Determinants of hearing impairment in Swedish hunters : an e-epidemiological approach

Abstract

Background: 290 236 persons bought a hunting license in Sweden 2014/15. Hunters are spread all over the country and come from all socioeconomic groups. Recreational fiearms can produce peak noise levels between 156-164 decibel sound pressure levels (dB SPL). Human challenge experiments in the past have demonstrated that a single gunshot can cause a severe temporary threshold shift (TTS). The recovery period after such acoustic trauma is unpredictable and a permanent threshold shift (PTS) cannot be excluded. Swedish and international surveys have shown a high usage of hearing protection at training but much less so at actual hunting. Although several studies have measured noise levels from diffrent types of weapons, there are only sparse observational data on dose-risk relationships and effctmodifying factors in humans engaged in real-life hunting. For continuous noise we know that diffrent factors, as tobacco use, affct the susceptibility to develop hearing impairment when exposed, but little is known of these factors when exposed to impulse noise as from hunting fie arms.

Aims: To construct and validate an internet-based screening tool for testing hearing ability among large populations; To examine the association between high frequency hearing Impairment (HFHI) among Swedish hunters, and their shooting habits and usage of hearing protection; To examine potential risk modifiation of HFHI by tobacco-use when exposed to impulse noise from hunting rifl calibers.

Materials and method: A secure website with a questionnaire and an internet-based audiometry test, the InternetAudio test, based on a JAVA platform, was constructed. The combination questionnaire and hearing test was tested for feasibility in a pilot study on 560 members of the Swedish Hunters’ Association (Study1). The hearing test was validated using clinical pure-tone air-conducted audiometry as gold standard on 72 participants (79% women) with a mean age of 45 years (range 19-71 years). Twenty participants had impaired hearing according to the gold standard test (Study 2). Finally in 2013 invitations to participate in the main survey was sent to 27 063 e-mail addresses retrieved from the Swedish hunters’ association membership roster. In all, 1937 persons started the survey, 1771 of them completed the questionnaire, and 202 also completed the InternetAudio test. In this fial survey the InternetAudio test was further validated while used under authentic conditions by 12 participants (Study 3). Associations, between the number of unprotected shooting with hunting rifl calibers (HRC) and HFHI, expressed as prevalence ratio (PR), were multivariately modelled using Poisson regression (Studies 3). In study 4 the possible effct modifiation of tobacco use in form om cigarettes/snus-use or both was examined with the same methods.

Results and conclusions: In the pilot study 162 out of 560 (29%) had answered the questionnaire, out of which 88 (16%) had completed the hearing test. Those who completed the hearing test were older than the non-participants, and had to a larger extent headphones (p=0.003) and the correct version of the JAVA program (p=0.007) than those who only answered the questionnaire (Study1). In the validation study of the Internet based hearing test the Pearson correlation coeffient was 0.94 (p< 0.0001) for the right ear and 0.93 for the left (p = 0.0001). The sensitivity for hearing loss was 75% [95% CI, 51%-90%], and the specifiity was 96% [95% CI, 86%-99%]. The test-retest reproducibility was excellent (Study 2). In Study 3 subjective severe hearing impairment was reported by 195/1771 (11%), while 23/202 (11%) exhibited HFHI upon testing with Internet-based audiometry. As many as 328/1771 (19%) had never used hearing protection during hunting. In the preceding 5 years, 785/1771 (45%), had fied >6 unprotected gunshots with HRC. The adjusted PR of HFHI when reporting 1-6 unprotected gunshots with HRC, relative to 0, was 1.5 [95% (CI) 1.1-2.1; P = 0.02]. We could not verify any excessive HFHI prevalence among 89 hunters reporting unprotected exposure to such gunshot noise >6 times. In Study 4, current daily use of tobacco was reported by 61 hunters (19 cigarettes, 47 moist snuff and 5 both. Tobacco users tended to be younger, to fie more shots with HRC weapons, and to report more hunting days. Their adjusted PR (1-6 unprotected HRC shots versus 0) was 3.2 (1.4-6.7), p <0.01. Among non-users of tobacco the corresponding PR was 1.3 (0.9-1.8), p= 0.18. P value for the interaction was 0.01. The importance of ear protection could not be quantifid among hunters with HRC weapons because our data suggested that the HFHI outcome had led to changes in the use of such protection. Among hunters using weapons with less sound energy, however, no or sporadic use of hearing protection was linked to a 60% higher prevalence of HFHI, relative to habitual use. Hearing ability can be accurately screened with a PC, an internet connection, and a pair of headphones among a willing population. The thesis results support the notion of a wide variation in individual susceptibility to impulse noise. Susceptible individuals may sustain long-lasting or possibly irreversible damage to the inner ear from just one or a few shots, furthermore Tobacco use modifis the association between exposure to unprotected impulse noise from HRC weapons and the probability of having HFHI among susceptible hunters. The mechanisms remain to be clarifid, but since the effct modifiation was apparent also among users of smokeless tobacco, combustion products may not be critical.