PEDIATRICS Vol. 108 No. 1 July 2001, pp. 40-43

Estimated Prevalence of Noise-Induced Hearing Threshold Shifts Among Children 6 to 19 Years of Age: The Third National Health and Nutrition Examination Survey, 1988-1994, United States

Amanda Sue Niskar, RN, BSN, MPH^*, Stephanie M. Kieszak, MA, MPH^*, Alice E. Holmes, PhD, CCC-A, Emilio Esteban, DVM, MBA, PhD^*, Carol Rubin, DVM, MPH^*, and Debra J. Brody, MPH^§

From the ^* National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, Georgia;  University of Florida, Gainesville, Florida; and the ^§ National Center for Health Statistics, Centers for Disease Control and Prevention, Hyattsville, Maryland.

	ABSTRACT

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Objective.  This analysis estimates the first nationally representative prevalence of noise-induced hearing threshold shifts (NITS) among US children. Historically, NITS has not been considered a common cause of childhood hearing problems. Among children, NITS can be a progressive problem with continued exposure to excessive noise, which can lead to high-frequency sound discrimination difficulties (eg, speech consonants and whistles).

Methods.  The Third National Health and Nutrition Examination Survey (NHANES III) was conducted from 1988 to 1994. NHANES III is a national population-based cross-sectional survey with a household interview, audiometric testing at 0.5 to 8 kHz, and compliance testing. A total of 5249 children aged 6 to 19 years completed audiometry and compliance testing for both ears in NHANES III. The criteria used to assess NITS included audiometry indicating a noise notch in at least 1 ear.

Results.  Of US children 6 to 19 years old, 12.5% (approximately 5.2 million) are estimated to have NITS in 1 or both ears. In the majority of the children meeting NITS criteria, only 1 ear and only 1 frequency are affected. In this analysis, all children identified with NITS passed compliance testing, which essentially rules out middle ear disorders such as conductive hearing loss. The prevalence estimate of NITS differed by sociodemographics, including age and sex.

Conclusions.  These findings suggest that children are being exposed to excessive amounts of hazardous levels of noise, and children's hearing is vulnerable to these exposures. These data support the need for research on appropriate hearing conservation methods and for NITS screening programs among school-aged children. Public health interventions such as education, training, audiometric testing, exposure assessment, hearing protection, and noise control when feasible are all components of occupational hearing conservation that could be adapted to children's needs with children-specific research.  Key words:  noise, hearing, NHANES, audiometry, compliance.

Noise-induced hearing threshold shifts (NITS) can be a progressive problem for children and adults subjected to continued exposure to excessive noise. Historically, the effects of environmental noise on hearing have not been recognized as a public health problem among children.^1,2 However, because noise has been recognized as an occupational hazard among adults, there are hearing conservation programs that include public health interventions such as education and NITS screening for adults in appropriate occupational settings.^1,2 Yet the public may not be aware of the many nonoccupational activities that can be sources of hazardous environmental noise for people of all ages. Examples of environmental sounds that may produce hazardous noise levels are musical concerts, fireworks, lawn mowers, stereos, and toys.^1-3

Exposure to hazardous sounds can damage the inner ear's hair cells, resulting in NITS. NITS is the hearing threshold level shift attributable to noise alone.^3,4 Depending on the loudness and duration of the hazardous sound, NITS can be temporary or permanent. The first audiometric sign of NITS usually is a threshold loss at 3, 4, or 6 kHz.^1-3,5 With continued harmful noise exposures, the threshold loss at 3, 4, or 6 kHz increases in severity, and the NITS can extend to include lower and higher frequencies.^2,3,5

Potentially hazardous sound levels may make it difficult for a person to hear conversation and may cause the person to hear ringing in the ears or muffled sounds after the sound exposure has ended.³ NITS can be a result of exposure to acute or chronic noise. Acute exposure, such as an explosion or gunfire, can produce immediate, permanent, severe NITS.^1-3 Chronic exposure to less intense sounds, such as loud music, can painlessly accumulate over a lifetime to gradually produce irreversible damage to the ear's inner hair cells.^1-3 This analysis is the first to estimate the nationally representative prevalence of NITS among US school-aged children using data from the Third National Health and Nutrition Examination Survey (NHANES III).

	METHODS

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NHANES III was conducted from 1988 to 1994 by the National Center for Health Statistics of the Centers for Disease Control and Prevention. The survey used a complex-stratified multistage probability design to examine a nationally representative sample of the US civilian noninstitutionalized population. National population estimates and estimates for the 3 largest race-ethnicity subgroups in the US population (non-Hispanic white, non-Hispanic black, and Mexican American) can be derived from the 6-year survey.⁶ Data were collected through household interviews followed by a physical examination in a mobile examination center (MEC).⁷ Standardized audiometric and tympanometry compliance examinations were conducted on 6- to 19-year-olds.

Tympanometric Compliance Measures

NHANES III included tympanometry compliance testing (pass/fail).⁸ Compliance testing measures the flexibility of the middle ear's tympanic membrane. Failure of compliance can indicate a middle ear problem, which can affect the results of the audiogram.⁵ The Tympanometer-TA-7A Automatic Impedance Meter (Avionic Specialties, Inc, Charlottesville, VA) was calibrated with the same specifications at the beginning of each examining day and at the start and end of testing at each field location. The compliance testing was conducted in a sound-treated room in the MEC by trained examiners using a standardized protocol. Each ear was tested separately.² Additional tympanometric procedure information is referenced elsewhere.⁸

Audiometric Measures

Audiometry was conducted in a sound-treated room in the MEC by trained examiners using a standardized protocol to measure the intensity (loudness level) in decibels (dB) at which a pure tone can be heard at a specific frequency.⁵ A Grason-Stadler audiometer (Model GSI 16) (Grason-Stadler Inc, Milford, NH) was calibrated with the same American National Standards Institute S3.6-1969 standard specifications at the start and end of testing at each field location. Air conduction thresholds were measured for each ear at 0.5, 1, 2, 3, 4, 6, and 8 kHz, with testing repeated at 1 kHz.⁸ The correlation of the threshold value for the 1-kHz first test with the retest was 0.9 (P = .0001) for the left and right ear for each child with complete data. The 1-kHz first test was the value used for this analysis.

All testing was done using the standard Carhart-Jerger method of determining pure tone thresholds.⁹ A threshold value was defined as the lowest signal intensity that the child detected at least 50% of the time, with a minimum of 3 trials. Threshold values were recorded in 5-dB increments. If an examinee had threshold values at a specific frequency that differed by 40 dB or more between ears, masking was performed to ensure accuracy in measurement. Threshold values were obtained between 10 and 110 dB hearing level (HL). If no response was obtained at the limits of the test protocol, a threshold of 105 dB HL was recorded for statistical purposes.⁶ In this sample, masking was performed for 69 children (1.3%), and the masked values were used for this analysis. Additional audiometric procedure information is referenced elsewhere.⁸

Sociodemographic Variables

Age at interview was categorized as 6 to 11 and 12 to 19 years, and self-reported race-ethnicity was grouped as non-Hispanic black, non-Hispanic white, and Mexican American. The all other race-ethnicity category (includes other Hispanics, Asians, and Native Americans) was too small to be analyzed separately but was included in all totals. The poverty-income ratio (PIR) was defined as the total family income divided by the poverty threshold, as determined by the US Bureau of the Census, for the year of the interview. To be consistent with major government food assistance programs that use a PIR cutoff of 1.3 to determine eligibility, PIR categories used in analyses were low (PIR  1.3), middle (1.3 < PIR  3.5), and high (PIR > 3.5).¹⁰ No family income data were available for 8.8% of the children available for this analysis. Urban status was grouped as metropolitan (1 million population) and nonmetropolitan (<1 million population).¹¹ Geographic region was grouped as Northeast, Midwest, South, and West.¹⁰

Analytic Sample

Of the 6908 children aged 6 to 19 years who were interviewed in NHANES III, 6497 (94.1%) children were examined. All 6- to 19-year-olds interviewed were eligible for the examination. Children who were interviewed but not examined did not differ by age, sex, race-ethnicity, or PIR from children who were interviewed and examined. Of the 331 children (5.1%) excluded from this analysis for incomplete or missing audiometric data, 44 reported ear drainage.^7,8 An additional 917 children were excluded from this analysis for incomplete or missing compliance data or for failing compliance testing in at least 1 ear. Therefore, our analyses excluded a total of 1248 children (19.2%) examined in NHANES III. Children excluded from the analyses did not differ by sex or race-ethnicity from children included in the analyses. However, children excluded from the analyses were younger and more likely to have a lower PIR than children included in the analyses. A total of 5249 children were available for this analysis.

NITS Criteria

NITS has a distinct audiometric pattern (noise notch), with 3, 4, or 6 kHz typically affected at the onset of NITS.³ A child was defined as having NITS when the audiogram met the following 3 criteria for at least 1 ear. First, threshold values at .5 and 1 kHz were 15 dB HL (better). Second, the maximum (poorer) threshold value at 3, 4, or 6 kHz was at least 15 dB higher (poorer) than the highest (poorest) threshold value for .5 and 1 kHz. Third, the threshold value at 8 kHz had to be at least 10 dB lower (better) than the maximum (poorest) threshold value for 3, 4, or 6 kHz. These 3 criteria describe a noise notch audiometric pattern.^5,12,13

Data Analysis and Statistical Methods

The children who met NITS criteria at any hearing level were described by reported sociodemographic characteristics, number of affected ears, and involved frequencies. The children who met NITS criteria were grouped into 4 hearing level categories determined by maximum threshold values: 15 dB HL (normal hearing level with early NITS), 16 to 25 dB HL (slight NITS), 26 to 40 dB HL (mild NITS), 41 dB HL (moderate to profound NITS).¹⁴

All prevalence estimates and 95% confidence intervals were derived using SUDAAN, a statistical package compatible with SAS that accounts for the complex survey design and weights.^15,16 Prevalence estimates were adjusted to the 1991 US Census data for the population of children aged 6 to 19 years.¹⁰

	RESULTS

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Among US children, the overall prevalence of NITS in 1 or both ears was 12.5%. This point estimate represents approximately 5.2 million children. NITS prevalence estimates by 6 sociodemographic characteristics are presented in Table 1. Boys (14.8%) had a significantly higher prevalence estimate of NITS than girls (10.1%). Children aged 12 to 19 years (15.5%) had a significantly higher prevalence estimate of NITS than 6- to 11-year-olds (8.5%). Non-Hispanic black children had a marginally lower prevalence estimate of NITS than the other 2 race-ethnicity categories. The prevalence estimate of NITS was lowest among children from families with high PIRs. Children residing in the Northeast geographic region had the lowest NITS prevalence estimate of the 4 geographic regions. There were no differences in the NITS prevalence estimates for children by urban status.

Among children meeting NITS criteria (N = 597), 14.6% had an audiogram showing a noise notch for both ears. Of the 597 children with NITS, 18.1% had a normal hearing level, 57.1% had slight NITS, 19.8% had mild NITS, and 4.9% had moderate to profound NITS. Assessment of threshold shifts at 3, 4, and 6 kHz in 1 or both ears showed that only a single frequency was involved for 88.4% of children with NITS, 8.2% had 2 involved frequencies, and 3.4% had involvement of all 3 frequencies. Of the children meeting NITS criteria, 3 kHz was involved in 14.1%, 4 kHz was involved in 23.8%, and 6 kHz was involved in 77.1%.

	DISCUSSION

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This is the first nationally representative report of the estimated prevalence of children 6 to 19 years of age who meet NITS criteria. These results suggest that children are being exposed to hazardous levels of noise, and children's hearing levels are vulnerable to these exposures. Public health interventions such as education, training, audiometric testing, exposure assessment, hearing protection, and noise control when feasible are all components of occupational hearing conservation that could be adapted to children's needs with children-specific research.

The majority of children had an early phase of NITS in only 1 ear and involving only a single frequency. Continued excessive environmental or occupational noise exposures could lead to progression of NITS to include other frequencies and to increase in severity. In studies completed on adult subjects, it is possible to recover from temporary NITS within a few minutes to several weeks after the termination of the noise exposure.⁵ The permanent NITS that remains is irreversible. The resiliency of a child's auditory system with respect to noise exposure is unknown and should be a topic of additional investigation. Any level of NITS may muffle high-frequency sounds such as whistles or buzzers and may result in difficulty discriminating speech consonant sounds such as those in the words fish and fist, particularly in noisy everyday environments with background noise, many voices, or room reverberation.^2,17,18

The sex difference in the point estimates of NITS may be explained by cultural differences in participation in noisy activities. The difference in the prevalence of NITS by age group is expected because the older children have had more years of noise exposure. Although the overall prevalence estimates are too small to present stable estimates by individual years of age, the continuous data suggest a trend that the prevalence of NITS increased by age. It is well documented among adults that noise-induced hearing loss often is the result of accumulated exposures over time.⁵ Similar differences by sex and age group have been reported in past studies.^19-21 Children who were interviewed but not examined did not differ by age or sex from the children who were interviewed and examined. Children who were interviewed and examined but were excluded from this analysis because of incomplete or missing audiometric or compliance data were younger and more likely to have a family with a low PIR, which may be explained by a higher prevalence of otitis media in young children and children with less access to health care services. Of the tested frequencies, 6 kHz was the most commonly involved frequency among US children meeting the NITS criteria. This finding supports including 6 kHz for audiometric screening of school-aged children, a method that typically does not test 6 kHz. The development of NITS (including the frequencies involved and the maximum threshold value) may be influenced by noise duration and loudness, quantity of noise exposures, the ear's distance from the sound source, ear canal length and volume, and other factors.^3,22 Because NITS may influence communication and behavioral skills, it can adversely affect education, social interactions, employment, and quality of life.^{3,14,17,18,23,24}

The findings of this study represent a point estimate of NITS in US children during the survey period (1988-1994). The cross-sectional study design of NHANES III did not allow follow-up testing or assessment of temporary or permanent NITS status. There are no planned follow-up surveys to retest the hearing levels of children who participated in NHANES III. Many children with NITS may not have been identified in this analysis because of failed compliance testing or incomplete or missing audiometric or compliance data. In addition, NITS could have been masked in children with unidentified ear disorders that may have been detected if follow-up testing were conducted, if an otoscopic examination were conducted, or if tympanic width could have been calculated. Noise sources were not identified, but aircraft, firearms, power tools, lawn care tools, firecrackers, portable stereos, referee whistles, and toys are examples of environmental noise sources that have measured sound levels above the allowable occupational standard.^1-3,1325-28

The hearing threshold shifts were designated as NITS in this analysis by an audiometric configuration displaying a noise notch pattern. Although this pattern could result from other sensorineural etiologies, such as hereditary factors, this audiometric configuration is most commonly associated with exposure to loud noise.^5,12 Without a noise history (prenoise exposure and postnoise exposure audiograms), it is not possible to confirm that all NITS identified in this analysis is strictly caused by exposure to loud noise.

There is no US federal mandate for childhood hearing screening, but there are recommended guidelines for school-aged children to be screened at 1, 2, and 4 kHz.²⁹ In addition, guidelines and standards for audiometric testing of adults in occupational settings where noise is a hazard can be adapted for children's needs in noisy school settings such as band practice or shop class.¹³ Epidemiologic and cost-benefit studies are needed to determine appropriate hearing screening methods among school-aged children. In addition, national routine childhood hearing screening that includes audiometric testing at 3, 4, and 6 kHz could allow assessment of differences over time in the prevalence of NITS. Periodic objective hearing screening among school-aged children, including adolescents, may be an important way to detect NITS and other sensorineural threshold shifts. Additional research is needed to determine whether the differences by sociodemographic characteristics could result from environment, behavior, individual susceptibility, or other factors. Appropriate interventions to prevent NITS progression, hearing impairment, and potential educational difficulties in children should be evaluated. Because NITS is preventable, methods of prevention specific to children (including education, training, audiometric testing, exposure assessment, hearing protection, and noise control when feasible) must be developed, researched, and applied to benefit all children.

	ACKNOWLEDGMENTS

We thank Maureen Hannley, PhD, consultant for NHANES III audiometry, from the American Academy of Otolaryngology-Head and Neck Surgery Foundation, Inc, Alexandria, Virginia.

	FOOTNOTES

Received for publication Jun 7, 2000; accepted Oct 17, 2000.

Reprint requests to (A.S.N.) National Center for Environmental Health, Centers for Disease Control and Prevention, 1600 Clifton Rd NE, MS E23, Atlanta, GA 30333. E-mail: aniskar{at}cdc.gov

	ABBREVIATIONS

NITS, noise-induced hearing threshold shifts; NHANES III, Third National Health and Nutrition Examination Survey; MEC, mobile examination center; dB, decibel; HL, hearing level; PIR, poverty-income ratio.

	REFERENCES

Top Abstract Methods Results Discussion References

1. Brookhouser PE, Worthington DW, Kelly WJ Noise-induced hearing loss in children. Laryngoscope 1992; 102:645-655 [Medline]
2. CDC Leading work-related diseases and injuriesUnited States. MMWR Morb Mortal Wkly Rep 1986; 35:185-188 [Medline]
3. Berglund B, Lindvall T, eds In: The World Health Organization WHO Healthy Cities Programme. Community noise. Arch Cent Sensory Res. 1995; 2:1-195
4. Stach BA. Comprehensive Dictionary of Audiology. Baltimore, MD: Williams & Wilkins; 1997:97, 144
5. Katz J, ed. Handbook of Clinical Audiology. Baltimore, MD: Williams & Wilkins; 1994:20-21, 283-291, 534-551, 627
6. National Center for Health Statistics. NHANES III Reference Manuals and Reports (CD-ROM). Hyattsville, MD: Centers for Disease Control and Prevention; 1996
7. National Center for Health Statistics. Plan and Operation of the Third National Health and Nutrition Examination Survey, 1988-1994. Vital Health Statistics 1:1994, No. 32. Hyattsville, MD: US Department of Health and Human Services, Public Health Service, National Center for Health Statistics; 1994. DHHS Publ. No. PHS 94-1308
8. National Health and Nutrition Examination Survey III Audiometry and Tympanometry for Health Technicians Manual. Rockville, MD: Westat, Inc; 1988
9. Carhart R, Jeger JF Preferred method for clinical determination of pure-tone thresholds. J Speech Hear Dis 1959; 24:330-345
10. Johnson C. NHANES III Analytic and Reporting Guidelines. Hyattsville, MD: US Department of Health and Human Services, Public Health Service, National Center for Health Statistics; 1994
11. Butler MA, Beale CL. Rural-Urban Continuum Codes for Metro and Nonmetro Counties, 1993. Washington, DC: US Department of Agriculture, Economic Research Service, Agriculture and Rural Economy Division; 1994. Staff Report No. 9425
12. Schmidt JM, Verschuure J, Brocaar MP Hearing loss in students at a conservatory. Audiology 1994; 33:185-194 [Medline]
13. CDC/NIOSH. Criteria for a Recommended Standard: Occupational Noise Exposure Revised Criteria 1998. Cincinnati, OH: US Department of Health and Human Services; 1998
14. Disorders of the eye and ear. In: Behrman RE, Kliegman RM, Nelson WE, Vaughan VC, eds. Nelson Textbook of Pediatrics. Philadelphia, PA: WB Saunders; 1992:1602-1608
15. SAS Institute Inc. SAS Language: Version 6. Cary, NC: SAS Institute; 1990
16. Shal BV, Barnwell BG, Hunt PN, Lavange LM. SUDAAN User's Manual, Release 5.50. Research Triangle Park, NC: Research Triangle Institute; 1991
17. Bess FH The minimally hearing-impaired child. Ear Hear 1985; 6:43-47 [Medline]
18. Smoorenburg GF Speech reception in quiet and in noisy conditions by individuals with noise-induced hearing loss in relation to their tone audiogram. J Acoust Soc Am 1992; 91:421-437 [CrossRef][Medline]
19. Niskar AS, Kieszak SM, Holmes A, Esteban E, Rubin C, Brody DJ Prevalence of hearing loss among children 6 to 19 years of age: The Third National Health and Nutrition Examination Survey. JAMA 1998; 279:1071-1075 [Abstract/Free Full Text]
20. Rytzner B, Rytzner C School children and noise. Scand Audiol 1981; 10:213-216 [Medline]
21. Axelsson A, Aniansson G, Costa O Hearing loss in school children. Scand Audiol 1987; 16:137-143 [Medline]
22. Hellstrom PA The relationship between sound transfer functions and hearing levels. Hear Res 1995; 88:54-60 [CrossRef][Medline]
23. Jarvelin MR, Maki-Torkko E, Sorri MJ, Rantakallio PT Effect of hearing impairment on educational outcomes and employment up to the age of 25 years in northern Finland. Br J Audiol 1997; 31:165-175 [Medline]
24. Crandell CC Speech recognition in noise by children with minimal degrees of sensorineural hearing loss. Ear Hear 1993; 14:210-216 [Medline]
25. Swift D, Molon T. Disorders of the ear and hearing. In: Brooks SM, Gochfeld M, Herzstein J, Jackson RJ, Schenker MB, eds. Environmental Medicine. St Louis, MO: Mosby; 1995:250-262
26. Holmes AE, Kaplan HS, Phillips RM, Kemker FJ, Weber FT, Isart FA Screening for hearing loss in adolescents. Lang Speech Hear Serv Sch 1997; 28:70-75 [Abstract/Free Full Text]
27. Passchier-Vermeer W. Noise From Toys and the Hearing of Children. Netherlands: TNO-Gezondheidsonderzoek; 1991:40-47
28. Yaremchuk K, Dickson L, Burk K, Shivapuja BG Noise level analysis of commercially available toys. Int J Pediatr Otorhinolaryngol 1997; 41:187-197 [CrossRef][Medline]
29. American Speech-Language-Hearing Association, Ad Hoc Committee on Screening for Hearing Impairment, Handicap, and Middle Ear Disorders Report on audiologic screening. Am J Audiol 1995; 4:24-40 [Free Full Text]