3 75.2 109.2 47.8 60.0 69.7 79.8 91.0 51.8 64.8 79.7 95.3 99.3
43.2 57.5 73.5 87.5 110.1 47.5 67.5 82.5 90.5
Rosenhall et al.
68.1 73.5 91.3 98.6 100.4
Ear and Hearing, Vol.11, No. 4, 1990
borg, 1988). Comparison of participants and nonparticipants showed that still at age 8 1, the participants were representative for the population at that age (Nilsson-Ehle et al, 1988). Two groups (cohorts) of elderly persons were systematically selected by day of birth from the Internal Revenue Register. The first cohort, F 01, consisted of 1148 persons born in 1901-1902. The second cohort, F 06, consisted of 1281 persons born in 1906-1907. From each cohort a subsample consisting of 2/5 of all the participants was selected systematically for evaluation of hearing. These subjects were tested at age 70 in 197 1- 1972 and 1976- 1977, respectively, for the cohorts. The participants of the F 01 cohort were followed longitudinally and tested at ages 70, 75, and 79. From the F 01 cohort 197 women and 180 men were tested at age 70, 141 women and 125 men at age 75, and 87 women and 70 men at age 79. From the F 06 cohort 171 women and 125 men were tested at age 70. The number of nonresponders was low (Mdler, 198 1 ; Pedersen, Rosenhall, & Mdler, 1989). The participants of the study were tested with pure-tone audiometry using standard test procedures at each test session (Pedersen et al, 1989). All the participants were interviewed regarding exposure to occupational noise at age 75 (F 01) and at age 70 (F 06). This part of the study emanated from a comprehensive questionnaire describing the vocational situation during the participants' active years before retirement. The enquiry explored the type of occupation, exposure to stress and different chemicals, and exposure to noise. The estimated duration in years of the noise exposure was asked for. The enquiry regarding noise exposure was compared with the professions of the participants. Seventeen noise-exposed occupations, for example, sheet-metal work, engineering industry, and shipbuilding were considered as heavily noise-exposed. A fair level of correlation ( r : 0.40) was found between noise-exposed occupations and reported exposure to noise. In addition, exposure to extraneous toxic factors, for example, smoking and alcohol as well as a large number of health factors, were also correlated to auditory function. The results of this study will be reported elsewhere (Rosenhall, Sixt, Sundh, & Svanborg, unpublished data). Each cohort was divided into three groups: participants who had not been exposed to noise, those who had been exposed to noise from 1 to 15 years, and those who had been exposed to noise for 15 years or more. The numbers of men and women who had answered the questionnaire and who had been exposed to noise of different duration are given in Table L. Comparisons were made between participants who had not been exposed to noise and participants who had been exposed to noise for 15 years or more. Rightlleft ear differences were studied. Furthermore, comparisons were made between men and women not exposed to noise. Possible age cohort differences were analyzed at age 70. STATISTICAL METHODS
A two-sided Pitrnan permutation test was used to test the statistical significance of carrelations between hearing acuity and exposure to nose expressed as a continuous variable (Cox & Hinkley, 1974); p < 0.05 was considered as statistically significant. RESULTS
The results of the questionnaire regarding occupational noise exposure are shown in Table 1. ApproxiEar and Hearing, Vol. 11, No. 4, 1990
1 "t 8 kHz
dB
20
--
40
--
60
--
80
-1
A
,
I
I
I
I
8
I
8 kHz
dB
8
dB
5 8 kHz
20
--
-_
40
--
-_
60
--
_-
80 --
__
C I
I
I
I
Figure 1. Median pure tone thresholds (dB HL) for men belonging to the F 01 cohort (born 1901-1902), left ear. (x), Men not exposed to occupational noise; (O), men exposed to occupational noise for 15 years or more. a, Age 79; b, age 75; c, age 79.
Presbycusis and NlHL
259
mately half of the men who reported occupational noise exposure had been exposed for long periods (515 years). The men born in 1901-1902 were more often exposed to noise than those born in 1906-1907. Men who had been exposed to occupational noise for 15 years or more had significantly poorer highfrequency hearing than those who had not been exposed to noise at age 70 and 75 (Tables 2 and 3 ) . The median pure-tone thresholds for men belonging to the F 01 cohort with prolonged noise exposure and without noise exposure at all are shown in Figure 1, a to c. At age 75, when the enquiry was performed, the differences between the two groups were considerable, 10 to 20 dB at 2 to 4 kHz (Fig. lb; Tables 2 and 3 ) . Moreover, the men who had been exposed to noise had significantly poorer hearing at 0.25 kHz and at 8 kHz than the nonexposed men (Table 2). At age 70 the difference between the groups was about 10 dB at 2 to 8 kHz (Fig. la; Table 3). At age 79 the differences between the groups were less pronounced, especially for the left ear (Fig. lc; Table 3 ) . At this age there were no significant differences in hearing acuity between noiseexposed men and men without exposure. The auditory function for 70 year old men belonging to the F 06 cohort are shown in Figure 2 and in Table 4. The men who had been exposed to noise had 10 to
15 dB more pronounced hearing losses at 2 to 4 kHz than those who had not been exposed to noise. Those men of both cohorts who had been exposed to noise for 1 to 15 years had pure-tone thresholds which were found between the other two groups. Only few women had been exposed to noise according to the questionnaire (Table I). There were no significant differences in pure-tone thresholds between women not exposed to noise and those who had been exposed to noise for 15 years or more (Fig. 3). A comparison was made between men and women without occupational noise exposure. For both cohorts the men had significantly poorer high-frequency hearing than the women. The difference was 10 to 15 dB at 4 kHz and 3 to 10 dB at 8 kHz (Figs. 4, a-c, and 5 ; Table 5). There were no threshold differences between the right and the left ears in any of the cohorts at any of the ages studied both for exposed and nonexposed men. No significant difference was observed between the two cohorts at age 70. DISCUSSION
Exposure to noise is a very important extraneous noxious factor involved in hearing loss in old age (Glo-
s t "t
8 kHz
8 kHz
dB O 20
dB
~
t
.Ot-
40
--
60
--
60
40
8o
i
--
t
t
Figure 2. Median pure tone thresholds (dB HL) for men belonging to the F 06 cohort (born 1906-1907) left ear. (x), Men not exposed to noise; (O),men exposed to noise 15 years or more.
80
I t
t
Figure 3. Median pure tone thresholds (dB HL) for women belonging to both the F 01 and the F 06 cohorts at age 70, left ear. (x), Women not exposed to noise; (O),women exposed to noise for 15 years or more.
Table 4. Pure-tone threshold values, left ear (medians and loth, 25th, 75th, and 90th percentiles) for men belongingto the F 06 cohort, at age 70 years. N-N, Subiects not exposed to noise; N, subjects exposed to noise for 15 years or more.
0.25 kHz Age, years
Percentile
N-N
0.5 kHz
2 kHz
1 kHz
4 kHz
8 kHz
N
N-N
N
N-N
N
N-N
N
N-N
N
N-N
N
9.1 13.4 17.5 26.3 41.0
4.2 8.0 12.5 20.0 28.5
6.3 10.7 17.5 26.7 39.8
2.8 6.5 14.7 23.4 31.2
3.6 8.9 19.2 37.5 48.2
5.7 10.8 25.4 40.6 54.0
8.3 19.5 41.5 55.5 65.5
17.3 31.7 49.6 66.3 76.5
35.5 56.3 64.6 73.5 78.2
31.5 48.1 65.8 79.4 89.0
41.5 59.5 69.8 76.9 88.0
~~~
70
260
10 25 50 75 90
Rosenhall et al.
5.2 9.5 14.8 21.3 30.5
Ear and Hearing, Vol.11, No. 4, 1990
dB
0.25
0.5
1
2
4
I
I
I
I
I
8 kHz I
dB 0
t
t
20
40
60
80
dB 0
tI A s
I
t I
I
I
t
1
;
I
I
I
I
1
Figure 5. F 06 cohort. Median pure tone thresholds (dB HL) for women (x)and men (0)not exposed at the age 70, left ear.
3 8 kHz
t
Table 5. Gender differencesfor pure-tone thresholds at 4 and 8 kHz for individuals not exposed to occupational noise. The women have better hearing capacity in the high frequency range than the men. The differencesin dB are aiven (DI and also the levels of sianificance. 70 years
20
L Freq. _ _ kHz D s
40
FO1 F 06
60
4 8 4 8
R ~
75 years
R
L
-
~
D s
D s
15.0**' 12.8"' 11.1"' 10.5**' 6.6* 4.9' 12.8*** 11.8*'* 8.7** 3.0 NS
D
79 years L __ s D s
12.2" 2.7'
R ~
D s
9.9" 11.5' 9.6NS 4.7 NS
< 0.05. * * p < 0.07. * * * pc 0.007. L, Left ear; R, right ear; NS, not significant. *p
80
B
t
dB 0
20
T
40
60
80
Figure 4. F 01 cohort. Median pure tone thresholds (dB HL) for women (x) and men (0)not exposed to noise, left ear, a, Age 70 years; b, age 75 years; c, age 79 years.
Ear and Hearing, Vol.11, No. 4, 1990
rig & Nixon, 1960). In the present study only a crude estimation of exposure to occupational noise and its duration was possible in retrospect. Exposure to other types of noise, for example, military or leisure-time noise, could not be estimated in this way. There was nevertheless a very clear difference in the high-frequency hearing for exposed and nonexposed men at ages 70 and 75. The auditory thresholds were considerably better, not only in the high frequency range (48 kHz) but also at 2 kHz in the nonexposed subjects. The same tendency was also observed, but to a lesser extent, in the low-frequency area. There were no or only insignificant differences in hearing between cohorts F 0 1 and F 06. The exposition to occupational noise was, however, different between the cohorts. The men belonging to the F 06 cohort had more often been exposed than those of the F 0 1 cohort. Acoustic overstimulation causes damage above all to the frequencies 4 to 8 kHz. The hearing loss reported here in the noise-exposed groups affected a wider frequency range than what can be anticipated caused by noise damage alone. It is therefore possible that other Presbycusisand NlHL
261
extraneous factors interact with noise exposure. One such plausible factor is smoking. In the same study population, smokers have more pronounced hearing loss than nonsmokers (Rosenhall et al, unpublished data). There is, however, a strong correlation between smoking habits and exposure to occupational noise. In the longitudinal study of the entire study population the hearing deteriorated gradually from age 70 to age 81 (Pedersen et al, 1989). In the group of men previously in their life exposed to long-lasting noise the hearing deteriorated much more from age 70 to age 75 at 2 kHz than in those not exposed to professional noise (about 20 and 10 dB, respectively). This further supports the concept that socioeconomic factors other than exposure to noise are of important in the group of males with a history of occupational noise. Another explanation is that the groups which were tested at age 70 and 75 were not completely identical. A number of subjects who answered the questionnaire were not tested at age 75, although they had been tested at age 70. At age 79, the hearing acuity in the high-frequency range (4-8 kHz) was without significant differences in the heavily exposed and not exposed groups. For the left ear there were no observed differences between the groups and for the right there were only minor differences at the frequencies 2 and 4 kHz (Figs. lc and 2). The deleterious effects of noise thus becomes less apparent in advanced age; presbycusis eventually catches up with NIHL. IS0 7029 (1984) and Robinson (1988) describe auditory threshold values from otologically normal persons with no history of undue noise exposure (data base A). A comparison between males from I S 0 7029 ( 1984) and men not exposed to noise in the present study shows that the I S 0 values were better than those presented here. These differences were found in the entire frequency range, but were most pronounced at the frequencies 2 and 4 kHz (from 6 dB to a maximum of 17 dB). One explanation for this discrepancy can be different selection criteria. The populations might also differ concerning nonoccupational noise exposure, incidence of infectious diseases, socioeconomic factors ( e g , smoking), and genetic factors. In ISO/DIS 1999 (1982) examples of noise induced permanent threshold shifts as a function of exposure time and daily noise exposure levels were given. An exposure duration of 20 to 40 years and a daily noise exposure level of 90 dB (A) gave a NIHL of 10 to 15 dB at 4 and 8 kHz according to ISO/DIS 1999. This figure is in accordance with the differences between nonexposed and exposed men at 4 and 8 kHz observed here. In contrast to other studies we have, however, also found a small discrepancy in hearing between the two groups in the low and middle frequencies. In the present study no difference between the right and left ears were observed in contrast to other studies in which the left ear had a more pronounced hearing loss that the right one (Baughn, 1966; Rudin, Rosen262
Rosenhall et al.
hall, & Svardsudd, 1988). A right/left ear difference possibly disappears in old age when presbycusis becomes more prominent. There were no differences in hearing acuity between women not exposed to noise and those exposed to noise. Even when test results from 70 year old women from both cohorts were pooled together no differences could be discerned (Fig. 3). A gender difference concerning susceptibility to noise can be one possible explanation for this observation. Another explanation might be that noise levels in factories where women worked were lower than in factories where men worked since a pronounced occupational segregation between the genders existed. According to Spoor (1967) and IS0 7029 (1984), over the age of 40 women have somewhat better highfrequency hearing than men of the same age. Both reports were based on studies of strictly selected subjects not exposed to noise and without history of middle-ear disease. In the present study the same tendency was seen in the F 0 1 cohort. The difference was 8 to 15 dB at 4 kHz, figures which are similar than those presented by Spoor ( 12- 15 dB) but somewhat smaller than those of IS0 7029 (1 9 dB). We found only a minor difference between the genders at 8 kHz, in contrast to the studies cited above. REFERENCES Axelsson A, Jerson T, Lindberg U, and Lindgren F. Early noise induced hearing loss in teenage boys. Scand Audio1 1981 ;10:9196. Baughn W. Noise control-percent of population protected. Int Aud 1966;5:33 1-338. Bergman M. Hearing in the Mabaans. Arch Otolaryngol 1966;84:411-4 15. Chung D, Willson G, Gannon R, and Mason K. Individual susceptibility to noise. In Hamernik R, Henderson D, and Salvi R, Eds. New Perspective on Noise-Induced Hearing Loss. New York Raven Press, 1982: 511-519. Cox D, and Hinkley D. Theoretical Statistics. London: Chapman and Hall, 1974. Glorig A, and Nixon J. Distribution of hearing loss in various populations. Ann Otol Rhino1 Laryngol 1960;69:497-5 16. I S 0 Acoustics-Determinations of occupational noise exposure and estimation of noise-induced hearing impairment. Draft International Standard ISO/DIS 1999. International Organization for Standardization. Geneva, 1982. IS0 Acoustics-Threeshold of hearing by air conduction as a function of age and sex for otologically normal persons. IS0 7029. International Organization for Standardization. Geneva, 1984. Kapur YP, and Patt AJ. Hearing in Todas of South India. Arch Otolaryngol 1967;85:400-406. Mdler MB. Hearing in 70 and 75 years old persons: Results from a cross sectional and longitudinal population study. Am J Otolaryngo1 1981;2:22-29. Nilsson-Ehle H, Jagenburg R, Landahl S, Svanborg A, and Westin J. Haematological abnormalities and reference intervals in the elderly. A crossectional comparative study of three urban Swedish population samples aged 70, 75 and 81 years. Acta Med Scand 1988;224:596-604. Pedersen K, Rosenhall U, and Mdller MB. Changes in pure tone thresholds in individuals aged 70 to 8 1. Results from a longitudinal study. Audiology 1989;28:194-204. Rinder R, Roupe S, Steen B, and Svanborg A. Seventy year-old people in Gothenburg. A population study in a industrialized Swedish city. Acta Med Scand 1975;198:397-407.
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Robinson DW. Threshold of hearing as a function of age and sex for the typical unscreened population. Br J Audiol 1988;22:5-20. Rosen S, Bergman M, Plester D, El-Mofty A, and Satty M. Presbycusis study of a relatively noise-free population in the Sudan. Ann Otol Rhino1 Laryngol 1962;71:727-742. Rosen S, Plester D, El-Mofty A, and Rosen H. High frequency audiometry in presbycusis. Arch Otolaryngol 1964;79:18-32. Rudin R, Rosenhall U, and Sviirdsudd K. Hearing capacity in samples of men from the general population. The study of men born in 1913 and 1923. Scand Audiol 1988;17:3-10. Spoor A. Presbycusis values in relation to noise induced hearing loss. Int Aud 1967;6:48-57. Svanborg A. The health of the elderly population results from longitudinal studies with age-cohort comparisons. Research and the
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ageing population. Chichester: Wiley (Ciba Foundation Symposium 134) 1988:3-16.
Acknowledgements: This study emanates from the gerontological and geriatric population study in Gothenburg, Sweden. Project leader: before 1988, Alvar Stenborg; from 1988, Bertil Steen. Address reprint requests to: Ulf Rosenhall, M.D., Department of Audiology and Otolaryngology, Sahlgren’s Hospital, S-4 I3 45 Gothenburg, Sweden. Received May 29, 1989; accepted January 22, 1990.
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