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

DISCUSSION

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,,22Because 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,,12Without 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.¹³Epidemiologic 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.