Noise Induced Hearing Loss

Dr. Vishal Sharma

Definitions

• Noise = wrong sound, in wrong place, at wrong

time (Park & Park)

• Acoustic Trauma = sudden, permanent sensori-

neural deafness due to single exposure to an

intense sound (130-140dB) of < 0.2 sec

• Chronic Noise Induced Hearing Loss = gradual

SNHL due to years of exposure to noise

History

• Ramazzini (1713): reported NIHL in copper workers

• NIHL recognized in US, Germany & UK in 1870s

• Thomas Barr of Glasgow (1886): did first survey

• Habermann (1890): histology of NIHL in cochlea

• Fowler (1929): first to comment on 4 kHz dip

• Bunch (1939): audiometric features of NIHL

Epidemiology• 30 million adults in United States are exposed to

hazardous occupational sound levels (National

Institute for Occupational Safety & Health, 2000).

• Among these 30 million, 1 in 4 will acquire a

permanent hearing loss (American Academy of

Audiology, 2003).

• 50 % of male miners have hearing loss by age of

50 & 70 % by age of 60 (NIOSH, 2001).

Classification of noise

A. Based on duration: B. Frequency based:

1. Continuous: cotton spinning 1. High: sawing

2. Interrupted: traffic 2. Low: grinder

3. Transient (< 0.2 sec) 3. White noise: boiler

a. Impulse: explosion

b. Impact: metal to metal collision

Sources of noise• Industrial noise

• Road traffic noise

• Aircrafts & Railways noise

• Entertainment noise: clubs, discos

• Residential noise: alarms, music systems, home

theatre, air conditioners,

generators

• Personal noise: personal stereos, mobile phones

• Firearms & bomb blast noise

Common Occupational Noise

• Agriculture

• Mining

• Construction

• Manufacturing

• Public Utilities

• Transportation

• Military

Volcano eruption 190 dB

Jet plane 120-150 dB

Thunder 120 dB

Factory boilers / trains 110 dB

Cars & bikes 90 dB

Loud radio music 85 dB

Grand piano 85 dB

Children crying 80 dB

Loud conversation 70 dB

Dog barking 70 dB

Sound Thermometer

Clinical Features of N.I.H.L.

• Similar to early ototoxicity & presbycusis

• Early NIHL limited to high frequency: no symptom

• First symptom: trouble in speech comprehension

with loud background noise

• As NIHL progresses, patients have difficulty in

understanding high-pitched voices (women &

children) even in quiet conversational situations

Clinical Features

• Telephonic conversation is unimpaired because

telephones don’t use frequencies > 3000 Hz

• Many patients experience tinnitus & hyperacusis

• Post-exposure tinnitus & temporary deafness are

warning signs of impending permanent NIHL

Impact of noise on children

• Household noise retards cognitive development of

children b/w 7-24 months. Brains of children cope

with loud noise by sound filtering which also

includes human speech. This leads to retardation

of intellectual skills.

• Foetus also may suffer from effects of noise.

Characteristics of chronic NIHL (Dobie, 1990)

• Always sensorineural, symmetric & bilateral.

• Greater SNHL present at 3, 4, & 6 kHz, with

recovery at 8 kHz ***. Usually greatest at 4 kHz.

***Presbyacusis has no recovery at 8 kHz

• Isolated NIHL is never > 75 dB in high

frequencies or > 40 dB in lower frequencies.

• Rate of hearing loss in chronic NIHL is greatest

during first 10-15 years of exposure & decreases

later as hearing worsens. Hearing loss does not

progress after noise exposure is discontinued.

• Commonest cause of asymmetric NIHL is

exposure to firearms. Right-handed shooters have

more severe left ear deafness (left ear faces barrel

while right ear is in acoustic shadow of head).

• Similarly, Tractor operators look over their right

shoulder, exposing their left ear to noise of prime

mover + exhaust & their right ear gets shielded.

Acoustic head shadow

Auditory Effects of Noise

• Per-stimulation fatigue or Adaptation:

Noise exposure of > 90 dB elevates hearing

threshold temporarily, especially at 4 kHz

• Post-stimulatory fatigue:

• Temporary Threshold Shift (reversible)

• Permanent Threshold Shift (irreversible)

Temporary Threshold Shift

• Exposure to loud noise for seconds to hours may

cause SNHL that recovers within 16-24 hours.

• Magnitude of TTS depends on:

• More intense sounds lead to larger shifts.

• Speech frequencies (500-3000 Hz) are most

susceptible to TTS.

• Interrupted exposures cause less TTS than

continuous exposures.

Permanent Threshold Shift• Repeated TTS over weeks, months & years fail to

recover completely & become noise-induced

permanent threshold shift (NIPTS)

• NIPTS is measured by subtracting amount of

hearing loss to be expected due to aging from

hearing threshold level

• Amount of NIPTS & frequencies involved depend

primarily on same parameters, as for TTS

Calculation of N.I.P.T.S.

Age related hearing loss

Progress of N.I.P.T.S.

Progress of N.I.P.T.S.

Male vs. female N.I.P.T.S.

Non-auditory effects of noise• Insomnia

• Anxiety & nervousness

• Coronary heart disease

• Lack of concentration

• Fatigue

• Irritability & impatience

• Indigestion

• Peptic ulcer

• Speech interference

• Hypertension

• ed heart rate

• ed intra-cranial tension

• ed breathing rate

• ed sweating

• ed color perception

• ed night vision

Factors influencing N.I.H.L.

1. Frequency: noise b/w 2-3 kHz more damage

2. Intensity & duration of exposure: noise > 85 dBA

for 8 hr time weighted average is unsafe

3. Intermittence: intermittent noise with quiet

intervals is safer than

steady noise

4. Age: children & elderly are more prone to NIHL.

Presbyacusis has additive effect not

synergistic.

5. Sex: males are more prone than females to NIHL

6. Individual Susceptibility: Tender ears are more

prone to NIHL than tough ears due to difference in:

Genetic: Ahl gene E.A.C. skin elasticity

area ratio of TM:stapes mass of ossicles

tension of stapedius outer hair cell density

basilar membrane stiffness cochlear vascularity

endolymph composition psychological

7. Experience: Green ears have bigger NIPTS than

those exposed to noise before (ripe ears). NIPTS

can be reduced by adding prior exposure at lower

levels (toughening or conditioning of ear).

8. Conductive deafness: gives protection (?) against

NIHL as less sound gets conducted to inner ear

9. Ototoxic drugs: enhance NIHL

10. Vibration: Raynaud phenomenon es NIHL

11. Absent Stapedial reflex: es NIHL for lower

frequency

12. Melanization: Albinos are more prone to NIHL

13. Diabetes mellitus, Hyper-cholesterolemia,

Cardiovascular disease: increase risk of NIHL (?)

14. Smoking: increase risk of NIHL (?)

15. Deep sea divers: are more prone to NIHL

Patho-physiology

NIHL damage is explained by:

• Macro-mechanical theory

• Micro-mechanical theory: produced by high

intensities

• Biochemical theory: produced by moderate

intensities

Macro-mechanical theory

• Traveling sound wave produces movement of

basilar membrane

• Central part of basilar membrane undergoes

maximum rocking vibration. This part houses

outer hair cells (especially inner row) hence they

are subjected to maximum damage.

Organ of Corti

Micro-mechanical Damage

Temporary Threshold Shift: moderate swelling of

hair cells + shortening of rootlets of hair cells +

small vacuoles in supporting cells

Permanent Threshold Shift: marked swelling of hair

cells + fracture of rootlets of hair cells + large

vacuoles in supporting cells + damage to outer

hair cells + damage to inner hair cells

Biochemical Damage

• Micro-mechanical damage causes interruption of

normal chemical gradient of cochlea which leads

to ionic poisoning

• Noise activity induced vaso-constriction leads to

cochlear ischaemia

• Metabolic exhaustion of activated hair cells

Investigations

A. Diagnostic B. Research only

1. Subjective Low power

Microscopy

Pure Tone Audiogram Electron Microscopy

2. Objective Cyto-cochleogram

Oto-acoustic emissions

Cortical Evoked Response Audiometry

Multiple Auditory Steady-state Evoked Response

Audiogram in Acoustic Trauma

Audiogram in Chronic N.I.H.L.

4 kHz (Boilermaker’s) notch, C5 dip

• characteristic audiometric

pattern of early NIHL.

• If exposure is continued,

notch gradually deepens

& widens.

• 4 kHz notch also seen in

head injury, barotrauma

or even in absence of any

explanatory history.

Reasons for 4 kHz notch1. Natural resonance of EAC being 2-3 kHz, 150% of

this produces maximum damage between 3-5 kHz

2. Protective effect of acoustic reflex below 2 kHz

3. Intermittent noise is more damaging for 3-5 kHz

4. Outer hair cells of 3-5 kHz region are more prone

to oxidative stress, have reduced vascularity (?)

& ed oxygen consumption (?)

Acoustic gain: middle ear & pinna

Otoacoustic Emissions (OAE)

• Spontaneous OAE: Sounds emitted without

stimulus. Presence indicates hearing < 25 dB HL.

• Transient evoked OAE: Sounds emitted in response

to short duration click stimulus. Presence = < 35 dB HL

• Distortion product OAE: Sounds emitted in

response to 2 tones of different frequency & intensity.

Presence = < 50 dB HL. Good for higher frequencies.

Normal Transient evoked OAE

Normal Transient evoked OAE

Reproducibility should be > 75 %

Early detection of N.I.H.L.

Early stage N.I.H.L.

Advanced stage N.I.H.L.

Malingering of N.I.H.L.

Auditory Evoked Potentials

• Auditory Brainstem Response: 1.5-10 ms post

stimulus; originates in 8th cranial nerve (waves I & II)

up to lateral lemniscus & inferior colliculus (wave V)

• Middle Latency Response (MLR): 25-50 ms post

stimulus; arises in upper brainstem & auditory cortex

• Slow Cortical Response: 50-200 ms post stimulus;

originating in auditory cortex

Auditory Evoked Potentials

Cortical Evoked Response Audiometry (CERA) or P1-N1-P2 response

• good specificity over speech frequency range

• recorded from higher auditory level than BERA, so

less subject to organic neurological disorders

• CERA must be done to evaluate accurate hearing

threshold in pt with flat audiogram & hearing

threshold of > 25 dB at 500 Hz

Multiple Auditory Steady-state Evoked Response audiometry

• Are responses to rapid stimuli where brain response to

one stimulus overlaps with responses to other stimuli

• Gives rapid, frequency specific & objective hearing

assessment by giving 4 continuous tones to each ear

• Slow rate responses (<20 Hz) arise in cortex & faster

rate responses (>70 Hz) originate in brainstem 

Multiple Auditory Steady-state Evoked Response audiometry

Photo-micrograph of Cochlea

Scanning electron micrographs

Cyto-cochleogram

Noise Exposure Evaluation

Definitions

• A scale (dBA): gives more weight to frequencies

b/w 1 to 5 kHz & less weight to other frequencies

• Time Weighted Average (TWA8): noise level (dB) if

kept constant for 8 hours would have same risk of

NIHL as briefer noise exposure in question

• Noise dose (D): Percentage of maximum allowed

noise exposure throughout the working day

Weighted Audiometry scales

Subjective EvaluationVoice level Distance of listener

from speakerNoise level

(dBA)

Loud voice 6 feet 85

Loud voice 4 feet 90

Loud voice 2 feet 95

Shout 4 feet 100

Shout 2 ft 105

Impossible to hear even when close > 110

Sound Level Meter

• Hand-held instrument

• Real-time display of

sound level (dBA)

coming from noise

sources

Noise Dosimeter

Worn by employees to determine TWA exposure

Sound meter

• Sound power doubles for each 3-dB increment

• Frequent intermittency in occupational noise

exposures, reduces risk of NIHL. Thus:

• O.S.H.A. reduces Permissible Exposure Level

(PEL) by 50% for every 5-dB increment

• 8-hour exposure at 90 dBA, 4-hour exposure at 95

dBA, 2-hour exposure at 100 dBA are considered

equally hazardous.

Occupational Safety & Health Administration regulations

Sound Intensity

Permitted exposure

Sound Intensity

Permitted exposure

85 dBA 16 hours 105 dBA 60 minutes

90 dBA 8 hours 110 dBA 30 minutes

95 dBA 4 hours 115 dBA 15 minutes

100 dBA 2 hours > 115 dBA < 1 second

Methods for noise control

• Reduction of noise production

• Reduction of noise transmission

• Protection of people exposed to noise

• Suitable legislation against noise pollution

• Worker’s rights to claim compensation

• Health education

General Noise Control

• Separation of Industrial & Transport areas

• Separation of residential areas from main streets

• House fronts should be > 15 m from street

• Intervening space planted with thick trees

• Acoustic insulation of buildings

• Use of public microphones with noise limiters

• Ban on bursting loud firecrackers

Industrial Noise Control

• Substituting new quieter equipment for old

• Isolate vibrating & noisy equipments

• Enclose noisy source or working personnel

• Attenuate noise energy at source via insulation

• Line surfaces with sound absorbing material

• Shield workers with sound barriers

• Use remote control systems for noisy operations

Traffic Noise Control• Heavy vehicles should not enter in narrow streets

• Restricted vehicular traffic on residential streets

• Prohibition on indiscriminate blowing of horns

• Banning use of pressure horns

• Encourage mass transportation & cut down on

personal vehicles

• Noisy vehicle testing program

Domestic Noise Control Reduce volume of:

• Music systems

• Home theatres

• Generators

• Alarms

• Power tools

• Personal stereos

• Cell phones

Personal Treatment

• Temporary Threshold Shift is reversible

• No treatment or recovery is expected once

Permanent Threshold Shift occurs

• Deterrence is only accepted management method

• Treat NIHL exacerbating conditions: smoking,

cardiovascular disease, diabetes mellitus,

hyperlipidemia, exposure to ototoxic drugs

Ear plugs

Proper use of Ear Plug

1. Roll earplug up into a small, thin "snake".

2. Pull your ear up & back with other hand & slide

in rolled-up earplug.

3. Count to 20 loudly while waiting for earplug to

expand & fill ear canal. Your voice will sound

muffled when earplug has made a good seal.

4. Check fitting of earplug. Most of foam body of

earplug should be within ear canal.

Proper use of Ear Plug

Proper fitting of Ear Plug

IMPROPERPROPER

Canal cap protectors

Ear muffs

Dual Protection (plug + muff)

Electronic Earmuffs

• Earmuffs permitting normal hearing in absence of

loud noise are now available

• They detect loud noise & attenuate it before it

reaches subject’s ear

• They permit normal hearing except in loud noise

• They allow wearer to hear environmental sounds

Electronic Earmuffs

Efficacy of earplugs & ear muffs

Performance of hearing protectors

• Hearing protectors are labeled with noise

reduction rating (NRR)

• Noise exposure in dBA – {(NRR-7) x .5} =

employee dose while wearing hearing protector

• Employee dose where exposure is 95 dBA, while

wearing earplugs with 29 NRR

= 95 – {(29-7) x .5} = 84 dBA dose

TWA8 ≥ 85 dBA or D ≥ 50%

• Worker’s option to wear hearing protection

TWA8 > 90 dBA or D > 100%

• Worker must wear either ear plug or ear muff

TWA8 > 105 dBA or D > 800%

• Worker must wear both earplug & earmuff

When to wear ear protection?

Hearing Conservation Program

• Baseline PTA within 6 months of onset of exposure

• Audiogram done when employee has been noise free

for at least 48 hours, (3 month for blast injury) - SB

Workers exposed to TWA of > 85 dBA, must have:

• Annual audiometric testing, annual training about

effects of noise on hearing, purpose of audiometry

testing & hearing protective devices

Medical care (?)• Hyperbaric oxygenation + corticosteroid therapy

• Intratympanic infusion of: JNK ligand, AM-111

• Neural stem cells injection into scala tympani

• Anti-oxidants (prevent oxidative stress of

cochlea): Acetyl-L-carnitine, Carbamathione, D-

methionine, N -acetyl-l-cysteine, alpha tocopherol

• Dexamethasone infusion into perilymphatic space

Accepted noise levelsResidential 25 - 40 dB

Commercial 35 - 45 dB

Industrial 40 - 60 dB

Educational 30 - 40 dB

Hospital 20 - 40 dB

Punishment for offenders

Hearing Impairment• Monaural hearing impairment calculated from four-

frequency (500, 1000, 2000, 3000 Hz) Pure Tone

Average

• Monaural hearing impairment (MHI) in % =

1.5

(Pure Tone Average - 25) %

• Binaural hearing impairment (BHI) in % =

5 (MHI better ear) + 1 (MHI worse ear)

_______________________________

6

• Hearing Impairment: Structural, functional or

psychological damage to hearing

• Hearing Disability: Hearing impairment affects

subject’s ability to perform normal body functions

• Hearing Handicap: Hearing disability prevents pt

from performing duties towards society

• Pure Tone Average (500, 1000, 2000, 3000 Hz) > 25

dB is considered as hearing handicap (A.A.O.)

Health Education

Hearing Rehabilitation “I would choose blindness

over deafness, because

blindness just separates

me from things, while

deafness cuts me off

from people, & exchange

of ideas about today,

tomorrow & yesterday.”

- Helen Keller

Assisted Listening Devices

• They are NOT hearing aids

• They are NOT used instead of hearing aids

• Help pt with hearing loss to function better in

communication situations to overcome distance,

background noise, or poor room acoustics

• Can be used with or without hearing aids

Vibrating wrist watch & alarm clock

CO2 & smoke alarm with strobe light

Amplified & captioned telephone

T.V. & F.M. amplifiers

Personal & multi-user amplifier

Alerting Devices

Amplified Stethoscope

Hearing Aids

References1. Scott-Brown: 6th edition; volume 2; chapter 11

7th edition; volume 3; chapter 238b

2. Cummings: 3rd edition; volume 4; chapter 162

3. Ballenger: 16th edition; chapter 15

4. Byron Bailey: 4th edition; chapter 147

5. Paparella: 3rd edition; volume 2; chapter 45

6. Ludman: 6th edition; chapter 35

7. Park’s textbook of PSM: 19th edition; p 598-600

8. Mathur NN, Roland P. Inner ear NIHL. emedicine

9. Ganguly SN, Reddy NS. JCOMS. 2008; 5:1 p 9-11