Occupational Hearing Loss, Noise Induced Hearing Loss, Hearing Handicaps and Presbycusis
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Exposure to loud noise is the second most common cause of hearing loss.  Approximately 30 million Americans are exposed to high intensity noise in their workplace (1,2) in one in 4 of these workers (or 7.5 million Americans) a permanent hearing loss will develop (1,2).  Much can be done to prevent noise-induced hearing loss but little can be done to reverse it.  Sometimes a single exposure to loud noise is all that is needed, a single hunting trip without ear plugs.  Loud noise damages the hair cells in the inner ear and can cause hearing loss, ear ringing and distortion of sounds.   

Franks JR, Stephenson MR, Merry CJ.  Preventing occupational hearing loss.  A practical guide.  DHSS (HIOSH) pub. No. 96-110.  Available at: https://www.cdc.gov/niosh/docs/96-110/   Accessed Mar. 7, 2015.

2) National Institute of Deafness and Other Communicative Disorders:  Noise Induced Hearing Loss.  Available at:  https://www.nidcd.nih.gov/health/hearing/Pages/noise.aspx   Accessed Mar. 7, 2015.

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The symptoms of noise induced hearing loss are subtle in the early stages.  Hearing loss tends to occur first for high-pitched sounds only.  Consequently, the volume of sound heard may be unchanged but the quality of it lessens.  Speech may be heard but not completely understood.  The presence of background noise can make speech hard to understand.  Noise induced hearing loss has been reported to be accompanied by a ringing in the ears (tinnitus) in 23% of subjects (Phoon, 1993).  Tinnitus can often be more annoying than the hearing loss itself.  Treatment of tinnitus is often unsatisfactory.   There has been an association between acoustic trauma (noise induced hearing loss) and Meniere's disease which has been reported in the a few research articles and text books.  However, more recent literature has shown that acoustic trauma is not a causative factor --  Read More.

Decibel Levels of Environmental Sounds

Source--Dangerous Level dBA SPL
12-Gauge Shotgun 165
Produces Pain  140
Jet Engine During Takeoff 140
Ambulance Siren 120
Discotheque/Chain Saw/Jackhammer 110
Tractor 96
Jackhammer (at 15 m) 95
Lawn Mower 90
Heavy Truck  (at 15 meters) 85
Source-- dBA SPL
Discomfort Level Above 80
Vacuum Cleaner (at 3 m) 73
Conversational Speech (at 1 meter) 60
Urban Residence 50
Whisper/Telephone 30 - 35
North Rim of Grand Canyon 25
Threshold of Audibility (1000 Hz)  0

Tables Adapted From
Occupational Noise Exposure, United States Department of Labor
https://www.osha.gov/SLTC/noisehearingconservation/#loud  &
Noise and Hearing Loss Prevention, United States Department of Labor


What Noises are Dangerous??

Physical measurements of the sound can be make to determine whether it exceeds dangerous levels, and most factories have access to the necessary equipment.  Radio Shack also sells a sound level meter for under $40 which will measure noise levels using the "A" Scale.  (This is what the designation dBA refers to -- decibels measured in the A Scale.)  However, without noise-measuring equipment, the following basic rules can be followed:

#1.  If it is necessary to shout to hear yourself over a noise, the level of the sound can be damaging.

#2.  Should ringing in the ears occur after exposure to a loud sound, damage has been done and that sound should be avoided or ear protection used in the future.

#3.  If diminished hearing or a sense of fullness in the ears is experienced after  noise exposure, the level of that noise is damaging.      There are several federal guidelines for noise exposure.

DHHS (NIOSH) Publication No. 98-126  ( Download PDF ) "The NIOSH recommended exposure limit (REL) for occupational noise exposure (85 decibels, A-weighted, as an 8-hour time-weighted average [85 dBA as an 8-hr TWA]) was reevaluated using contemporary risk assessment techniques and incorporating the 4000-hertz (Hz) audiometric frequency in the definition of hearing impairment. The new risk assessment reaffirms support for the 85-dBA REL. With a 40-year lifetime exposure at the 85-dBA REL, the excess risk of developing occupational NIHL is 8%—considerably lower than the 25% excess risk at the 90-dBA permissible exposure limit (PEL) currently enforced by the Occupational Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA)."

NIOSH HEALTH HAZARD EVALUATION REPORT  HETA 95-0406-2609  "The OSHA standard for occupational exposure to noise (29 CFR 1910.95)38 specifies a maximum PEL of 90 dB(A)-slow response for a duration of eight hours per day. The regulation, in calculating the PEL, uses a 5 dB time/intensity trading relationship, or exchange rate. This means for a person to be exposed to noise levels of 95 dB(A), the amount of time allowed at this exposure level must be cut in half in order to be within OSHA's PEL. Conversely, a person exposed to 85 dB(A) is allowed twice as much time at this level (16 hours) and is within his daily PEL. NIOSH, in its Criteria for a Recommended Standard,39 proposes an exposure limit of 85 dB(A) for 8 hours, 5 dB less than the OSHA standard. In 1995, NIOSH recommended a 3 dB exchange rate. In 1994, the ACGIH changed its TLV to a more protective 85 dB(A) for an 8-hour exposure, with the stipulation that a 3 dB exchange rate be used to calculate time-varying noise exposures. Thus, a worker can be exposed to 85 dB(A) for 8 hours, but to only 88 dB(A) for 4 hours or 91 dB(A) for 2 hours."   (ACGIH: American Conference of Governmental Industrial Hygienists')

Duration of Exposure (hrs/day)

Sound Level dB(A)

16 82 82 85
8 85 85 90
4 88 88 95
2 91 91 100
1 94 94 105
1/2 97 97 110
1/4 100 100 115*
1/8 103 103 ---
  ***   **

*   No exposure to continuous or intermittent noise in excess of 115 dB(A).
**  Exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure level.
*** No exposure to continuous, intermittent, or impact noise in excess of a peak C-weighted level of 140 dB.


Maximum Recommended Noise Exposure
  NIOSH 1997 and ACGIH 

 Enter Decibels: 
Maximum Recommended Noise Exposure
 Enter Decibels: 
There are also many agents found in industry can also damage hearing in addition to industrial noise exposure.  The most common is tobacco
View Abstract
bulletOrganic Solvents found in Industry which are ototoxic include.

--Toluene:  Found in paints, thinners, rubbers and in the printing industry. View Abstract
--Stryene:  Found in plastics, rubbers, resins, insulating materials. 
--Carbon Disulfide:  Found in the textile industry and insecticides
View Abstract
View Abstract
--Xylene:  Found in paint and lacquer industry
View Abstract
bullet Metals: -- Lead may also cause hearing loss View Abstract 
bullet Industrial Solvents:  The combination of solvents and noise exposure exceeds the damage produced by either also.  The effect of solvents is potentiated even more by exposure to ethanol.   View Abstract 
bullet Asphyxiants:  Carbon Monoxide  View Abstract

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earplugs.jpg (29066 bytes)Prevention     Search PubMed for Ear Plugs

Since little can be done to treat long standing noise induced hearing loss, the best prevention is to avoid loud noises.  Compressible foam ear plugs (not water plugs) and ear muffs can decrease the noise exposure level by over 20 dB.  Ear plugs and ear muffs are about equally effective.  Muffs cost more but can be used in patients with ear canal disease. Muffs are also hot in warm weather.  For hunting, electronic plugs and muffs can be obtained which make surrounding noise louder so game can be heard, but when firing a gun they muffle the loud noise. 
Many people will not wear ear plugs in a noisy environment because they think it will make it more difficult to hear others talking.  Actually the reverse is true.  Ear plugs reduce noise most efficiently in the low frequencies, below the speech range.  This will increase the signal to noise ratio of the speech and makes it more easily heard.
New research indicates that several types of drugs when taken before or immediately after noise exposure may mitigate the damage to the
inner ear.  These drugs fall into three categories:
bullet Anti-oxidants:  These drugs may be protective based on the theory that one of the mechanisms in noise inducted hearing loss is the generation of free oxygen radicals.  Vitamin E given at 10mg/kg/day to 40mg/kg/day in the guinea pig was protective.  View Abstract ;  Acetyl-L carnitine View Abstract a mitrochondrial stabilizer for damage induced by free oxygen radicals, and N-L-acetylcysteine View Abstract , an antioxidant, has been shown to mitigate noise induced hearing loss in the chinchilla.  

Glutathione is an antioxadant which has been show to reduce the damage of noise exposure.  View Abstract  Researchers have also found that noise induced hearing loss is characterized by a glutathione deficiency state and increase glutathione levels may be protective.  View Abstract    Glutathione monoethylester and in combination with R-N6-phenylisopropyladenosine has also been studied in the chinchilla and has been found to be protective.  View Abstract 
Glutamate Receptor Antagonist:  These drugs may be protective based on the theory that one of the mechanisms in noise induced hearing loss is the generation of Glutamate which binds to post-synaptic glutamate receptors which leads to degeneration of the neurons.   Investigated drugs include:  caroverine View Abstract ; carbamathione  View Abstract

bullet Neurotrophins:  There is also evidence that neurotrophins (neurotrophin-3) may also be protective. View Abstract

Additional References:  

Noise Induced Hearing Loss References:  Go To NIOSH References

Franks JR.  Four Earplugs in search of a rating system.  Ear & Hearing.  21:218-226, 2000.

Phoon WH, Lee HS, and Chia, SE.  Tinnitus in noise-exposed workers.  Occup Med 43:35-38. 1993.

Remember the most common outcome to noise exposure is a permanent hearing loss. 
When this occurs the only effective treatment is the use of hearing aids. 
This is why prevention by avoiding loud noises and wearing ear protectors is so important.

Kevin Kavanagh Copyright  2003.   All rights reserved.   Webpage last updated: 08/24/2023    Search Website

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