Epidemiological Investigation on Symptoms and Inuencing Factors of Noise-Induced Deafness in Ocers and Soldiers After Live Firing Training

Objective To study the symptom and inuencing factors of acute acoustic injury in soldiers after shooting training. Methods A questionnaire survey was conducted among 571 soldiers who participated in live ring training. The necessary information of soldiers, the history of noise exposure before enlistment, and acute acoustic injury symptoms after shooting practice were collected and analyzed statistically. Result The soldiers seldom contact with impulse noise before enlistment. Another environmental noise intensity is medium, and contact time is short; after enlistment, most of them have received shooting training. The most common symptom of acute acoustic injury is tinnitus. The low-frequency tinnitus is more common, and the degree of discomfort is low; A few soldiers wear protective equipment. Statistics found that military age, whether to wear protective equipment, blood type, is related to acute acoustic injury. Conclusion Acute acoustic injury is more likely to occur after exposure to impulse sound in shooting training, and tinnitus is the most common symptom. With the increase of military age, it is more likely to have acoustic injury symptoms. Wearing personal protective equipment can effectively reduce acoustic damage and the susceptibility to the noise of type A blood is low. people with acute acoustic injury symptoms after high level noise exposure. B The proportion of tinnitus due to acute acoustic injury before enlistment.


Background
Military noise is a particular type of noise in a special environment, mainly including high-intensity impulse noise. The level of noise generated by military guns can be as high as 150-180 dB SPL, far exceeding the safe limits -85 dB SPL. Such high-intensity noise will seriously damage the hearing of unprotected people [1] . Acute acoustic trauma symptoms can range from mild reversible hearing loss and tinnitus to severe hearing loss with long-term tinnitus [2] . In foreign military combat training, the utilization rate of personal hearing protection devices (HPD) has reached more than 90%, effectively reducing the occurrence of various acoustic traumas. However, hearing protection is still low in the training of our army's basic units, mainly because of restricted equipment conditions poor protection awareness, and other aspects. Our study conducted a eld survey on soldiers after shooting training to investigate the current status and the in uencing factors of noise-induced hearing loss in our army o cers and soldiers.

Research methods
After the shooting training, the "Noise-induced Hearing loss Questionnaire" was designed and distributed to soldiers for the surveys to obtain basic information, the history of noise exposure before enlistment, and symptoms of noise-induced hearing loss after shooting training. After the questionnaire recovered, the relevant information was analyzed statistically.

Statistical methods
This study's data were statistically analyzed using SPSS 20 statistical software, and χ2 tested the incidence between symptoms and various related factors of noise-induced hearing loss. P < 0.05 indicates that the difference is statistically signi cant.

Results
3.1 High level noise exposure and symptoms before joining the army Before joining the army, only 14.3% of soldiers had been exposed to high level noise environments, and the main noise was daily life noise. Only about 2% of soldiers had experienced strong impulse noises such as explosions in industrial and mining operations (Fig. 1). In the independent rating of the noise intensity, 45.9% of the o cers and soldiers believed that the noise intensity before joining the army was moderate ( Fig. 2A). The cumulative exposure time of high level noise was relatively short, and 63.3% of the o cers and soldiers claimed that the cumulative exposure time was less than one year (Fig. 2B).  Only 13.4% of the o cers and soldiers participating in the investigation developed ear symptoms after being exposed to high level noise before enlisting (Fig. 3A), and 21.1% of them had tinnitus (Fig. 3B). Figure 3 The status of symptoms due to acute acoustic injury after high level noise exposure before enlistment. A The proportion of people with acute acoustic injury symptoms after high level noise exposure. B The proportion of tinnitus due to acute acoustic injury before enlistment.
3.2 High level noise exposure and symptoms after enlistment 88.8% of O cers and soldiers have had a history of exposure to strong impulse noise such as weapon explosion or shooting. 38% of o cers and soldiers have symptoms such as tinnitus, ear fullness, and hearing loss after training (Fig. 4A), of which tinnitus is the most common (56%). Hearing loss accounted for 14%, ear swelling, and fullness 20% (Fig. 4B). Figure 4 The status of symptoms due to acute acoustic injury after enlistment. A The proportion of people with acute acoustic injury symptoms after high level noise exposure among all solders. B The proportion of speci c symptoms among solders who have acute acoustic injury after shooting training.
In tinnitus rate classi cation, High-frequency tinnitus accounted for 41%, low-frequency tinnitus accounted for 49% (Fig. 5A); in the independent rating of discomfort, mild discomfort accounted for 44%, extreme discomfort accounted for only 5%, and the average discomfort was level 2 (Fig. 5B). 3.3 HPD wearing situation when exposed to strong impulse noise During shooting training, only 20% of o cers and soldiers actively wear HPD, and they have various styles and non-standard equipment (Fig. 6A). Among them, earplugs account for up to 41%, earmuffs account for 28%, cotton balls account for 15%, and bullet shells account for 11% (Fig. 6B). 3.4 The relationship between military age, HPD wearing, blood type, and hearing loss symptoms after noise 221 people lled the form about military age and symptoms after shooting. The average military age is 5.81 ± 7.03, and the median military age is two years. Because the standard deviation of military age is large, it is believed that the median is more representative of the military age distribution of the people participating in this research. Therefore, chi-square statistics were conducted on the relationship between military age and hearing loss symptoms after noise by taking two years as the limit. The probability of noise damage symptoms after serving more than two years was 45.1%, and the probability of occurrence of noise damage symptoms less than two years was 31.1%. The difference between them was statistically signi cant (chi-square value = 4.594, P = 0.032 < 0.05); 215 people lled the form about the symptoms after shooting and HPD wearing status, 65.8% of those who wore HPD were asymptomatic, those who did not wear HPD and with symptoms accounted for 51.9%. The difference was statistically signi cant (chi-square value = 4.255; P = 0.039 < 0.05); Among the people with acute noise injure after shooting, the proportion of blood type O appears the highest and rising, reaching 39%, which is compared with the proportion of blood type O (35.6%) in the total population participating in shooting training. The proportion of blood type A with symptoms after the shooting is 18%, which was lower than the proportion of the total population (24%) (Fig. 7). Statistics found no statistical difference between the proportion of blood type O with symptoms and other blood types (chi-square value = 0.073, P = 0.787 > 0.05. There is a statistical difference in the proportion of blood type A without symptoms and other blood types (chisquare value = 4.138, P = 0.042 < 0.05).

Discussion
Noise is a very special item in a severe military operating environment. In addition to direct damage to the hearing system, it can also cause serious damage to multiple systems throughout the body. In recent years, with the rapid development of army's new equipment and weapons, o cers' and soldiers' ratio to military noise has increased signi cantly. As an example, the percent of US Marines with NIHL increased by almost 68% in recent years. The Department of Veteran Affairs states that hearing loss and tinnitus are the third most common form of disability within the military, representing approximately 680, 000 individual cases. Hazardous noise exposure in the US military is greater than it has been in more than 30 years, with 30% of all post-deploying soldiers reporting noise exposure and 28% reporting corresponding hearing loss and/or tinnitus. While noise exposure is a staggering problem for deployed soldiers, it is also a problem for non-deployed soldiers. Recently, it was reported that 11% of Marines sustain hearing loss during basic training, despite the use of hearing protection [3] . As all known, high level noise is the direct cause of acute acoustic injury. When o cers and soldiers operate live ammunition, they produce highintensity impulse sound. The blast wave generated by it can directly act on the human body and cause damage. The alternating overpressure and negative pressure in the blast wave are the main factors leading to hearing organs' damage. The instantaneous overpressure blast wave moves in the air at supersonic speeds, causing violent uctuations in the lymph uid in the labyrinth of the inner ear, which leads to mechanical damage and metabolic disorders such as basement membrane, vestibular membrane, and stria vascularis, and nally leads to hearing impairment [4] Research by the U.S. military has shown that some veterans can tolerate hearing loss to a certain extent but can hardly tolerate long-term tinnitus. Not only does it sound like sleep, but it can also even cause hallucinations and depression, which seriously affects the quality of life. According to the statistics of o cers and soldiers' symptoms after shooting training, tinnitus has the highest incidence, accounting for 56%, which is consistent with other literature reports. However, the degree of self-rating discomfort is lower, which may be related to most o cers' and soldiers' shorter military age. In the classi cation of tinnitus sounds, low-frequency sounds are the most common, which may be related to conductive damage caused by strong impulse sounds.
This study shows that the probability of noise damage symptoms in service for more than two years is much higher than that of less than or equal to 2 years. Past studies have shown that the severity of noise-induced hearing loss increases with exposure time [5] , which is consistent with this study's results. It is suggested that the noise exposure time should be reduced as much as possible during o cers' and soldiers' training, and the prevention work should be focused on those who have served longer. Data shows that 18% of the U.S. military has signi cant noise-induced hearing loss [6] . According to our survey, 38% of the o cers and soldiers participating in shooting training have different noise-induced hearing loss degrees. This signi cant difference may be related to the lack of protective measures in our army.
This survey shows that only 20% of the o cers and soldiers wear HPD, which is much lower than that of foreign soldiers. In the foreign army,90% of the military wear HPD. Whether or not to wear HPD is highly correlated with the appearance of symptoms. This study shows that wearing HPD and asymptomatic accounted for 13.9% higher than those without HPD and asymptomatic. The difference is statistically signi cant, suggesting that wearing HPD is very important for preventing noise-induced hearing loss.
This study shows that blood type has a certain correlation with the population's susceptibility to noise. Among people with noise-induced hearing impairment after shooting, the proportion of blood type O appears the highest, which is higher than the proportion of blood type O (35.6%) compared to the total population participating in shooting training. The proportion of people who appear symptoms after shooting with type A blood is 24%, lower than that of the general population. Previous studies have shown that blood type, and noise susceptibility of people are related. Several previous studies have shown that blood type is highly correlated with people's noise susceptibility. Blood type O is more susceptible to noise [7,8] . However, in this study, there was no statistical difference in the proportion of people appear symptoms with type O blood and other blood types, which may be related to the particularity of the population under investigation. Simultaneously, the ratio of no symptoms with type A blood and no symptoms with other blood types appeared statistically signi cant differences. The difference suggests that type A blood may be more able to tolerate high-intensity noise.

Conclusion
In this study, a large sample epidemiological survey was conducted among the o cers and soldiers who participated in the ring training to understand the in uencing factors and related symptoms of acute acoustic injury, We found that military noise is more likely to cause noise induced hearing loss, among which tinnitus is the most common symptoms. Military age is one of the susceptible factors of noise induced hearing loss. The blood group is related to the susceptibility of noise induced hearing loss, and the blood type A is relatively low.