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PEDIATRICS Vol. 108 No. 1 July 2001, pp. 40-43
,
From the * National Center for Environmental Health, Centers for
Disease Control and Prevention, Atlanta, Georgia; 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.
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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Abstract
Methods
Results
Discussion
References
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).
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 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 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 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: 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
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.
TABLE 1
METHODS
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Abstract
Methods
Results
Discussion
References
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
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
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
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
RESULTS
Top
Abstract
Methods
Results
Discussion
References
Prevalence Estimates of NITS Among US Children Aged 6 to 19 Years by
Sociodemographic Characteristics: NHANES III, 1988-1994 (N = 5249)
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%.
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DISCUSSION |
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Results
Discussion
References
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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.
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ACKNOWLEDGMENTS |
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We thank Maureen Hannley, PhD, consultant for NHANES III audiometry, from the American Academy of Otolaryngology-Head and Neck Surgery Foundation, Inc, Alexandria, Virginia.
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FOOTNOTES |
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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
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ABBREVIATIONS |
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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.
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REFERENCES |
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United States.
MMWR Morb Mortal Wkly Rep
1986;
35:185-188 [Medline]This article has been cited by other articles:
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