| [Download PDF]
|Year : 2005 | Volume
| Issue : 28 | Page : 1--15
The Nord-Trondelag Norway Audiometric Survey 1996-98 : Unscreened thresholds and prevalence of hearing impairment for adults > 20 years
HM Borchgrevink1, K Tambs2, HJ Hoffman3,
1 Rikshospitalet University Clinic, Oslo, Norway
2 Norwegian Institute of Public Health, Division of Epidemiology, Oslo, Norway
3 Epidemiology and Biostatistics, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Bethesda, MD, USA
H M Borchgrevink
The Research Council of Norway, P.O. Box 2700 St Hanshaugen, 0131 Oslo
As supplement to a general health screening examination (HUNT-II), we conducted a pure-tone audiometry study in 1996-98 on adults ( > 20 years) in 17 of 23 municipalities in Nord-Trondelag, Norway, including questionnaires on occupational and leisure noise exposure, medical history, and symptoms of hearing impairment. The study aims to contribute to updated normative hearing thresholds for age and gender, while evaluating the effects of noise exposure, medical history, and familial or genetic influences on hearing. This paper presents the unscreened hearing threshold data and prevalence of hearing impairment for different age groups and by gender.
Valid audiometric data were collected from 62% (n=50,723) of 82,141 unscreened invited subjects (age-range 20-101 years, mean=50.2 years, SD=17.0 years). Two ambulant audiometric teams each conducted 5 parallel self-administered, pure-tone hearing threshold examinations with the standard test frequencies 0.25-0.5-1-2-3-4-6-8kHz (manual procedure when needed). Tracking audiometers were used in dismountable booths with in-booth noise levels well within ISO criteria, except being at the criterion around 200 Hz. The data were electronically transferred to a personal computer. Test-retest correlations for 99 randomly drawn subjects examined twice were high.
The mean thresholds recorded were some dB elevated from «DQ»audiometric zero«DQ» even for age group 20-24 years. As also found in other studies, this might indicate too restrictive audiometric reference thresholds. Males had slightly better hearing < 0.5kHz for all age groups. Mean thresholds were poorer in males > 30 years from > 2kHz, with maximal gender differences of ~20dB at 3-4kHz for subjects aged 55-74 years. Weighted prevalence data averaged over 0.5-1-2-4kHz showed hearing impairment > 25dB hearing threshold level of 18.8% (better ear) and 27.2% (worse ear) for the total population - for males 22.2% and 32.0%, for females 15.9% and 23.0%, respectively. Mean hearing loss > 10dB at 6kHz registered for both genders even in age groups 20-24 years may be partly due to calibration artefacts, but might possibly also reflect noise-related socioacusis.
|How to cite this article:|
Borchgrevink H M, Tambs K, Hoffman H J. The Nord-Trondelag Norway Audiometric Survey 1996-98 : Unscreened thresholds and prevalence of hearing impairment for adults > 20 years.Noise Health 2005;7:1-15
|How to cite this URL:|
Borchgrevink H M, Tambs K, Hoffman H J. The Nord-Trondelag Norway Audiometric Survey 1996-98 : Unscreened thresholds and prevalence of hearing impairment for adults > 20 years. Noise Health [serial online] 2005 [cited 2020 Oct 21 ];7:1-15
Available from: https://www.noiseandhealth.org/text.asp?2005/7/28/1/31632
Normative age-related hearing thresholds are needed as reference for the definition of normal hearing and for identification of presence and degree of hearing loss. Knowledge about normal hearing threshold levels and deterioration by age may also contribute to our understanding of potential hazards to hearing - - a prerequisite for preventive initiatives. Hearing impairment continues to be the most prevalent disability in Western societies (Davis, 1997; Wilson et al., 1999). It is estimated that 11% of the total population in the US and Europe are at risk for hearing loss from noise (Alberti, 1998). The National Institute of Occupational Safety and Health (NIOSH), USA, rates noise induced hearing loss among the top ten work-related problems, involving at least 11 million workers in the US (Ishii and Talbott, 1998). Compensation for acquired occupational hearing loss is estimated by comparing with normative thresholds for age and gender. Thus, incorrect normative hearing thresholds may have major consequences for society, industry, and affected individuals.
"True" normative age-related hearing thresholds have been difficult to establish. The reason is that hearing thresholds collected in population studies run the obvious risk of being "contaminated" by hearing loss from factors such as occupational noise, societal noise (socioacusis) and disease/trauma (nosoacusis). The usual approach has been to compare hearing threshold data from otologically screened and unscreened populations. However, screening to eliminate risk factors may implicitly eliminate the poorer part of the normal population, leading to a too good normative hearing threshold for age and consequent overestimation of acquired hearing loss in an individual. Conversely, norms based on thresholds from an unscreened population may underestimate the magnitude of an acquired hearing loss and thus leave effects of risk exposure undetected.
Current international standards partly define normal hearing in relation to "audiometric zero" (ISO 389, 1994), partly as normative hearing thresholds as a function of age and gender relative to a group of persons 18 years of age for an otologically highly screened population free of any known environmental factors (ISO 7029, 1984 - which is identical to database A of ISO 1999, 1990). This gives the basis criteria for estimation of hearing threshold impairment due to noise for each age and gender. In addition there are databases from populations unscreened except for occupational noise exposure, e.g. database B of the ISO 1999 (1990) based on US data. ISO 1999 recommends the use of nationally based values for unscreened populations. If ISO A is applied as reference, a greater part of a registered threshold shift would of course be attributed to acquired hearing loss and e.g. lead to greater compensation for occupational hearing loss - compared with the ISO B reference. The above standards are based on population studies with data collected several decades ago and are often criticized for being outdated and not based on representative populations (for further discussion, see Engdahl et al. 2005).
In audiometric surveys from various countries the median hearing threshold levels of any group of 18-20 year olds are typically not zero dB - as would be expected - but on the order of + 5 dB for most frequencies 0.25-8 kHz and with higher values (e.g., 13.5 dB) for 6 kHz (discussed by Ward, 1990; 1993). For example, in UK, adults 18-30 years show 4.3 dB poorer thresholds than "audiometric zero" (ISO 389, 1994) for the average of 0.5, 1,2, and 4 kHz with even greater shifts for 0.25, 6 and 8 kHz (Lutman and Davis, 1994). Corresponding findings have been reported e.g. by Robinson et al. (1981) and Buren et al. (1992). The "audiometric zero" (normal threshold for young adults) may thus be too restrictive (Smith et al, 1998). Or, it might reflect hearing deterioration even in young subjects with little or no occupational noise exposure, indicating socioacusis. If met by revision of normative thresholds to new, less restrictive norms, this would conceal the possible effect of socioacusis, fail to elicit preventive initiatives, and move the normative threshold away from the "true" valid threshold.
The present study aims to provide recent audiometric reference data from a large Caucasian population sample as basis for evaluation of current normative standards, and to contribute to updated normative hearing thresholds and prevalence of hearing impairment for age and gender while evaluating the effects of noise exposure, medical history, and familial or genetic influences on hearing. This paper presents the unscreened hearing threshold data and prevalence of hearing impairment for different age groups and by gender.
A pure-tone audiometry study was conducted from January 1, 1996 through February 1998 on invited unscreened adults aged > 20 years in 17 of 23 municipalities in the county of Nord-Trondelag, Norway - a rural county with a rather stationary population, scattered farms, some forestry and five small towns, some with heavy (e.g., paper mill) industry. The audiometry was performed as part of a general health screening, the Nord-Trondelag Health Survey (HUNT-II). The invitation lists for HUNT-II were based on the continuously updated electronic public address registry provided by the governmental Statistics Norway. The attending subjects gave informed consent for electronic collection and storage of audiometric data. A questionnaire (Q1) on occupational and leisure noise exposure, medical history and symptoms of hearing impairment was included. A second, more detailed, questionnaire (Q2) was applied in a nested case-control study on a subset of participants. (Questionnaire issues, socioeconomic circumstances and the case-control study are not dealt with in the present paper.)
Of the 23 municipalities in the county of Nord-Trondelag, two municipalities did not accept the invitation to take part in the audiometry survey.
The general health survey (HUNT-II, conducted August 1995 through June 1997) had visited Levanger town and four sparsely populated municipalities before the audiometry study started. The Levanger subjects were re-invited for a separate audiometry session August 1997 through February 1998 after the ordinary HUNTII was finished. The audiometry survey thus included 17 of the 23 municipalities in Nord-Trondelag. [Figure 1] shows the number of subjects participating at the various stages of the study.
A total of 63% (n=51,975) of 82,141 invited and other attending subjects gave consent to attend the audiometry survey. Audiometric data are missing for 2.4% (n=1,252) of which around 300 were due to technical / computer problems, a few refused or discontinued the examination, and some had left the waiting room before audiometry. Valid audiometry data were thus obtained from 50,723 subjects (62%) of the 82,141 invited and other attending subjects (age range 20-101 years, mean=50.2 years, SD=17.0 years).
Questionnaire data were missing or incomplete for 1.2% (n=638) due to refusal or reduced capability. Valid audiometry and questionnaire data were thus obtained from 50,132 subjects.
The attendants from the town of Levanger examined in a separate audiometry session were 5,110 subjects. All the other participants, i.e. around 90 % of the sample, had audiometry in the same session as their general health screening, making selection towards participants with poor hearing unlikely in this sample. [Figure 2][Figure 3] show the attendance rates for different age groups and gender.
The low attendance for age groups 30 years (less than 50%) partly reflects that students and young adults are formally registered as residents in their home county during education and compulsory military service even when living elsewhere - thus many of them were prevented from attending their home-town screening. For age groups 50-80 years the attendance rates are high, 75-87%. The attendance rate for all municipalities except Levanger was 69% (males 65%, females 73%), and for Levanger 42% (males 39%, females 45%). As the Levanger results did not differ significantly from the rest of the sample, all subjects attending were treated as one sample.
Two ambulant audiometric teams (both with one or two authorized operators and one assistant) each conducted five parallel pure-tone hearing threshold examinations with five self-administered automatic audiometers electronically linked to a personal computer in five dismountable audiometric booths. Personal identification data were attached by "bar-code" scanning from an electronic population register file prepared by the governmental Statistics Norway. One audiometry team stayed for 4-6 months in each of the five towns, whereas the other team stayed for shorter periods (from a few days up to two months) as needed in each rural village.
The test frequencies were the standard 0.25-0.51-2-3-4-6-8 kHz, examined first on the right, then on the left ear in accordance with ISO 82531 (1989). The duration of the audiometry procedure was around ten minutes per subject. Interacoustics-AD25 self-administered automatic fixed-frequency tracking audiometers
(S.nr. 030035-030044; the model is now superseded by AD226) with TDH-39P headphones and MX 41/AR cushions were used in dismountable Tegner T-booths (95x105x210cm) with ventilation fan "on" in rooms specially selected for low background noise. For each test frequency the signal was automatically increased by 5 dB steps until the test person responded, then decreased by 10 dB, and again increased until response. The threshold was recorded when two out of three responses were consistent. A maximum threshold level of 120 dB was recorded, and no response to 120 dB was registered as a 120 dB loss. Weber's test (256 Hz) was included. Masking or bone conduction was not included due to time restrictions. Manual audiometry with corresponding ascending procedure was applied right away for the few subjects who did not master the self-administered procedure. Once the audiometry session was ended for a subject, the next subject was ushered in from the waiting room.
The audiometers and equipment were new at the start of the study and were calibrated before the start and every 6 months during the study according to ISO 389-1 (1994). Each audiometer was checked by the operators every day prior to audiometry and sent for service and calibration in case of failure. In-booth noise levels were measured for all five town sites and for three randomly selected village sites and were well within ISO 8253-1:1989 [Table 2] (test-tone frequency range 250-8000 Hz), but were at the criterion around 200 Hz. (Norsonic SA110 sound level meter calibrated on-site, dB Fast, 1/3 octave band 31.5-8000 Hz, each band tested for 10 consecutive 1 sec periods with the microphone in right ear position, with ventilation fan "on" and audiometry going on in the other 4 booths. Noise registration periods with occasional single ambient noise events inevitable in out-of-lab audiometry, e.g. door banging, were omitted and replaced by immediate repetition of that 1 second registration period.)
To study test-retest reliability, retest audiometry with the same procedure was performed for 99 randomly drawn subjects some minutes after the ordinary (first) session, including repositioning of headsets.
The test-retest correlations 0.25-8 kHz for the 99 randomly drawn subjects examined twice were, for the right ear: .88-.91-.95-.96-.98-.97-.95-.97, for the left ear: .68-.79-.94-.97-.98-.97-.97-.97. Further details on test-retest differences are given elsewhere (Engdahl et al. 2005).
[Figure 4]a-h show the mean thresholds (mean of both ears) for age and gender with 95% confidence intervals for each five-year age group for the eight test frequencies 0.25-8 kHz. The 95% confidence intervals are narrow for both genders, although somewhat wider above 80 years of age. Even for the youngest age group, 20-24 years, the thresholds recorded were some dB elevated compared with audiometric zero for all test frequencies 0.25-8 kHz. Mean thresholds for different age groups and gender showed slightly better values for males for frequencies 0.5 kHz, but were poorer in males > 30 years from > 2 kHz with maximal gender differences around 20 dB at 3-4 kHz for subjects age 55-74 years. Both genders showed > 10 dB mean hearing loss at 6 kHz even in the youngest age group 20-24 years [Figure 4]g.
[Figure 5]a and b show the percentage of subjects in each age group and gender with hearing loss 25 dB or more in the better ear for each of the eight test frequencies. In males the percentage of hearing loss > 25 dB was highest for 6 kHz below age 60 years, and nearly as high for 4 kHz and 8 kHz. Females older than 25 years showed lower prevalence of hearing loss > 25 dB than males for frequencies > 3 kHz, with the highest prevalence for 6 kHz below age 50 years and for 8 kHz above 50 years. An elevated prevalence for > 25 dB loss at 6 kHz was most expressed for the younger male subjects 25-39 years.
[Table 1]a-c show the prevalence (%) of hearing impairment (dB hearing threshold level averaged over the frequencies 0.5, 1, 2 and 4 kHz) exceeding > 25 dB, > 30 dB, > 35 dB, > 40 dB, > 45 dB, and > 65 dB for the better ear and worse ear for each gender and age group and for overall genders and age groups. Weighted by proportion of participants relative to population size for each age group and gender, the overall data showed that 18.8% of our total population sample have an averaged hearing impairment > 25 dB hearing threshold level in the better ear and 27.2% in the worse ear. The corresponding prevalences were 22.2% (better ear) and 32.0% (worse ear) for males, and 15.9% (better ear) and 23.0% (worse ear) for females, respectively.
The audiometry was performed as part of a general health screening and in the same session for around 90% of the subjects. The findings did not differ significantly for the remaining 10% of the sample (from Levanger), who had audiometry in a separate session. Thus, in this study selection towards participants with poor hearing is unlikely. The large population sample (n=50,723) recruited from urban and rural areas, the relatively high participation rate (69% for 9/10 of the sample), and the high test-retest reliability (apart from 250 Hz left ear) indicate reasonable validity, reliability and representativeness of the recorded data. The in-booth ambient noise levels were satisfactory, well within the ISO 8253-1:1989 standard [Table 2], test-tone frequency range 250-8000 Hz) for all 1/3 octave bands 31.5-8000 Hz, except being at the criterion around 200 Hz (low-frequency ambient noise level is hard to avoid in ambulant out-of-lab audiometry). Some of the threshold elevation registered at 250 Hz may thus be attributed to background noise. The 0.68 test-retest correlation for 250 Hz left ear might be due to ambient noise influence combined with impaired concentration for the frequencies tested last in the retest audiometry, which immediately followed the first one, as the test-retest correlation for 250 Hz right ear was 0.88. It might also partly reflect random fluctuations in the small sub-sample tested twice (n=99).
The overall prevalence (%) of hearing impairment (dB hearing threshold level averaged over the frequencies 0.5, 1, 2 and 4 kHz) exceeding > 25 dB hearing threshold level was 18.8% for the better ear and 27.2% for the worse ear, respectively. Our data compare reasonably well with the corresponding data from Great Britain (16.1% / 26.1%) and from South Australia (16.6% / 22.2%), taking into account the differences in age profiles of the studies. The prevalence decreases to 0.5% (better ear) and 2.2% (worse ear) for overall averaged thresholds exceeding > 65 dB hearing threshold level, compared with 1.1% and 3.5% for Great Britain and 0.5% and 2.5% for South Australia (Davis, 1989; Wilson et al., 1999).
The thresholds recorded were some dB poorer than audiometric zero for any of the test frequencies 0.25-8 kHz, even for the youngest age group 20-24 years [Figure 4]a-h. For 250 Hz in this study this may partly be due to low frequency ambient noise (cf. above), but hardly for the higher frequencies where the ambient noise was negligible. Corresponding elevated thresholds are found in a number of audiometric surveys from various countries (Ward, 1990; 1993). Our findings are in line with the 4.3 dB poorer average of 0.5, 1, 2 and 4 kHz, and the even greater shifts for 0.25, 6 and 8 kHZ found also for young adults 18-30 years in Great Britain (Lutman and Davis, 1994).
Theoretically, a general threshold elevation relative to audiometric zero for age and gender in a population may reflect 1) too restrictive reference thresholds, 2) calibration artefacts, 3) hearing deterioration due to modern society noise exposure, or 4) combinations of these elements. Regarding 2), calibration errors may partly be related to the introduction of the ISO 389 (1994) standard, which includes use of artificial ear, which may contribute some few dB to the threshold elevation according to Smith and Lutman (1992) and Smith et al. (1998). Threshold errors at 6 kHz may also be specifically related to resonance phenomena in the TDH-39P earphones used in the present study (further discussed by Engdahl et al, 2005). However, calibration errors would hardly be expected to fully account for the > 10 dB shift recorded at 6 kHz even for the youngest age group 20-24 years in the present study, or for the high-frequency threshold shifts recorded in other studies using other earphones, e.g. the British study on young adults (Lutman and Davis, 1994). Quantification of possible system-related calibration errors should be dealt with in specific studies.
Regarding 3), the specific elevated prevalence for > 25 dB loss at 6 kHz in the better ear most expressed for the younger males 25-39 years old in the present study [Figure 5]a, cannot be explained by calibration error and most likely reflects societal and/or occupational noise exposure. A corresponding specific high frequency threshold elevation recorded by otoacoustic emissions has been reported in Australian young adults 15-45 years (Murray and LePage, 1993). In a more recent study 12.5% of US children 6-19 years show a 6 kHz noise "notch" in one or both ears (n=5,249; Niskar et al., 2001), which might be taken to reflect societal noise exposure. In contrast, in a Swedish study on a population unscreened except for occupational noise exposure - i.e. a true database B according to ISO 1999 - the median thresholds at 6 kHz were not poorer than at 8 kHz. For the youngest age group the median thresholds differed less from audiometric zero than in the British (Lutman and Davis, 1994) and the present study. Thus, the magnitude and reasons for threshold deviation from audiometric zero at 6 kHz in various studies may differ.
Regarding 1), whether the general threshold elevation relative to audiometric zero for age and gender recorded in this study reflects too restrictive reference thresholds, can hardly be concluded at present. The fact that threshold elevation is also recorded in corresponding surveys might indicate that the present audiometric reference thresholds may be too restrictive. Further discussion of possible effects of calibration artefacts and automated audiometry, and presentation of the relation between screened and unscreened thresholds in our study related to other studies are given by Engdahl et al. (2005).
In an earlier report from the present study the male median thresholds at 4 kHz were found to be some few dB poorer, but rather parallel to ISO A (screened population) up to age group 45-54 years, sloping to poorer than ISO B (unscreened) for 55-64 years (Borchgrevink and al, 2001). The fact that the threshold elevation exceeded ISO B for the older age groups in our sample may reflect higher noise exposure in our sample. This may also agree with recent indications that aged subjects seem more susceptible to hearing loss, possibly due to impaired spontaneous cochlear repair mechanisms (Miller et al., 1998), especially those with a prior NIHL history (Gates et al., 2000; 2001). Race and ethnicity, and possibly socio-economic circumstances, also seem to influence age related hearing thresholds for age and gender (Rosen et al., 1964; Clark and Bohl, 1996; Lee et al., 1996; Ishii and Talbott, 1998). The present study population was almost entirely Caucasian.
Hearing thresholds for age and gender were recorded on an unscreened Caucasian population > 20 years (n=50,723) as part of a county general health screening. The study offers a recent, large and reasonably valid reference base that may contribute to evaluation and adjustment of the present normative hearing thresholds for age and gender. The mean unscreened pure-tone thresholds recorded for age and gender were in general some dB higher than the present audiometric zero for all test frequencies, even in young adults. Mean thresholds exceeded 10 dB at 6 kHz for both genders even for the youngest age group 20-24 years. Mean thresholds were poorer in males > 30 years from > 2kHz, with maximal gender differences of ~20dB at 3-4kHz for subjects aged 55-74 years. The results largely agree with corresponding surveys, which might indicate that the present audiometric reference thresholds may be too restrictive. Further studies are needed to sort out to what extent the recorded high-frequency threshold elevation found e.g. at 6 kHz even in young adults may be due to e.g. calibration artefacts, or whether it may partly reflect hearing deterioration in the population due to modern society noise exposure.
The study was funded by the National Institute on Deafness and Other Communication Disorders (NIDCD), National Institutes of Health, Bethesda, Maryland, USA, research contract No. N01-DC-6-2104. The Nord-Trondelag Health Study (HUNT), of which the Hearing Loss Study is a part, was conducted in collaboration with the National Institute of Public Health, Oslo, the National Health Screening Service, Oslo, Nord-Trondelag County Council, and The Norwegian University of Technology and Science, Trondheim. The Nord- Trondelag County Health Officer and the Community Health Officer in Levanger and in other municipalities provided organizational and other practical support. We also want to thank the NTHLS team for their diligence.
|1||Alberti P.W. (1998) Editorial: Noise, the most ubiquitous pollutant. Noise & Health 1:3-5.|
|2||Bohnker B.K., Page J.C., Rovig G., Betts L.S., Muller J.G., Sack D.M. (2002b). Navy Hearing Conservation Program: threshold shifts in enlisted personnel, 1995-1999. Mil Med 167(1):48-52.|
|3||Borchgrevink H.M., Tambs K., Hoffman H.J. (2001). The Nord-Trendelag Norway audiometric survey 1996-98: Unscreened adult high-frequency thresholds, normative thresholds and noise-related socio-acusis. In Henderson D., Prasher D., Kopke R., Salvi R., Hamernik R., eds. Noise Induced Hearing Loss: Basic mechanisms, prevention and control. Noise Research Network Publ., London, pp377-385.|
|4||Buren M., Solem B.S. and Laukli E. (1992). Threshold of hearing (0.125-20 kHz) in children and youngsters. Br J Audiol 26:23-31.|
|5||Clark W.W., Bohl C.D. (1996) Hearing levels of US industrial workers employed in low-noise environments. In Scientific basis of noise-induced hearing loss. Axelssoon A., Borchgrevink H.M., Hamernik R.P., Hellstram P.A., Henderson D., Salvi R.J., eds. Thieme, New York, pp 397-414|
|6||Davis A.C. (1989) The prevalence of hearing impairment and reported hearing disability among adults in Great Britain. Int. J. Epidemiol. 18(4): 911-917.|
|7||Davis A.C. (1997) Epidemiology of hearing disorders. In Scott Brown's Otolaryngology. Kerr A.G., ed. Butterworth-Heinemann, Boston.|
|8||Engdahl B., Tambs K., Borchgrevink H.M., Hoffman H.J. Screened and unscreened hearing threshold levels for the adult population: Results from the Nord-Trandelag Hearing Loss Study. Int J Audiol 2005;44:213-230.|
|9||Gates G.A., Schmid P., Kujawa S.G., Nam B., D'Agostino R. (2000). Longitudinal threshold changes in older men with audiometric notches. Hear Res 141(1-2):220-228.|
|10||Gates G.A., Schmid P., Kujawa S.G., Nam B., D'Agostino R. (2001). The effects of aging on noise-damaged ears. In Henderson D., Prasher D., Kopke R., Salvi R., Hamernik R., eds. Noise Induced Hearing Loss: Basic mechanisms, prevention and control. Noise Research Network Publ., London, pp513-522.|
|11||Ishii E.K., Talbott E.O. (1998) Race/ethnicity differences in the prevalence of noise-induced hearing loss in a group of metal fabricating workers. J Occup Environ Med 40(8):661-666.|
|12||ISO 389. (1994) Acoustics - - Reference zero for the calibration of audiometric equipment. International Organization for Standardization, Geneva.|
|13||ISO 7029. (1984) Acoustics - Threshold of hearing by air conduction as a function of age and sex for otologically normal persons. International Organization for Standardization, Geneva.|
|14||ISO 8253-1:1989 (E). Acoustics - Audiometric test methods. International Organization for Standardization, Geneva.|
|15||ISO 1999. (1990) Acoustics - - Determination of occupational noise exposure and estimation of noiseinduced hearing impairment. International Organization for Standardization, Geneva.|
|16||Johansson M.S.K. & Arlinger S. Hearing threshold levels for an otologically un-screened, non-occupationally noiseexposed population in Sweden. Int J Audiol 2005;41:180194.|
|17||Lee D.J., Gomez-Marin O., Lee H.M. (1996) Prevalence of childhood hearing loss. The Hispanic Health and Nutrition Examination Survey and the National Health and Nutrition Examination Survey II. Am. J. Epidemiol. 144(5): 442-449|
|18||Lutman M.E., Davis A.C. (1994). The distribution of hearing threshold levels in the general population aged 1830 years. Audiology 33(6):327-350.|
|19||Miller J.M., Dolan D.F., Raphael Y., Altschuler R.A. (1998). Interactive effects of aging with noise induced hearing loss. Scand Audiol Suppl, 48:53-61.|
|20||Murray N.M., LePage E.L. (1993) Age dependence of otoacoustic emissions and apparent rates of ageing of the inner ear in an Australian population. Aust. J. Audiol. 15: 59-70|
|21||Niskar A.S., Kiesak S.M., Holmes A.E., Esteban E., Rubin C., Brody D.J. (2001). Estimated prevalence of noiseinduced hearing threshold shifts among children 6 to 19 years of age: he Third National Health and Nutrition Examination Survey, 1988-1994, United States. Pediatrics 108(1):40-43.|
|22||Robinson D.W., Shipton M.S. and Hinchcliffe R. (1981). Audiometric zero for air conduction. A verification and critique of international standards. Audiology 20: 409-431|
|23||Rosen S., Plester D., El-Mofty A., Rosen H.V. (1964) High frequency audiometry in presbycusis. Arch Otolaryngol 79: 18-32|
|24||Smith P.A., Lutman M.E. (1992) Consequences of the change to standards ffor airconduction hearing levels. A cautionary note. Br J Audiol 26(1):59-61.|
|25||Smith P.A., Davis A.C., Ferguson M.A. and Lutman M.E. (1998). Hearing in young adults. Report to ISO/TC43/WG1.|
|26||Ward W.D. (1990) Presbyacusis and NIHL, current DRC and the validity of EEH, noise susceptibility and new psychoacoustic methods. In Noise as a public health problem. New advances in noise research, Part 1. Berglund B., Lindvall T., eds. Swedish Council for Building Research, Stockholm, pp 167-177.|
|27||Ward W.D. (1993) Current exposure standards; interaction of exposures; susceptibility and vulnerability. In Noise and man '93. Noise as a public health problem. Vallet M., ed. Arcueil: INRETS 3:152-160.|
|28||Wilson D.H., Walsh P.G., Sanchez L., Davis A.C., Taylor A.W., Tucker G., Meagher I. (1999) The epidemiology of hearing impairment in an Australian adult population. Int. J. Epidemiol. 28: 247-252|