The effect of recreational gunfire noise on hearing in workers exposed to occupational noise.

Abstract

Hearing sensitivity for an experimental group of 278 industrial workers who engaged in recreational shooting was compared with that of an age-matched control group of 278 nonshooting industrial workers to examine the effect of gunfire exposure on auditory thresholds. The influence of age and the number of unprotected gunshot exposures per year also was examined. The subjects in the experimental group completed a questionnaire to define the types of firearms used, the number of years shooting, and the number of protected and unprotected gunshot exposures per year. Results revealed that hearing sensitivity for frequencies 3.0, 4.0, and 6.0 kHz for both ears in the experimental group was an average of approximately 5 to 10 dB poorer than that of the control subjects. Although both subject groups demonstrated poorer 3.0-to-6.0-kHz hearing sensitivity for left-ear compared with right-ear listening, the degree of asymmetry between ears was greater for the experimental subjects. Older shooters had more hearing loss and greater threshold asymmetry than younger shooters, but there was no significant relationship between hearing sensitivity and the number of unprotected exposures per year. This latter finding was attributed to a fourfold increase in the number of unprotected annual exposures that were reported by younger shooters, who had inherently better hearing than did the older shooters.

Introduction

Hearing conservationists have long been concerned about the confounding relationship between occupational and recreational noise exposures and noise-induced hearing loss (NIHL). Because noise generated from either source is capable of causing NIHL, knowledge of the relationship between these two forms of noise exposure is critical for effective management in hearing conservation programs (HCPs). For example, in order to provide effective counseling about the appropriate use of hearing protection devices (HPDs), managers of HCPs must not only be aware of employees who are exposed to potentially hazardous levels of workplace noise, but also those employees who concomitantly engage in noisy recreational activities.

Recreational firearm noise has been cited as a primary cause of NIHL incurred during leisure activity. [1] It is estimated that in the United States, 60 million Americans shoot firearms as part of recreational target practice or when hunting various wildlife. The use of firearms while hunting is of special concern because it has been reported that only about 1% of hunters use HPDs. [2] The NIHL associated with firearm noise in right-handed rifle shooters has been described as a bilateral high-frequency sensorineural loss in the left ear, which is more affected than the right ear. [3-10]

Although hearing sensitivity has been investigated extensively in various populations exposed to either occupational or firearm noise, only limited data have been reported on individuals who are exposed to both recreational shooting and workplace noise. Prosser et al found that railway workers who engaged in recreational hunting had significantly poorer left-ear high-frequency hearing sensitivity than age-matched coworkers who did not hunt. [8] Differences in right-ear hearing sensitivity were not significantly different between the two groups. Furthermore, the amount of asymmetry noted in the group of hunters increased with age, the number of gunshot exposures per year, and the cumulative number of gunshot exposures over a lifetime. Based on these findings, Prosser et al concluded that railway workers who hunted were at risk of developing greater degrees of hearing loss than coworkers who did not hunt.

Pirila et al reported hearing sensitivity data for two groups of men who were exposed to occupational noise. [11] One group comprised men who had substantial shooting histories, and the other group was made up of men who denied using firearms. Unlike Prosser et al, Pirila et al found that both groups had similar degrees of ear-specific hearing loss and that each group demonstrated significant asymmetry, with the left ear being more impaired. Kryter, as part of a larger study, reported that railway workers who participated in hunting activities and/or target practice or who had used guns as part of their military service had greater degrees of hearing loss, especially in the left ear, than did railway workers of similar age who had not engaged in shooting activities. [12] Finally, Pekkarinen et al examined the hearing sensitivity of 150 forest workers who had been exposed to chain-saw noise. [13] The forest workers were divided into two groups based on their exposure to recreational firearm noise. Foresters wi th high firearm noise exposures had mean hearing thresholds that were 9 dB poorer at 4.0 kHz and 10 dB worse at 8.0 kHz than did forest workers with low firearm noise exposures.

The paucity of published data on the combined effect of recreational firearm and occupational noise on NIHL is only one limitation to our understanding of the relationship. Those studies that have been conducted have been limited solely to comparisons of hearing sensitivity. Other potentially important audiometric indicators, such as standard threshold shift (STS) rates and STS laterality, have not been examined. STS is defined by the Occupational Safety and Health Act (OSHA) as an average 10-dB or greater shift from the baseline audiogram at 2.0, 3.0, and 4.0 kHz for either ear. [14] This measurement is of particular importance to industrial hearing conservation programs because one of the main goals in industrial settings is to minimize STS rates. For example, the use of STS rates and laterality data as measures of HCP outcomes would be compromised if industrial workers who shoot firearms experience greater degrees of hearing loss that occur at faster rates than those of nonshooting industrial personnel.

The purposes of this investigation, therefore, were fourfold: (1) to compare the degree of hearing loss and threshold asymmetry in an experimental group of 278 industrial workers who engaged in recreational shooting with those of an age-matched control group of 278 industrial workers who did not shoot; (2) to identify and compare the occupational HPD compliance in the two groups; (3) to examine the influence of age and the number of unprotected and protected firearm noise exposures per year on hearing sensitivity in the experimental group; and (4) to identify STS rates and STS laterality in both groups.

Subjects and methods

Subjects. The study group consisted of 556 right-handed male industrial workers drawn from a larger pool of 1,830 workers who were enrolled in HCPs. The men were divided equally into an experimental group (n = 278; mean age: 32.5 years) who reported that t they had engaged in recreational firearm activity within the previous 12 months and a control group (n = 278; mean age 32.1 years) who had denied ever using firearms or being exposed to firearm noise. Subjects in the control group were agematched to within 1 year of the subject in the experimental group--that is, for each shooter selected for the experimental group, a nonshooter of the same age was selected for the control group. No subject in ei er group reported otologic symptoms that required medical referral based on American Academy of Otolaryngology--Head and Neck Surgery guidelines. [15]

Because it was reasoned that the sample size of each group was sufficient to minimize any systematic influence caused by potentially confounding variables, no attempt was made to control for factors such as salicylate consumption, blood pressure and cholesterol levels, and smoking habits, either within or between the two groups. Likewise, with the exceptions of age and the number of years enrolled in an HCP, subjects were not matched for the level and type of occupational noise exposure; however, because all subjects were enrolled in HCPs, they presumably were exposed to at least an 8-hour time-weighted average of 85 dBA. More than 90% of all U.S. workers who are exposed to occupational noise are in levels that do not exceed a time-weighted average of 95 dBA. [16] Thus, it was assumed that the level of occupational noise exposure in both groups was similar and that their time-weighted average ranged between 85 and 95 dBA.

Methods. In addition to providing routine demographic data, all subjects completed a case history checklist that included items related to otologic history, general illness, occupational noise exposure, firearm noise exposure, and the use of occupational HPDs. The experimental subjects also completed a questionnaire regarding their shooting habits, including the types of guns fired (i.e., pistols, rifles, calibers, etc.), their estimated, number of protected and unprotected gunshot exposures ithin the previous year, and their total number of years hooting. Pure-tone air conduction thresholds were obtained for all subjects with Telephonics TDH-39 earphones housed in MX 41-AR cushions for octaves from 0.5 to 4.0 kHz and for half-octaves at 3.0 and 6.0 kHz. Audiometric equipment was calibrated to American National Standards Institute [17] criteria, and audiologic assessment were completed in compliance with OSHA guidelines. [14]

Analysis. Hearing sensitivity and threshold asymmetry in the two groups were compared by computing the mean right- and left-ear thresholds at each test frequency. A series of t tests was performed to determine if there were any statistically significant differences (p[less than]0.05) in hearing sensitivity or threshold asymmetry between the two groups at any of the test frequencies.

All subjects indicated on a five-point scale (never, seldom, half the time, almost always, and always) how often they used HPDs while working. The frequency of occupational HPD use was tabulated for both groups by calculating the percentage of subjects who responded to each of the five qualifiers.

Twelve four-way analyses of variances (ANOVAs) were performed on the shooter group data to examine the effects of age, the number of protected and unprotected firearm exposures per year, and the frequency of occupational HPD use on hearing sensitivity at each audiometric test frequency for both ears (i.e., age x the number of unprotected shots x the number of protected shots x the frequency of occupational HPD use at each test frequency for both ears). Experimental subgroups were classified by age (17 to 25, 26 to 35, and [greater than or equal to] yr), the number of reported protected and unprotected gunshot exposures during the previous year (1 to 25, 26 to 100, and [greater than or equal to] 101), and the extent of occupational HPD use (never, seldom, half the time, almost always, or always).

As noted earlier, STS rates and STS laterality were defined in accordance with OSHA guidelines--that is, an STS was recorded whenever an average 10-dB or greater shift occurred at 2.0, 3.0, and 4.0 kHz for either ear. [14] STS rates and lateralities were computed for both groups based on the number of subjects in each group who had completed at least two audiometric assessments (experimental: n = 250; control: n = 216). STS rates were expressed in two ways: (1) as the ratio of STSs in a group to the total number of audiograms performed in that group and (2) as the ratio of subjects with STS in a group to the total number of subjects in that group. STS lateralities for right, left, and binaural shifts were expressed as the ratio of right, left, and binaural STSs for each group to the total number of STSs within the same group.

Results

Air conduction thresholds. Mean right- and left-ear air conduction thresholds were computed from the pure-tone data for both groups (figure 1). At octaves 0.5 and 1.0 kHz, the mean intra- and intergroup differences in hearing sensitivity in both ears were minimal (p = NS for all comparisons). At audiometric test frequencies 2.0 through 6.0 kHz, however, the mean threshold differences within and between groups became more pronounced, especially at 3.0, 4.0, and 6.0 kHz. Thresholds were significantly different for within-group, between-ear comparisons at 3.0, 4.0, and 6.0 kHz among the controls, and at 2.0, 3.0, 4.0, and 6.0 kHz among the experimental subjects (p[less than]0.05). Except for the right ear at 0.5 kHz, between-group, within-ear comparisons revealed that the mean hearing sensitivity for the shooters was consistently poorer in each ear than that of the nonshooters and that these differences were statistically significant at 2.0, 3.0, 4.0, and 6.0 kHz (p[less than]0.05).

Estimates of the influence of firearm noise exposure within the experimental group were calculated by subtracting the mean thresholds for the control group from hose of the experimental group in each ear (figure 2). The nean thresholds at 0.5 and 1.0 kHz were within 1 dB for each ear. At the higher frequencies (2.0, 3.0, 4.0, and 6.0 kHz), however, the mean hearing sensitivity among the shooters was an average of approximately 5 dB poorer in the right ear and slightly more than 7 dB poorer in the left ear (p[less than]0.05 for both ears).

Asymmetry. Asymmetry, determined by subtracting the mean pure-tone threshold in the right ear from the mean threshold in the left ear, was computed for each audiometric test frequency in both groups (figure 3). Neither group exhibited significant asymmetry at 0.5, 1.0, or 2.0 kHz. Asymmetries were more pronounced in each group at the higher frequencies. At 3.0,4.0, and 6.0 kHz, the shooters had an average asymmetry of 5 dB (left ear poorer) and the nonshooters had an average asymmetry of 2 dB (left ear poorer) (p[less than]0.05).

Hearing protection devices. As a group, the shooters appeared to be more conscientious about using HPDs on the job than were the nonshooters (table 1). Approximately one-half of the experimental group (46.0%) reported that they always wore HPDs in the occupational setting, compared with only about one-fourth of the controls (26.9%).

The twelve separate four-way ANOVAs revealed that only age had a significant influence on hearing sensitivity among the shooters (p [less than] 0.05). (The other ANOVA analyses concerned the number of protected and unprotected firearm noise exposures and the frequency of occupational HPD use among the shooters.) Among the two younger subgroups of shooters, there was a trend toward a slight impairment of hearing sensitivity at the higher frequencies (2.0 to 6.0 kHz) and a small but consistent left-ear asymmetry (figure 4), but the differences between these two subgroups were not statistically significant. In the older shooters, thresholds at 1.0 through 6.0 kHz were an average of 6 to 15 dB poorer in the right ear and 10 to 25 dB poorer in the left ear compared with the two younger subgroups. Mean thresholds for both ears in the older group were significantly poorer than those of either of the younger groups (p [less than] 0.05).

Standard threshold shifts. When expressed as the ratio of within-group STSs to the total number of audiograms, the STS rates were 1.96% in the shooter group and 0.8 6% in the nonshooter group (table 2). When STS rates were expressed as the ratio of within-group STSs to the total number of subjects in the group who had completed at least two audiograms, they were 8.0 and 3.2%, respectively. STS lateralities in the experimental group were 5, 35, and 60% for right, left, and both ears, respectively.

The corresponding data in the control group were 14, 29, and 57%.

Discussion

This investigation compared the hearing sensitivity of two groups of industrial workers drawn from a single HCP that provided services to several companies. All subjects had been enrolled in the HCP as a consequence of their histories of occupational noise exposure. One group comprised workers who reportedly engaged in recreational firearm use, while those in the control group denied using firearms. The subjects in this study worked for different companies, and the companies were represented fairly equally in the two study groups. Thus, given the sample size of each group, it is logical to assume that the overall profile of occupational noise exposure in the two groups was similar.

Based on the findings of this investigation, it can be estimated that workers exposed to occupational noise can expect to incur an additional 5- to 10-dB high-frequency loss of hearing sensitivity if they shoot firearms on a recreational basis. The significant differences between shooters and nonshooters in hearing sensitivity in both ears at 2.0, 3.0, 4.0, and 6.0 kHz, as well as the significantly greater interaural asymmetry at 3.0, 4.0, and 6.0 kHz among the shooters, most likely reflects exposure to recreational firearm noise. In general, these findings support those previously reported by Prosser et al [8] and Kryter. [12]

The observation that older shooters exhibited a significantly greater hearing loss at the higher frequencies than did younger shooters was also reported by Prosser et al. [8] This finding provides support for the contention that the aging process enhances the degree of industrial NIHL among shooters.

The finding that the number of unprotected firearm exposures among the shooters did not have a significant effect on hearing sensitivity is most likely explained by the higher incidence of unprotected exposures reported by younger shooters, whose hearing sensitivity was inherently superior to that of the older subjects. The youngest group of shooters had four times as many unprotected exposures during the previous year as did the oldest shooter group (table 3). Because it is reasonable to hypothesize that the middle-aged and older shooters were exposed to a similar number of unprotected gunshots when they were the same age as the younger shooters, it follows that the influence of the aging process coupled with the cumulative exposure to both occupational and firearm noise served to obscure the relationship between hearing sensitivity and the reported number of unprotected exposures during the previous year. Also, the number of unprotected exposures in this study occurred in a rather limited 1-year period. An exact determination of the cumulative effect of repeated fire noise on hearing sensitivity would require accurate records of all exposures to all types of firearms over each subject's entire shooting history. This would be an impossible task in a retrospective study.

Although STS rates and lateralities ere significantly greater in shooters than nonshooters, these findings should be treated cautiously because of the relatively small number of STSs. Nonetheless, the STS data suggest that industrial workers exposed to recreational firearm noise might experience a decrease in hearing at a faster rate than nonshooters. Such a circumstance could result in an inflation in the rate of injuries caused by NIHL.

It is interesting that the shooters reported a higher compliance rate with occupational HPD use than did the nonshooters. One possible explanation is that shooters might be more conscientious about wearing HPDs at work because they might have received more training in their use during firearm safety classes, which are mandatory in the U.S. before any person can obtain a hunting license. Also, HPDs are almost always used by shooters during target practice. [18] This might have made them more cognizant of their importance in the occupational setting.

Finally, it should be noted that there were two primary limitations to this investigation that require further consideration, especially with respect to future research. First, data for this investigation were obtained by a retrospective examination of audiometric test results and paper-and-pencil-type questionnaires. Because practical considerations precluded an in-depth, face-to-face questioning of subjects to verify shooting habits and noise exposure histories, it is possible that some individuals who indicated negative case histories for shooting might, in fact, have had some experience with firearms. Although the potential influence of forgotten firearm noise exposures on the hearing sensitivity of our control subjects cannot be quantified, it is logical to assume that such unknown exposures would have minimized rather than enhanced the differences in hearing sensitivity between the two groups. The reason is that the relative differences in hearing sensitivity found in this investigation might have been even greater if the hearing of shooters had been compared with an age-matched group of persons who had absolutely never experienced even a single firearm noise exposure.

A second limitation relates to the overall variability observed in shooting habits. For example, the annual number of protected and unprotected gunshot exposures and the type and caliber of the firearms used varied dramatically. This high degree of variability, coupled with the limited 1-year shooting history, precluded any meaningful comparison of shooting habits and hearing sensitivity. Regardless, future investigators are encouraged to consider carefully these observations in order to delineate further the relationship between recreational gunfire and occupational noise exposures and hearing handicap.

The findings of this investigation serve to emphasize to hearing conservationists the importance of including in their educational efforts information on recreational firearm activities. Specifically, it would be prudent to stress the potentially hazardous effects of gunshot noise and to emphasize the use of HPDs during both work and recreation. Special attention should be given to younger workers, who not only report the highest number of annual gunshot exposures, but also the highest incidence of unprotected exposures.

From the Division of Audiology, Department of Communication Disorders, Central Michigan University, Mt. Pleasant, Mich. (Dr. Stewart and Dr. Konkle), and the Department of Audiology and Speech Language Pathology, Wayne State University, Detroit (Dr. Simpson).

Reprint requests: Dan P. Konkle, PhD, Moore 466, Central Michigan University, Mt. Pleasant, MI 48859. Phone: [phone]; fax: [phone]; e-mail: [e-mail]

References

(1.) Clark WW. Noise exposure from leisure activities: A review. J Acoust Soc Am 1991;90:175-81.

(2.) Kramer WL, Updike CD. Recreational shooters and their use of hearing protection. Presented at the Annual Meeting of the American Speech-Language-Hearing Association; November 1991; Seattle, Wash.

(3.) Ogden SW. Effect of gunfire upon auditory acuity for pure tones and efficacy of earplugs as protectors. Laryngoscope 1950;60:993-1012.

(4.) Ward DW. Hearing of naval aircraft maintenance personnel. J Acoust Soc Am 1957;29:1289-301.

(5.) Taylor GD, Williams E. Acoustic trauma in the sports hunter. Laryngoscope 1966;76:863-79.

(6.) Keim RJ. Sensorineural hearing loss associated with firearms. Arch Otolaryngol 1969;90:581-4.

(7.) Odess JS. Acoustic trauma of sportsman hunter due to gun firing. Laryngoscope 1972;82:1971-89.

(8.) Prosser S, Tartan MC, Arslan E. Hearing loss in sports hunters exposed to occupational noise. Br J Audiol 1988;22;85-91.

(9.) Ylikoski J. Acute acoustic trauma in Finnish conscripts. Etiological factors and characteristics of hearing impairment. Scand Audiol 1989;18:161-5.

(10.) Dancer A, Grateau P, Cabanis A, et al. Delayed temporary threshold shift induced by impulse noises (weapon noises) in men. Audiology 1991;30:345-56.

(11.) Pirila T, Sorri M, Jounio-Ervasti K, et al. Hearing asymmetry among occupationally noise-exposed men and women under 60 years of age. Scand Audiol 1991;20:217-22.

(12.) Kryter KD. Hearing loss from gun and railroad noise--relations with ISO standard 1999. J Acoust Soc Am 1991:90:3180-95.

(13.) Pekkarinen J, Iki M, Starck J, Pyykko I. Hearing loss risk from exposure to shooting impulses in workers exposed to occupational noise. Br J Audiol 1993;27:175-82.

(14.) United States Department of Labor. Occupational Safety and Health Administration: Occupational noise exposure; hearing conservation amendment; final rule. Federal Register 1983;48:9738-85.

(15.) Otologic Referral Criteria for Occupational Hearing Conservation Programs. Washington, D.C.: American Academy of Otolaryngology--Head and Neck Surgery, 1983.

(16.) Franks JR. Number of workers exposed to occupational noise. Seminars in Hearing 1988;9:287-98.

(17.) American National Standard Specifications for Audiometers, ANSI S3. New York: American National Standards Institute, 1989.

(18.) Stewart M, Ball L, Simpson T. Shooting habits and demographic risk patterns for recreational firearm users. Presented at the Annual Meeting of the National Hearing Conservation Association; February 1998; Albuquerque, N.M.

Table 1. Frequency (%) of the use of occupational hearing protection
devices in the experimental and control subjects
                             Half the  Almost
              Never  Seldom    time    always  Always
Experimental  27.3    12.2     2.8      11.7    46.0
Control       47.8    16.9     2.7       5.7    26.9
Table 2. Standard threshold shift (STS), the number of
completed audiograms (A), and the total number of subjects
who had at least two completed audiograms in the
experimental and control group
              STS*                  A        Subjects
               R    L   B   T     n (%)        n (%)
Experimental   1    7  12  20  1,022 (1.96)  250 (8.0)
Control        1    2   4   7    811 (0.86)  216 (3.2)
(*)R = right ear;
L = left ear;
B = bilateral;
T = total.
Table 3. Mean number of unprotected
firearm noise exposures in the
experimental group during the 1-year
study period, classified by age
                                 Subjects          Mean
Age                                 n              (SD)
17 to 25 yr                         49     193.6 ([+ or -]28.1)
26 to 35 yr                        154     106.5 ([+ or -]15.8)
[greater than or equal to]36 yr     75      48.6 ([+ or -]22.7)

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