The Definition of Noise-Induced Hearing Loss (NIHL)

Definition of Noise-Induced Hearing Loss (NIHL)

Noise-induced hearing loss (NIHL) is permanent hearing damage caused by exposure to loud sound, either over months and years or from a single extremely loud event such as an explosion. Loud noise damages the tiny hair cells in the inner ear, called the cochlea, that turn sound vibrations into nerve signals. Once these cells are destroyed, they do not grow back.

Occupational hearing loss is one of the most common work-related illnesses. It develops gradually and painlessly, so many workers do not notice it until significant damage has been done. Early signs include muffled hearing after a shift, ringing in the ears (tinnitus), and difficulty following conversations in noisy places. NIHL typically affects higher-pitched sounds first, which is why speech can sound clear but hard to understand.

Industries with high noise exposure include manufacturing, construction, mining, agriculture, oil and gas, utilities, transportation, the military and entertainment. Some chemicals, called ototoxic substances, such as certain solvents and lead, can also damage hearing and may make noise damage worse.

Noise Exposure Limits

Noise damage depends on both loudness, measured in decibels (dBA), and how long the exposure lasts. Because the decibel scale is logarithmic, a small increase in decibels means a large increase in sound energy.

• OSHA: under 29 CFR 1910.95, the action level is an 8-hour time-weighted average of 85 dBA, which triggers a hearing conservation program. The permissible exposure limit is 90 dBA. OSHA uses a 5 dB exchange rate, meaning allowed time halves for every 5 dB increase.
• NIOSH: NIOSH recommends a limit of 85 dBA over 8 hours with a 3 dB exchange rate, which is more protective. Under the NIOSH approach, exposure at 88 dBA should be limited to 4 hours.

A simple rule of thumb: if you have to raise your voice to talk to someone an arm's length away, the noise level is probably 85 dBA or higher.

Hearing Conservation Programs

Where exposure reaches the action level, OSHA requires a hearing conservation program that includes:

• Noise monitoring to identify exposed workers, repeated when equipment or processes change.
• Audiometric testing: a baseline hearing test within six months of first exposure and annual tests after that.
• Hearing protection offered to all exposed workers and required for those above the limit or with a standard threshold shift. See noise reduction rating (NRR).
• Training every year on the effects of noise, the purpose of testing and how to use hearing protectors.
• Recordkeeping of exposure measurements and audiograms.

A standard threshold shift is an average change of 10 dB or more at 2000, 3000 and 4000 Hz compared with the baseline. Under 29 CFR 1904.10, a work-related standard threshold shift is a recordable hearing loss case when the worker's overall hearing level is also 25 dB or more above audiometric zero in the same ear.

Preventing Noise-Induced Hearing Loss

Follow the hierarchy of controls:

• Eliminate or substitute: buy quieter equipment and processes when purchasing.
• Engineering controls: enclose or isolate noisy machines, add sound-absorbing barriers, fit mufflers and vibration mounts, and maintain equipment to reduce rattles and leaks. See engineering controls.
• Administrative controls: limit time in noisy areas, schedule noisy work when fewer people are present and provide quiet break areas.
• Hearing protection: earplugs and earmuffs, properly selected, fitted and worn for the full time in noise.

Example of Noise-Induced Hearing Loss

A machine operator worked 8-hour shifts beside stamping presses that produced about 95 dBA. He wore foam earplugs some of the time but often removed them to talk with coworkers. His annual hearing test showed a standard threshold shift in both ears, mostly at 4000 Hz, and he reported ringing in his ears after work.

The plant recorded the case, retested the operator to confirm the shift and refit him with earmuffs that included a communication feature so he could talk without removing them. It also installed acoustic enclosures around the two loudest presses, cutting levels at the operator station to about 86 dBA, and added fit testing for hearing protectors to its program.

Frequently Asked Questions: Noise-Induced Hearing Loss

At what noise level does hearing protection become required?

OSHA requires hearing protection to be made available at an 8-hour average of 85 dBA and required at 90 dBA, or at 85 dBA for workers who have had a standard threshold shift. Many programs set stricter internal limits.

Can hearing loss from noise be reversed?

No. Once the inner ear's hair cells are destroyed by noise, they do not regenerate, so NIHL is permanent. Hearing aids can help, but they do not restore normal hearing.

What is a hearing conservation program?

It is an OSHA-required program for workplaces where noise meets or exceeds the action level, including noise monitoring, annual hearing tests, hearing protection, training and recordkeeping.

How is workplace noise measured?

Noise is measured with a sound level meter for spot checks, or a noise dosimeter worn by a worker to capture their full-shift exposure. Results are expressed as a time-weighted average.

Does hearing protection alone solve a noise problem?

No. Hearing protection is the last line of defense. Quieter equipment, enclosures and other engineering controls should come first, because they protect everyone without relying on perfect fit and use.