damage during development. Damage can also occur after birth as a result of severe jaundice. Long-term exposure to loud noise can also damage the cochlea and labyrinth, causing sensorineural hearing loss. Certain drugs, such as streptomycin and gentamycin, can damage the auditory nerve. The most common natural cause is age-related degeneration of the cochlea and labyrinth, known as natural hearing loss due to aging. About 25% of people over the age of 65 have irreversible hearing loss due to this degeneration [90].
b) Extra-otologic diseases
Certain chronic conditions that are not directly related to the ear can cause hearing loss. Some conditions cause damage to the ear by reducing blood flow to the inner ear or brain. These conditions can include heart disease, stroke, high blood pressure, and diabetes. Autoimmune diseases such as rheumatoid arthritis can also be associated with some forms of hearing loss.
1.3.1.2. Noise
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Noise is an important factor that directly damages hearing, and is also the main cause of occupational deafness. People who are regularly exposed to loud sounds, especially over long periods of time, can damage auditory cells and cause hearing loss. About 30 million Americans are exposed to harmful levels of noise in the workplace. Nearly 17% of adults in the United States have some hearing loss. Sometimes it is due to very loud and sudden explosions. Fireworks, gunshots, or other explosions create strong sound waves. They can rupture the eardrum or damage the inner ear. The consequences can appear immediately, leading to permanent damage and deafness [57],[99],[103].
1.3.1.3. Vibration in a noise environment: when there is a vibration factor in a noise environment, hearing loss will be more severe than when exposed to noise alone [33].
1.3.1.4. Drugs and chemicals: High doses of aminoglycosides increase the risk of hearing loss when exposed to noise. Exposure to Toluene and Carbon monoxide is more likely to cause hearing loss than exposure to noise alone [82].
1.3.1.5. Sensitivity: each individual has a different level of hearing loss due to noise [20].
1.3.1.6. Age: people aged > 35 are more vulnerable to noise [20, 30].
1.3.2. In armored troops
1.3.2.1. Harmful effects of noise
The higher the noise intensity, the more dangerous it is, above 85 dB is harmful.
- The higher the frequency, the more likely it is to cause deafness.
- Rhythm: intermittent, rhythmic noise that increases or decreases in intensity is more harmful than steady, continuous noise, especially impulse noise which has the greatest impact.
- The longer the exposure time, the more harmful it is.
- The older you are, the more susceptible you are to the effects.
- Hearing organ condition: people with otitis media are more severely affected, on the contrary, people with otosclerosis and stapedial stiffness can tolerate and are less affected by noise.
- Noise not only has a negative impact on the hearing organs but also through the nervous system affects the whole body and causes fatigue, reducing the ability to work. On the other hand, vehicle noise is complicated by the communication system in combat vehicles, making it difficult to convey instructions and orders verbally or via radio.
- For the central nervous system: causes central nervous system stimulation or inhibition, unstable emotions, poor concentration, reduced reflex speed and poor mood (sadness).
- For other physiological functions: rapid heart rate, rapid breathing, increased energy consumption, poor secretion and motility of the digestive tract.
- Noise can cause vestibular-vegetative reflexes with symptoms of dizziness and nausea.
- For hearing: can cause hearing fatigue, hearing loss, long-term can cause occupational deafness [4].
1.3.2.2. Effect of acceleration
Indicates a change in speed or direction of the vehicle. Acceleration disrupts a series of body functions: stimulates the vestibular system, creating motion sickness. Impact on the central nervous system causes dizziness, heaviness in the head, headaches, insomnia, fatigue, panic/drowsiness, and possible unconsciousness. Impact on the digestive system causes increased salivation, taste disorders, loss of appetite, heartburn, and can cause nausea and vomiting. Impact on the cardiovascular system: at a mild level, it causes heart rhythm and blood pressure disturbances, and in severe cases, it can cause cardiovascular collapse. In addition, acceleration also makes it difficult to control the tank, and body collisions with metal objects can cause injury [4].
1.3.2.3. Impact of temperature
a) In summer: the intensity of solar radiation is high, the car wall absorbs the heat of the sun's radiation. On the other hand, the heat generated from the operating engines is transmitted into the car, causing the temperature inside to increase, greatly affecting the body's thermal balance. The temperature inside the car can be 7 degrees higher than the outside temperature.
- 10 0 C. High temperature and humidity in the car cause serious physiological disorders (loss of sweat, salt...), reducing the ability to work.
The soldiers' movements and combat skills quickly tire them out and even cause heatstroke.
b) In winter: the outdoor temperature is low, the temperature of the vehicle's shell increases lower than the body's surface temperature, so the vehicle's shell absorbs heat from the body through radiation and conduction. At that time, the body must increase metabolism to increase heat production and mobilize mechanisms to reduce heat loss to maintain body temperature at a relatively constant level. This is one of the reasons why the body gets tired quickly. If training and fighting for a long time can disrupt the thermoregulatory mechanisms, causing hypothermia and developing colds [4].
1.3.2.4. Impact of high CO 2 concentration
In normal air, CO 2 has a content of 0.03%, but in an operating tank, the CO 2 concentration increases (can be up to 0.5%) due to the large number of people in the narrow and closed volume of the vehicle, combined with the amount of CO 2 emitted by the engine. High CO 2 concentration in exhaled air leads to increased CO 2 pressure in the alveolar air, hindering the release of CO 2 from venous blood to the alveoli. Therefore, the blood accumulates CO 2 leading to physiological changes: initially causing a state of stimulation, then a state of inhibition of functions.
1.3.2.5. Impact of smoke
When firing artillery shells and machine gun bullets, gunpowder smoke spreads through the breech into the tank and through the shell casing. Gunpowder smoke includes vapors, gases and dust created in the barrel when firing. Gunpowder smoke can pollute the air in the vehicle and cause poisoning of the body. In the composition of gunpowder smoke, carbon oxide (CO) and nitrogen oxides (NO, NO 2 , N 2 O 5 ), cyanide and sulfur compounds are the most toxic substances. Especially carbon oxide, they enter the body through the respiratory tract with clinical manifestations of poisoning. CO gas is a systemic toxic gas. Due to its high affinity for
hemoglobin (300 times more than oxygen), CO combines with hemoglobin to form carboxyhemoglobin (HbCO). Because CO gas occupies hemoglobin, it limits the transport of oxygen to the tissues, causing tissue hypoxia. Symptoms of CO poisoning often include: headache, dizziness, blurred vision, nausea, circulatory disorders, respiratory disorders; severe cases can lead to coma, respiratory arrest and death [4].
1.3.2.6. Impact of dust
When tanks are moving, the amount of dust entering the vehicle can reach 12 - 100 mg/m3 of air. The amount of dust in the vehicle depends on the speed of the vehicle, the number of vehicles moving on the road, the composition and humidity of the air at the combat site, the speed and direction of the wind, the position of the vehicle in the convoy, and the dust filter. When the vehicle moves in a convoy, the amount of dust in the air will gradually increase from the first vehicle, from vehicle number 5 to the last vehicle, the amount of dust in the air is at the same level. Dust entering the eyes causes tears, difficulty in observing and performing operations, causing conjunctivitis, blepharitis, and in some cases, corneal trauma. Dust entering the respiratory tract causes cough, upper respiratory tract infection, pharyngitis, rhinitis, etc. Dust stains clothes, reduces ventilation, absorbs sweat, and limits the body's heat dissipation process [4].
1.3.2.7. Limited visibility and lack of light in the vehicle
From inside the vehicle, the outside can be observed through small gaps, so the field of vision is very limited. The glass covering the vehicle's windows is covered with dust in the summer and blurred by water spray in the winter, so the visibility is very limited. On the other hand, when the tank moves, it causes vibrations, making it even more difficult to observe. When it is too bright outside, it creates a large difference in light inside and outside the vehicle, making it difficult for the eyes to adapt, causing eye fatigue. When the vehicle moves at night and in smoke and dust, the driver's visual activity is very stressful. Night lighting reduces the ability to distinguish details of the vehicle.
objects and spatial concepts are transformed. Objects feel closer and larger than they actually are, and the movement of objects in low illumination feels faster than it actually is [4].
NOISE
HEARING IMPAIRMENT
EAR DISEASES
NOSE AND THROAT
AGE,
MILITARY AGE
PATHOLOGY
FULL BODY
HEREDITARY
TOXIC TO THE INNER EAR
Figure 1.4. Factors affecting hearing loss
1.4. Measures to prevent hearing loss
Although not all forms of SGTL can be prevented, there are easy and effective measures such as regular blood pressure control and timely treatment of cardiovascular disease, not smoking, blood sugar control, exercise, a diet with adequate vitamins and minerals, and caution when using drugs that are toxic to the inner ear.
Because occupational deafness treatment is ineffective even when noise exposure is stopped, noise prevention measures in training are still the main method today.
1.4.1. Personal measures
Based on the psychological and physiological characteristics of tank troops as well as the impact of adverse factors on the body, ensuring the working capacity of tank troops must be carried out using many comprehensive measures.
a) Health assessment selection
- Select people who have the physical requirements to perform heavy work and have a stature suitable for the tank structure.
- Select people with suitable psychological and physiological characteristics to perform well the information operation functions in controlling tanks and using equipment in tanks (ability to quickly receive and process information).
- ENT requirements:
+ Stable state of the vestibular system prevents motion sickness caused by vibration and acceleration.
+ Good hearing to work in conditions of loud noise and sudden changes in pressure when shooting.
+ Upper respiratory tract function must be good, especially without chronic rhinitis or pharyngitis.
- Visual analysis: vision, color sense and good dark adaptation.
b) Training and physical training
General physical training combined with vestibular function training. Physical training and vestibular function training will create the body's ability to adapt to occupational conditions.
c) Ensure proper nutrition
d) Use earplugs or earmuffs to reduce noise and have a rest period after exposure to high-intensity noise. However, while using noise-canceling devices can reduce noise, the disadvantage is that they cause discomfort to soldiers, especially in the confined, hot, and vibrating space of a vehicle.
e) Have a healthy lifestyle, do not smoke, do not drink alcohol, have regular check-ups to detect early hearing loss if any. Have regular health check-ups, promptly detect and treat chronic diseases that can affect hearing.
1.4.2. Collective measures
1.4.2.1. Technical and technological measures
a) Perfecting the tank structure to limit the impact of some adverse factors and increase the operational capacity of tank troops.
b) Isolate sound sources from the engine compartment to other compartments
c) Securely secure machinery and equipment in the tank
d) Ensure proper tension of the chain to the wheel
1.4.2.2. Environmental sanitation technical measures
As a measure to prevent noise transmission or partially absorb the generated noise, this is a research direction to improve protective helmets for armored personnel carriers, ensuring both noise protection and convenience in training and combat.
1.4.3. Medical measures
Currently, the best preventive measure is regular check-ups to detect hearing loss early and provide treatment. However, regulations for regular hearing check-ups have not been established and applied routinely.