Medical Issues 3
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Human subjects participated in these studies after giving free and Informed voluntary consent. Investigators adhered to AR 70-25 and USAMRDC Reg 70-25 on Use of Volunteers in Research.
Bruce C. Leibrecht, PhD
Director, Sensory Research Division
J. D. Lamothe, PhD
Chairman, Scientific Review Committee
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ABSTRACT: This study provided hearing threshold data for Army aviators stationed at Fort Rucker, Alabama, from February through August 1982. The mean pure tone thresholds here found to be improved when compared to data gathered by Walden and McCurdy in 1971. This improvement partially was attributed to redesign of the aviation helmet and increased awareness and compliance with hearing conservation measures. It Is possible that tighter administrative controls also contributed to the reduced threshold values. This study further indicated that, for Fort Rucker aviators, there exist three threshold regions correlated with flight hours: 50-400 flight hours, 401-3000 flight hours, and 3001-6000 flight hours. Each region has a specific range of hearing loss measured by comparing 2000 and 4000 Hz thresholds for the left ear. Anyone falling outside the threshold range for her/his respective region could be identified for possible
The authors would like to thank CPT Graham Wilde for providing his expertise in the auditory evaluation of the selected aviators, and Dr. H. Jones for his co Writer management support. LTC J. Goldstein also should be recognized for his initial work which led to the Implementation of this project and design of the questionnaire.
The Occupational Health Report (AEHA, 1981) states that hearing loss continues as the number one occupational health injury In the Army. Ten years earlier, a survey report (Walden and McCurdy, 1971) identified aviation as a military occupation with a high Incidence of hearing loss. The Air Force, as well as the Navy, also has identified their hearing hazardous occupations and the Air Force even has tracked these data by age group (Sutherland and Gasaway, 1978). Although hearing loss long has been associated with the military, the amount of hearing loss per length of exposure never has been reported. This project has attempted to establish a value of hearing loss for a specified number of flight hours.
This study sought to quantify the extent of hearing loss among aviators at Fort Rucker. Alabama, and to identify factors which may have contributed to this loss. Standard audiometric test procedures and questionnaire techniques were used in a stratified random sample approach. Values found in this study can serve as Initial normative hearing threshold levels distributed by flight hours. This is important because it Is not possible to discuss risk and excess hearing loss by flight hours until a norm is established among rotary wing aviators. This study also should provide insight into the effect the SPH-4 flight helmet has had on the reduction of noise-induced hearing loss.
METHODS AND MATERIALS
A listing of permanent party military aviators was provided by the Automation Management Office (A140) at Fort Rucker. These printouts Indicated that 98 percent of Fort Rucker's military aviators could be placed In a range between 50 and 7000 flight hours. A stratified random sample (Table 1) was developed by separating this military aviation population into 10 cells or strata according to flight hours. To obtain a confidence interval of 95 percent, a N of 145 was selected using a standardized sampling scheme (Cochran, 1977). The weight which each stratum represented to the total population was determined (Table 1). These Individual stratum weights then were multiplied by the number of aviators assigned to each stratum, resulting in ~h the required sample size of each stratum. (Appendix A lists the population distribution by stratum.) Aviators selected at random were asked to fill out a questionnaire and take a hearing test as a part of their scheduled flight physical. The questionnaire (Appendix B) attempted to identify a subject's hearing history and exposure to hazardous noise. Although an audiologist or technician reviewed each questionnaire to eliminate obvious errors or omissions, the questionnaire data should be considered subjective In nature and only the best recollection of the subject.
A pure tone threshold audiometrlc test was administered to each subject using either a Grason-Stadler Audiometer Model 1701 with TDFI 49 earphones or
Grason-Stadler GSI 10 with TDH 50P earphones. The audiometers had received electro-acoustic calibrations in accordance with American National Standards Institute (ANSI) S3.6-1969 prior to the start of the survey. In addition, daily biological listening checks (self-administered audiometric tests to insure consistent audiometric output) were performed by examiners prior to testing during the conduct of the survey. P11 hearing thresholds were obtained using the Carhart-Jerger preferred clinical method of threshold determination (Carhart and Jerger, 1959) with the subjects seated In a double-walled IAC sound-treated audiometric examination booth.
French and Steinberg (1947) demonstrated that speech signals above 2500 Hz do not significantly improve speech Intelligibility in quiet. However, in noise lower frequency speech components are masked by competing background signals and the higher frequencies become more important. Since high levels of background noise exposure result in high frequency hearing loss and signals below 2000 Hz easily are masked, this study limited its investigation to 2000, 3000, 4000, and 6000 Hz bilaterally.
Questionnaire results and threshold values were stored In a Systems Engineering Laboratories Model 85 Digital Computer. All data were double-checked to eliminate any entry errors and then sorted for analysis.
The analysis of data considered mean and median threshold levels, standard deviations, and standard errors as a function of flight hours. Questionnaire and threshold data then were tabulated.
RESULTS AND DISCUSSION
Hearing threshold data for all subjects are summarized in Table 2. In general, increased flight hours are associated with an increase in hearing loss, with the greatest loss occurring at 6000 Hz in pilots with 3001-4000 flight hours: 34 dB In the right ear.
Table 3 lists the relationship between flight hours, age, years In service and years rated as an aviator. With this Information, It was determined that aviators with over 3000 hours (Table 3) bad, on the average, more than 10 years as a rated aviator. Results from a 1971 US Army survey (Walden and McCurdy, 1971) which recorded threshold data for aviators with over 10 years rated service then were compared (Table 4) with the threshold data for the 10 years plus aviators from the current study.
As indicated by Table 4, mean hearing threshold levels a pp ear to be lower for today's aviators than for those of a decade ago. This could be the result of complex factors which include the redesign of aviator helmets and increased emphasis on hearing conservation. The difference in sample populations may be a contributing factor In any attempt to view Table 4 as displaying absolute reduction levels. However, this population variation is not felt to be significant in identifying a trend.
As further evidence of a reduction in hearing loss, Table 5 compared mean threshold data by age against thresholds reported by the U.S. Public Health Service (USPHS, 1965) study of the general non industrial civilian population. Not until the 5th stratum (1001-2000 flight hours) did mean hearing threshold values for Army aviators significantly exceed the civilian normative data. This excess probably was due in part to the aircraft noise environment in combination with small arms and artillery exposure. These results were similar to a 1978 US Air Force study (Sutherland and Gasaway, 1978) which found that Air Force aviation personnel reflected better hearing than either noise-exposed Air Force civilians or the general U.S. population.
Table 2 (threshold data) also Indicated a greater amount of mean hearing loss in the left ear than the right ear from 2000 through 4000 Hz. At 6000 Hz the right ear exhibited the most damage. These results held true for all categories of flight hours. This result has not been reflected in the literature; other studies (Corso, 1963; Spoor, 1967) did not report the ear Involvement to be frequency dependent, but indicated rather that the left ear most consistently was involved. However, a one-way analysis of variance of all frequencies (2, 3, 4, and 6 khz) did not find the threshold differences between ears significant at the .05 level.
Table 6 displays the difference in dB between 2000 and 4000 Hz for the left ear broken out by flight hours. This table indicates an increase in threshold between the 50-400 and 401-600 flight hour strata and again between the 2001-3000 and 3001-4000 strata. The 4 dB difference for 801-1000 flight hours was felt to be artificially low due to the high incidence of exposure to weapons fire: eight out of 10 subjects had been exposed to significant small arms or artillery fire. It was felt that this greater than normal exposure elevated thresholds for 2000 Hz as well as 4000 Hz and thus artificially reduced the expected range. One-way analysis of variance did not find a significant difference for threshold values among strata at the .05 level for either 2000 or 4000 Hz. However, a one-way analysis of variance of difference scores (4000 Hz threshold minus 2000 Hz threshold) did find a significant effect of flight hour strata (F=2.5, dF=l35. p<.O5).
Questionnaire data concerning the 145 aviators who participated in this study were reviewed and although all data were entered into the computer, only selected items are reported in Appendix C (Data and Summary Statistics). This decision was based on the fact that most of the questionnaire items could not be verified and represented only the best recollection of the subject.
Flight hours, rank, and age were the administrative questionnaire items selected for analysis. Rank was found to be heavily polarized - 69 percent of the sample population (57/83) with more than 2000 flight hours was warrant officers. Of subjects with less than 2000 flight hours. 85 percent (53/62) were found to be officers. The age of the 145 subjects ranged from 24 to 45 with a mean of 32 years.
Questionnaire items No. 7 (earplugs worn with the helmet), No. 16 (years exposed to small arms), No. 17 (years exposed to artillery), and No. 28 (years smoked), are tabulated in Appendix C. The following is a summary of the self-reported responses to these items: 39 aviators, or 27 percent, of the sample population admitted to wearing earplugs with the SPH-4 helmet. Seventy-one aviators, or 49 percent, had at least one year of frequent exposure to small arms fire and 56, or 39 percent, had received frequent exposure to artillery fire for at least one year. Smoking was defined as the use of one-half package of cigarettes a day for at least one year. Using this criteria, only 40 pilots, or 28 percent, were Identified as smokers.
This study provided hearing threshold data for Army aviators stationed at Fort Rucker, Alabama, from February through August 1982. This population displayed the following characteristics: nonsmoking, 72 percent; average age, 32; earplugs typically not worn with SPH-4- helmets, 73 percent; and substantial exposure to small arms and artillery fire, 54 percent.
Three major findings emerged from the threshold data. The present mean pure-tone thresholds were found to be lower than data gathered by Walden and McCurdy in 1971. The authors speculate that this Improvement can be attributed to redesign of the SPH-4 helmet and increased awareness of and compliance with hearing conservation measures. It is possible that tighter administrative controls also contributed to the reduced threshold values through the elimination of aviators who failed to meet the criteria required to remain on flight status.
The second finding, that the threshold values for the right ear exceeded those for the left ear at 6000 Hz across all strata of flight hours, cannot be readily explained. This result may be peculiar to the aircraft's acoustic environment.
Perhaps most Important was the finding of three threshold regions which may have possible use as discriminators to highlight the individual who has increased susceptibility to noise damage. Currently, a 20 dB elevation in threshold is used to indicate a significant threshold shift In hearing acuity. The data from this study indicated that, for Army aviators at Fort Rucker, there exist three threshold regions correlated with flight hours: 50-400 flight hours, 401-3000 flight hours, and 3001-6000 flight hours. These regions each have a specific range of hearing loss measured by comparing 2000 and 4000 Hz thresholds in the left ear. These normative data establish an expected amount of hearing loss according to the amount of flight hours. Therefore, anyone falling outside of his/her respective range could be identified for possible follow up procedures prior to incurring a 20 dB shift.
It is recommended that additional studies be undertaken to enlarge the data base for the aviation population. Similar 2000-4000 Hz range results from an enlarged sample would serve to validate the initial normative data and allow hearing threshold levels to be predicted. Aviators exceeding those predictor values would be considered “at risk” and follow-up procedures would be recommended prior to the onset of a significant threshold shift.
AEHA, 1981. Occupational Health Report. U.S. Arniy Environmental Hygiene Agency, Aberdeen, Maryland.
American National Standards Institute, 1969. American National. Standard Specification for Audiometers. New York. American National Standards Institute. ANSI S3.6-1969.
Carhart, R. and Jerger, J.. 1969. Preferred method for clinical determination of pure tone threshold. Journal of Speech and Rearing Disorders. Volume
Cochran, IL, 1977. Sampling Techniques. 3d ed., John Wiley and Sons, New York, NV. pp. 73-76, 96-98.
Corso, J.., 1963. Age and sex differences in pure tone thresholds. Archive.
of Otolaryngology. Volume 77:385-405.
French. N. and Steinberg, 3,, 1947. Factors governing the Intelligibility of speech sounds. Journal. of the Acoustical Society of America. Volume 19:90.
Spoor. A., 1967. Presbycusis values In relation to noise induced hearing loss. International Audiology. Volume 6:48-57.
Sutherland, H. and Gasaway, D., 1978. Current Hearing Threshold Levels for Noise exposed US Air Force Personnel: One Year's .Reporting. Report SAM-TR-78-39, USAF School of Aerospace Medicine, San Antonio, Texas.
U.S. Public Health Service, 1965. Hearing Level's of Adults by Age and Sex. Vital and health statistics, USPHS. Pub. No. 1000, Series 11 Washington, DC: US Government Printing Office.
Walden, B. and McCurdy 0., 1971. The Extent of Hearing Loss in The Army, A Survey Report. USAMRDCI Washington, DC.
Webster, 3., 1969. Effects of noise on speech Intelligibility. American Speech and Hearing Association. Report No. 4, p. 49.
Fort Rucker Aviation Population and Sample Size
Army Aviator Questionnaire
Data and Summary Statistics
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