Noise Induced Hearing Loss

The word noise is derived from the Latin word "nausea" meaning impulsive, unwanted, and unpleasant. Noise pollution can be defined as any unwanted elec­tromagnetic signal that produces a jarring or dis­pleasing effect and which interferes with human com­munication, comfort and feeling of wellbeing.  Noise-induced hearing loss (NIHL) is the most common cause of Sensorineural hearing loss (SNHL). NIHL is one of the most common workplace disorders and the second most self-reported occupational injury. In U.S. alone thirty million workers are at risk for noise-induced hearing loss, and 22 million American adults ages 20 to 69 already have it. Forty-four percent of carpenters and 48 percent of plumbers report having a hearing loss. By age 25, the average carpenter has the same hearing as a 50-year-old person who does not work around hazardous noise.

World Health Organization (WH)) estimated 360 million individuals in the world with disabling hearing loss, out of which 91% are adults and only 9% are children.1 Prevalence of disabling hearing impairment in South East Asia is 2.4% in children; 9.5% in adults below 65 years and up to 48% in adults above the age of 65.2 The most common etiological factor for this age related deafness is noise pollution.  National Institute on Deafness and Other Communication Disorders (NIDCD) estimates that 15% of population has got decreased hearing due to loud noise which may be noise at work, leisure activity or use of mobile phone only3. Two publications in Indian Journal of Otology state that use of mobile phone may lead to sensorineural deafness4 and if continuously for more than 30 minutes, may lead to 10 dB transient hearing loss.5

As per data available 3% average population suffers from deafness6 which includes both conductive and sensorineural hearing loss,7 but the incidence is rising due to noise pollution which may be much higher in metros, in genetically prone families and in persons with anemia and vitamin D deficiency,8,9 or with other associated factors.   

Exposure to sound above a level of approximately 85 dB initially manifest as a temporary hearing loss or dullness of hearing that is known as temporary threshold shift (TTS), which may have fast resolution within first 10-15 days of the exposure. However, a repeated or sustained exposure of noise to the hair cells and associated nerve fibers leads on to degenerative changes and the TTS becomes permanent threshold shift (PTS). The effect of excessive noise could be so devastating that it can cause permanent memory loss or psychiatric disorder. There are many hypothesis that include mechanical injury from basilar membrane motion, metabolic exhaustion, activity induced ischemia and ionic poisoning from breaks in cell membrane.

Noise-induced hearing loss is 100% preventable, but once you have hearing loss, you'll have it for life. Exposure to harmful sounds causes damage to the sensitive structures of the inner ear. These structures can be injured instantly from an intense, brief impulse, such as the explosion of a firecracker10, or gradually from continuous exposure to noise, such as in a woodworking shop.

Noise exposure can cause two kinds of health effects. These effects are nonauditory effects and auditory effects. Nonauditory effects include stress-related physiological and behavioral effects, and safety concerns. Auditory effects include hearing impairment resulting from excessive noise exposure and tinnitus. Noise-included permanent hearing loss is the main concern related to occupational noise exposure.

The main auditory effects include:

1. Acoustic trauma
2. Tinnitus (Ringing or buzzing in the ear)
3. Temporary hearing loss / temporary threshold shift (TTS): It occurs immediately after exposure to a high level of noise or a sustained sound by ear phone or mobile phone. These shifts usually involve the 3-8 kHz range. They tend to resolve within minutes to hours while complete recovery depends on the degree of shift & type of noise trauma.
4. Permanent hearing loss /permanent threshold shift (PTS): If exposure to loud noise continues at a rate of eight hours a day and five days a week, it can be a threat to develop permanent hearing loss. The hearing impairment is noticeable only when it is substantial enough to interfere with routine activities. At this stage, a permanent and irreversible hearing damage has occurred. When noise exposure stops, the person does not regain the lost hearing sensitivity. As the employee ages, hearing may worsen as "age-related hearing loss" adds to the existing noise-induced hearing loss. This loss, usually, is permanent and is not satisfactorily cured by medicines or any devices such as, hearing aids. There is focal loss of hair cells and degeneration of nerve terminals.

We should protect hearing of our children as voice development is dependent upon proper hearing of a child. Pre-lingual deaf child will have to go for Cochlear implantation before the age of 5 years for proper voice development.

Two types of hearing loss are: conductive and sensorineural.In conductive deafness sound-pressure waves are unable to reach the cochlea due to external ear canal blockage, ruptured tympanic membrane (TM) or a defect in the ossicles of the middle ear.

The function of middle ear structures (TM, malleus, incus & stapes) is to convert ear pressure changes into the vibrations of the inner ear fluids. During sound transmission through the middle ear, the tensor tympani and the stapedius muscles play an important role in inner ear protection from high intensity acoustic stimuli of moderate frequencies.

Sensorineural hearing loss can be noise-induced or increasing age-related. The pathology in NIHL lies with the outer hair cells of the cochlea causing interference in the mechano-electrical transduction. The Organ of Corti is the site for mechano-electrical transduction of the sound signal as this organ contains hair cells and supporting cells. Possible mechanisms involved are mechanical injury from basilar membrane motion, metabolic exhaustion, activity-induced ischaemia, and / or ionic poisoning from cell membrane breaks.11 Pathological changes range from bending or fusion of cilia, up to the total destruction of supporting cells. Other hypotheses for NIHL include nerve rootlet damage by overstimulation from outer hair cell, over secretion of glutamate or cellular damage from glutathione depletion. Sensorineural deafness by virtue of acoustic trauma is more common type of loss in teenagers today listening to loud rock music.

Hazardous sound levels are louder than 80 decibels. That's not as loud as traffic on a busy street. Listening to loud music, especially on headphones, is a common cause of noise-induced hearing loss. The degree of a hearing hazard is related to both the level of the noise and the duration of the exposure. Sound is measured in decibels (dB). Prolonged (long or repeated) exposure to noise at or above 85 dB can cause hearing loss. A normal conversation takes place at about 60 dB. A hand drill measures 98 dB, a spray painter 105 dB, a hammer drill 114 dB, and a pneumatic percussion drill 119 dB. (Table-I)  

Industrial Noise Levels

Table-I

The National Institute of Occupational Safety and Health has issued guidelines about the amount of time you can be exposed to different noise levels safely. If the level of noise in your workplace averages 85 decibels, you are at risk for noise-induced hearing loss after eight hours of exposure. If the average level of noise is 88 decibels, you are at risk after only four hours of exposure. Remember—the greater the noise level, the less time before hearing damage can occur. (Table-II) In the initial stages, noise-induced hearing loss is most pronounced at 4000 Hz but it spreads over other frequencies as noise level and/or exposure time increases. Noise trauma is known to cause 4 kHz notch or Aviators notch. Government and regulatory beets have fixed the permissible limit of sound levels.

 

Permissible Sound Levels

Table-II

Sound Intensity & Permissible Time Limit

Table-III

 

 

 

Non audiological Pathological disorders:

(i)         High frequencies or ultrasonic sound above the audible range damages inner ear and induces nau­sea and dizziness in man.

(ii)        Exposure to mid-audible frequency affects brain and nerv­ous system with significant impact on thinking and coordination of limbs.

(iii)       Moderate vibration can lead to pain, numbness and cyanosis of fingers and toes. It causes contraction of blood vessels, makes the skin pale leads to excessive secretion of adrenaline hormone into blood thereby inducing high blood pressure, along with thickening of blood and changes in breathing amplitude.

(iv)       Severe vibration results in damage to bones and joints in the hands along with swelling and stiffness.

(v)        Exposure to low frequency noise can reduce heart beat, variation in blood pressure and breathing difficulties.

 (vi)      Noise of a jet engine may aggravate or may lead to gastric ulcer and thymus gland atrophy.

 (vii)     Depression, anxiety, insecurity, and paranoia, and involvement in unsocial activities.

 

As required by government agencies, workplaces that involve exposure to noise have a mandated hearing conservation program (HCP). HCPs feature a number of components included sound management, engineering and administrative controls, audiometric monitoring, and use of personal hearing protection. Understanding of the process, evaluation and treatment of NIHL is of vital importance to the practicing otorhinolaryngologists. You can protect your hearing by-

• Keeping the volume down while  listening to music
• The safest way to protect your ears is to always wear hearing protectors anytime you are around loud noises.

Work places where noise and ototoxic chemical combine include painting, printing, construction, furniture, manufacture of metals fuelling vehicles, fire fighting, pesticide spraying needs special mention and their employee need particular attention. Organic solvent seeks special mention as they are two fold harmful one as in manufacturing unit second as a consumer as it is used in detergents, medicines, perfumes, fabric, paper coating and laser printing inks and insect repellents.

In environmental problems leading to health hazards noise is the easiest to control but awareness among the public and strict regulatory authority is required. No significant attention has been drawn towards preventive measures. No concrete efforts have been made to decrease the intensity of sound (Noise) in industrial area even the manufacture are not made aware to decrease the noise level of industrial machines. Workers at many places are not provided with preventive devices, ear plugs or ear muffs. Remember noise level drops about 6 dB each times if distance is doubled.

Methods of minimizing Industrial noise pollution:

The noise pollution in industries can be lowered down by adopting improved techniques, Sound proofing, and Transmission control. The level of noise can be minimized by covering the walls with acoustic tiles, introducing gaskets at the doors and windows, sealing the outlets and putting carpet, drapery and acoustic materials inside the room.

Green vegetation cover by planting the trees should be along highways, streets and indus­trial areas as vegetation covers absorb and dis­sipate sound energy and acts as a buffer zone.

Operations in open space: Noisy industrial operations may be conducted at open spaces far off from resi­dential areas in day time only.

Protective devices use (ear plugs or ear muffs or even cotton balls) in the industry can protect from hazardous effects of noise. Government should provide subsidiary for it. NGO must be promoted to join hands in public education and awareness program. We must remember with increase in life span after ten years we will be having more than 18% citizens and up to 40% above the age of 75 years having disabling deafness all the more out of which only 20% will be benefited by hearing aids hence we have to take all precaution and necessary measures to prevent all preventable or avoidable type of hearing loss especially taking care of noise pollution. 

Start protecting your ears now by avoiding noise pollution and hearing relaxing technique yoga. Meditation and weekly off in quit areas.

References:

1-         World Health Organizaiton. Deafness and Hearing Impairment Fact Sheet; 2014. Available from: https://www.who.int/mediacentre/factsheet/fs300/index.   

2-         WHO. Global Estimates on Prevalence of Hearing Loss; 2012. Available from: https://www.who.int/pbd/deafness/WHO_GE_HL_pdf.

3-         Health Info Statistics and Epidemiology. Quick statistics, National Institute on Deafness and Other Communication Disorders (NIDCD). Available from: http:/www.nidcd.nih.gov/health/statistics/pags/quick/aspx.

4-         Sahoo GC, Sebastian H. Prevalence of sensorineural deafness in habitual mobile phone users. Indian J Otol 2011;17:97-100.

5-         Ramya CS, Karthiyanee K, Vinutha S. Effect of mobile phone usage on hearing threshold: A pilot study. Indian J Otol. 2011;18:159-61.

6-         Taneja MK. Role of ENT surgeons in the national program for prevention and control of deafness. Indian J Otol 2012;18:119-21.

7-         Taneja MK. Deafness, a social stigma: Physician perspective. Indian J Otolaryngol Head Neck Surg 2014; 66:353-8. 

8-         Taneja MK. Role of vitamin D in prevention of deafness. Indian J Otol 2012;18:55-7.

9-         Taneja MK, Taneja V. Vitamin d deficiency in E.N.T. Patients. Indian J Otolaryngol Head Neck Surg 2013;65:57-60.

10-       Taneja MK. Diwali-fire crackers and deafness. Indian J Otol 2004;10:3-5.

11-       Bugliarello, G Alexendre, A Barnes, J & Wakstein C. 1976. The impact of Noise Pollution : A socio-technological introduction, New York : Pergamon Press.

 

Compiled by Dr M K Taneja