Prevention of acute acoustic trauma by earmuffs during military training.

BACKGROUND Impulse noise produced from firearms can potentially damage hearing organs resulting in sensorineural hearing loss (SNHL). In Chiang Mai during 1997, Detphithak et al reported a prevalence of acute acoustic trauma (AAT) in 22% of new military privates using an H.K.33 for shooting practice without hearing protection. The H.K.33 renders an average sound pressure level of 128 dBA. Earmuffs are claimed by their manufacturing company to attenuate noise 27 dB, and they have been recently introduced in Chiang Mai to prevent AAT. OBJECTIVE To study the outcomes of hearing protection against AAT using earmuffs during shooting training (H.K.33) in the Military Reserve Officer Training Corps (MROTC). MATERIAL AND METHOD A quasi - experiment study was conducted among 267 normal hearing MROTCs in the Reserve Officers Student Training Center (ROSTC) of the 33rd Military Circle, Chiang Mai, Thailand. Their condition of hearing was examined by an otolaryngologist and an audiologist using pure tone audiometry (PTA) before shooting practice, within 2 hours after the practice, 3 days after the practice at the ROSTC; and 1 month later at Fort Kawila Hospital, Chiang Mai, Thailand. RESULTS All 267 MROTCs used earmuffs during shooting practice. Their mean age was 17.95 years. Average sound pressure level of an HK.33 was 127 dBA (range; 126.5-130 dBA). AAT was found in 4 persons (1.5%) within two hours after shooting. Repeat testing 3 days later revealed that only one person (0.37%) still had SNHL. Fortunately, this case reached complete recovery within 1 month. When comparing these findings with those of 1997, earmuffs can reduce AAT by 15 times (95% confidence interval 5.3-40.5, p-value < 0.0001) to a negligible level. CONCLUSION Shooting training with an H.K. 33 produces a dangerous sound pressure level at the shooting site, resulting in a possible high risk of acute acoustic trauma. Earmuffs can effectively reduce this risk by approximately 15 times, and they should be widely used in all shooting training.