Hearing in young men : The influence of military noise and epidemiological aspects

During compulsory military service, the hearing capacity of the conscripts is at risk, because of exposure to continuous and impulse noise that exceeds the threshold levels of the European Union and the Swedish Work Environment Authority. In young adult life, a multitude of other risk factors also might result in auditory symptoms and hearing impairment. A Military Hearing Conservation Program is of the utmost importance to prevent auditory complications caused by the military exposure. Aims: - to assess the prevalence values of hearing impairment in young men at conscription in the period 1971-1995. to analyse relations between causative factors and hearing symptoms/hearing impairment before military service. to estimate the incidence and relative risk of hearing decline during military service (the efficacy of the Hearing Conservation Program). Methods: In Paper I we obtained cross-sectional data on prevalence values of hearing impairment at conscription, at age 18, from six complete age cohorts born in 1953-1977, in all 337 986 men. Of these, 89.3% were tested with screening audiometry. A group of 839 men from three different military units were included in Paper II and IV. They all performed mandatory screening audiometry and filled in a questionnaire at reporting to training in 2002- 2004. The same procedure was repeated at discharge. In Paper III a group of 747 conscripts from three regiments were included. They performed mandatory screening audiometry at reporting to training in 1999 and at discharge in 2000. In study II, III, and IV the cross sectional data were evaluated as prevalence values and the longitudinal data as incidence values. The comparison groups with no military noise exposure made the first audiometric test at the Centre of Conscription and the second at the army unit at reporting for training. Results: The prevalence values of hearing impairment decreased in the period 1971-1981 from 15.7% to 8.3 and increased in 1986-1995 from 9.8% to 16.3%. This pattern was caused by changes of mild high frequency elevations. The threshold elevations of 45 dB HL or more decreased over the entire study period. Most often only one ear was affected. High frequency hearing impairment was more common in the left ear. Before military service 6.8% of the conscripts reported one or more auditory symptoms often or always. They had been exposed to a multitude of excessive noise levels, the most common being rock concerts or discotheques. TTS after noise exposure was related to elevated prevalence values of hearing impairment and auditory symptoms. The conscripts who often played loud music had an elevated risk for tinnitus but not for hearing impairment. At discharge from military service we observed a relative risk of hearing decline compared to the contrast group of 2.7 in 2000 and of 1.8 in 2003-05. In 2000 we observed an elevated risk of hearing decline during military service for those of the conscripts who had a mild hearing impairment already at reporting to training. This effect was not observed in the more strictly screened group in 2003-2005. Conclusions: TTS after noise exposure was related to elevated hearing thresholds and auditory symptoms. To perform loud music was related to increase in tinnitus but not in hearing thresholds. Prevalence values of hearing impairment, tinnitus and sensitivity to noise were elevated at discharge compared to at reporting to service. Incidence values of hearing decline were elevated during military service compared to before