Anatomy and Physiology of How We Hear

The ear is a complex system that allows you to collect and hear sounds from the environment around you.

A normal-hearing person can hear sounds in frequencies from 20 Hz all the way to 20,000 Hz. Without the ear’s different parts and systems, meaningful signals and communication would just be vibrations in the air. 

The ear is responsible for understanding communication and balance, and it is vital to protect the ear from any damage!

Different Parts to Ear

Different parts of the ear are responsible for different aspects of the hearing and balance systems. There are three sections of the ear:

• Outer ear
• Middle ear
• Inner ear

The outer ear includes the pinna, or the part of the ear you see, the ear canal, and the ear drum. This part of the ear collects the sound and helps amplify sounds. When the ear collects the sound, it travels through the ear canal and to the eardrum.

The ear canal extends from the pinna to the eardrum, and it is where wax can build up. The eardrum vibrates when sound hits it; this structure is responsible for protecting the middle and inner ear areas from any foreign objects.

Damage to Anatomy from Noise

Noise induced hearing loss (NIHL) is caused by exposure to excessively loud sounds and cannot be medically or surgically treated. NIHL can result from a one-time loud sound exposure to a very loud sound, blast, or impulse, as well as by repeated exposure to loud sounds over an extended period. Exposure to harmful sounds can cause damage to the sensitive hair cells in the cochlea, the hearing nerve fibers and structures in the middle ear space.

On average, a person is born with approximately 16,000 hair cells within their cochlea. These hair cells play a major role in the hearing system because they allow your brain to detect sound. However, they are quite sensitive, especially to loud sounds.

Sound Over 85 Decibels

When exposed to harmful loud sounds (sound over 85 decibels), the hair cells are hit with and cause them to bend, break, and in some cases, completely shear off. Up to 30-50% of hair cells can be damaged before any changes in hearing can be measured by a hearing test.

Once damage to the hair cells occurs, the hair cells do not regenerate and therefore, once damage occurs the damage is permanent.

Hazardous levels of Noise, PTS and TTS

The National Institute for Occupational Safety and Health defines hazardous levels of noise as 85 dBA within an 8-hour time-weighted average using a 3 dB exchange rate. This means that significant noise-induced hearing loss will occur when individuals exceed an average of 85 dB of noise within 8 hours. Every increase by 3 dBA doubles the amount of the noise and halves the recommended amount of exposure time.

Short bursts of dangerous noise levels from a gunshot or loud machinery, for example, are also referred to as impulse noise. When compared to continuous noises at the same loudness, impulse noise has also been shown to cause more severe and more permanent hearing loss at higher rates.

Although continuous, or steady state noise, affects a wider range of frequencies than impulse noise, impulse noise resulted in more permanent losses.

Changes in hearing sensitivity, or thresholds, result from over exposure to noise, and may be temporary or permanent.

Threshold shifts are a sensitive precursor to underlying cell damage. Temporary threshold shift is caused by temporary swelling of the auditory nerve and does not require inner hair cell death to be classified as a temporary threshold shift.

In some cases of acoustic trauma, however, the inner hair cell will survive but communication between the inner hair cell and the nerve is interrupted. Although the hair cell survived, once the connection is lost, neurodegeneration begins, weakening the connection and eventually killing the cell. This causes a permanent threshold shift.

Additional Resources

• Noisy Planet
• OSHA
• CDC

Work, Hobbies and Noise

The noise we experience on a day-to-day basis is typically at a safe volume, unlikely to damage our hearing. However, when sounds are too loud, whether for a brief time or are long-lasting, the structures of the inner ear can become damaged and cause noise-induced hearing loss (NIHL).

NIHL can be immediate or it can take a long time to be noticeable. It can be temporary or permanent, and it can affect one ear or both ears.

Even if you can’t tell that you are damaging your hearing, you could have trouble hearing in the future, such as not being able to understand other people when they talk, especially on the phone or in a noisy room.

Jobs and Hobbies When You Need Protection

But when exactly would you need ear protection? Certain jobs and hobbies are more likely to expose you to noise, for example:

• Target shooting and hunting
• Snowmobile riding
• Listening to MP3 players at high volumes through earbuds or headphones
• Playing in a band or attending loud concerts
• Yardwork (Lawnmowers, weed eaters, leaf blowers, etc.)
• Woodworking tools

Hearing Protection Device Types

When surrounded by noise the best thing to do is to wear hearing protection. Hearing protection is an important aspect of hearing conservation. There are different types of hearing protection that can be used.

So, what type of hearing protection is available?

Do Research and Be Prepared

Take the time to research on the best hearing protection for you and the type of sound you will be exposed to. If you are concerned about being in a noisy environment and there is no hearing protection provided, be prepared to bring your own hearing protection.

If this occurs in a work environment, contact your supervisor about changes that can be made to the environment or about providing hearing protection as needed. Here is some helpful information on the hearing protection devices mentioned in this section.

Ear Plugs

These are inserted in the ear canal and are pre-molded or moldable foam ear plugs. The most come of ear plugs are disposable, however there are reusable or custom molded ear plugs.

Advantages

• Small
• Inexpensive
• Portable
• Comfortable
• Can be worn effectively without interference from hair or glasses

Disadvantages

• Can be difficult to fit without proper instruction 
• May be too large for small ear canals
• Level of protection depends on insertion depth

How and When to Use Hearing Protection

When to Wear Hearing Protection

Situations include

• Working with power tools or loud machinery
• Attending loud sporting events
• Attending loud music events (i.e. loud bars or concerts)
• Around firearms or explosives of any kind 
• Operating or performing noisy maintenance on a motorcycle
• Any time sound has become uncomfortable and/or painful to hear
• In an otherwise loud environment

How to Wear Hearing Protection

Earmuffs/ Over the ear hearing protection - Should be worn snug against the head over both ears. The padding should sit securely and comfortably around the entirety of each ear. These protective ear muffs can be worn with a secure strap that fits over the ear, or can be attached to a helmet.

Insert foam ear plugs - Should be rolled between th first two fingers and thumb to compress the foam without creating wrinkles. Next reach behind your head and pull on your ear "back and up" to straighten out the ear canal.

Quickly insert the ear plug fully and allow it to expand within the ear canal. Finger can be held against the ear plug to allow it to expand inward into the ear canal.

The outermost portion of the ear plug should sit just the tragus (the flap next to the ear canal). As the earplugs expands, it will fill up the circumference of the entire ear canal which will ensure sounds is decreased to an appropriate level.

Reusable plastic ear plugs - Should be inserted until "suction" is accomplished. Insert the ear plugs until they feel secure within the ear canal. A retrieving tab will be visible outside of the ear, along with the largest dome depending on the size of the ear canal.

Some models have a cord which can be worn around the neck. This makes removal and reinsertion convenient in the workplace where sound levels may change rapidly.

Music Induced Hearing Loss

Music induce hearing loss (MIHL) is different from noise induced hearing loss (NIHL) as noise is typically considered to be unwanted and/or bothersome. Montgomery & Fujikawa coined the term sociocusis, which is hearing loss that occurs due to potentially noxious levels of noise of modern civilization.

Listening to Music

With all of the listening technology that is around, and an estimated >90% of college students owning some type of music playing device (Torre, 2008), hearing loss particularly in the high frequencies is a greater concern with prolonged music listening.

Orchestras

MIHL is an occupational hazard for musicians. It has been found that at a distance of 2 meters, orchestras have been report to cause an output of 120dB(B) and hearing changes after rehearsal (Jatho & Hellmann, 1972). Individuals who play in the orchestra have been found to have hearing losses in the higher frequencies from 4k-8kHz with the most prominent noise notch occurring at 6kHz in each ear (Jansen, et al.).

Sing or Choir

For individuals who sing in a choir, studies have shown that there were hearing losses in the lower frequencies from 125-500Hz (Steurer et al., 1998).

Music induced hearing loss, like noise induced hearing loss, results in the destruction of the outer hair cells within the cochlea. These hairs are responsible for making fine distinctions of pitch. To protect these hair cells, take precautions with loud noise by wearing the proper hearing protection.

When listening to music through headphones and personal listening devices, make sure to keep the volume down to protect your hearing!

Annual Monitoring

OSHA states that employers must provide annual audiograms within 1 year of the baseline audiogram. Workers’ hearing must be tested annually to identify deterioration in the hearing ability as early as possible. This allows employers to begin protective follow-up measures before the hearing loss progresses. The employers must compare the annual audiograms to baseline audiograms to determine if the audiogram is valid or if the employee has lost hearing ability or experienced a standard threshold shift (STS). A STS is an average shift in either ear of 10 dB or more at 2, 3, and 4 kHz.

NIOSH:

Audiometric testing should consist of air-conduction, pure-tone, hearing threshold measures at no less than 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz. Right and left ears should be tested individually.

A baseline audiogram should be obtained before employment or within 30 days of employment for all workers who must be enrolled in the hearing loss prevention program. Workers should not be exposed to noise levels at or above 85 dBA for a minimum of 12 hrs before receiving a baseline audiometric test. Hearing protection should not be used instead of the required quiet period.

All workers enrolled in the hearing loss prevention program should have hearing threshold levels measured every year. The tests should be conducted during the worker’s normal work shift. The audiogram should be referred to as the “monitoring audiogram.” This audiogram should be examined immediately to determine whether a worker has a change in hearing relative to his/her baseline audiogram.

When there is a threshold change in the audiogram in either ear that equals or exceeds 15 dB at 500, 1000, 2000, 3000, 4000 or 6000 Hz, an optional retest may be conducted immediately to determine whether the significant threshold shift is persistent. Most cases will show that the worker does not have a persistent threshold shift, eliminating the need for a confirmation audiogram and follow-up action. However, if a persistent threshold shift has occurred, the worker should be informed that his/her hearing may have worsened and require additional testing.

When there is a threshold shift, the worker should receive a confirmation audiogram within 30 days. This audiogram should be conducted under the same conditions s those of the baseline audiogram. If a persistent threshold shift is shown, the audiograms and other appropriate records should be reviewed by an audiologist or physician.

If the threshold shift is validated, it is considered a significant threshold shift. This should be documented in the worker’s medical record and the confirmation audiogram should serve as the new baseline and used to calculate any subsequent significant threshold shift. Once a threshold shift has been validated, the employer shall take action to protect the worker from additional hearing loss due to occupational noise exposure.

The employer should obtain an exit audiogram from a worker who is leaving employment or whose employment no longer involves exposure to hazardous noise. This audiogram should be conducted under the same conditions as those of the baseline. More information can be found on the NIOSH website at https://www.nonoise.org/hearing/criteria/criteria.htm

Heavy Metals

Heavy metals can be encountered in a variety of environments and forms. Exposure to heavy metals can cause several health issues including an impact on an individual’s hearing sensitivity. Some of these heavy metals include:

• Mercury
• Arsenic - can be encountered in the environment
• Cadmium - can be encountered through dietary inteske, tobacco smoking habits, and air pollution
• Lead - the most common heavy metal poisoning

Even low levels of exposure to these heavy metals can cause hearing loss. When heavy metals settle into the blood, the affected blood can travel to the structures of the inner ear, causing damage to the structures which can result in hearing loss. Heavy metals also have a tendency to linger in the blood supply of the inner ear which results in further damage to the impacted structures. The effect on an individual’s hearing can be seen in the high frequencies of conventional audiometric testing. These effects can be further exacerbated when the presence of heavy metals is combined with noise exposure due to a synergistic effect.

Although there are other heavy metals such as chromium, cobalt, nickel, zinc, or manganese, these heavy metals have not been researched concerning their impact on hearing or have not been found to cause a toxic effect the hearing structures. Although these heavy metals may not impact hearing, they may still be dangerous depending on factors such as ingestion and amount of exposure.

Heavy Metals Can Be Toxic

It is important to reaffirm that heavy metals can be toxic to other aspects of an individual’s health, not just hearing. If you suspect heavy metal exposure or heavy metal poisoning, the appropriate medical attention should be sought.

Poison Control can be contacted at 1-800-222-1222 or can be visited online at https://www.poison.org/ If poisoning of any kind is suspected, help can also be reached at https://triage.webpoisoncontrol.org/#!/exclusions 

OSHA and NIOSH

In the Occupational Safety and Health Act of 1970, Congress created the Occupational Safety and Health Administration (OSHA) to make sure safe and healthful working conditions for working men and women were being implemented by establishing and enforcing standards and by providing training, outreach, education and assistance. This Act also declared that its main goal was to assure safe and healthy working conditions for every working individual to preserve our human resources. To achieve this, the National Institute for Occupational Safety and Health (NIOSH) is responsible for recommending occupational safety and health standards and defining levels of hazardous exposure that are safe for certain periods of employment. This includes but is not limited to concentrations at which no worker will suffer declining health, functional capacity, or life expectancy as a result of his or her work experience. NIOSH uses criteria documents to communicate these standards to regulatory agencies, such as OSHA, and to others in the occupational safety and health community.

OSHA Standards

With OSHA, they require employers to establish a hearing conservation program when noise exposure is at or above 85 decibels averaged over 8 working hours, or an 8-hour time-weighted average (TWA). Hearing conservation programs are intended to prevent initial occupational hearing loss, maintain and protect remaining hearing, and provide workers with the education and hearing protection devices necessary to protect themselves.

OSHA’s permissible exposure limit (PEL) for noise exposure is 90 dBA for an 8-hour TWA and the standard uses a 5 dBA exchange rate. This means that when the noise level is increased by 5 dBA, the amount of time a person can be exposed is cut in half. For example, if an individual is exposed to noise levels of 95 dBA (5 dBA above the OSHA PEL of 90 dBA), they can be exposed for only 4 hours in order to be within the daily OSHA PEL.

NIOSH Standards

The NIOSH recommended exposure limit (REL) for occupational noise exposure is 85 decibels, A-weighted, as an 8-hour time-weighted average. Levels at and above this level are considered hazardous. This level was reevaluated and reaffirmed 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%, which is substantially lower than the 25% excess risk at the 90-dBA permissible exposure limit (PEL) that was enforced by the Occupational Safety and Health Administration (OSHA).

Synergistic Effects of Drugs and Noise

The impact of noise and ototoxic drugs on their own can significantly damage an individual’s hearing and balance systems. In some instances when some traumatic agents are combined, more severe damage can occur. Some types of medications that are known to be toxic to hearing and balance include: aminoglycoside antibiotics, loop inhibiting diuretics, salicylates and certain chemotherapy drugs. The damage to the hearing and balance systems can be temporary or permanent depending on the drug used, duration and dosage.

Ototoxicity due to aminoglycosides most often will present in the higher frequencies and progress to the lower frequencies with time. Previous studies have examined how much of an impact a combination of aminoglycosides and noise have on hearing. In these studies the combined exposure showed more hearing deficits than when drugs or noise occurred separately.

Loop-inhibiting diuretics are another group of drugs that can damage hearing and balance. This type of ototoxicity is most often temporary and occurs in the higher frequencies. Both noise and these types of diuretics can cause hearing loss individually, but studies revealed that a combined exposure does not make the hearing deficits worse.

Salicylates can cause hearing and balance issues as well. The degree of hearing loss and the time of onset correlates to the amount of salicylates that are taken. Many studies reported no significant increase in hearing loss with noise exposure and salicylates concurrently.

Cisplatin is a chemotherapy drug that can cause issues with hearing and balance. The use of this type of drug typically causes a symmetrical hearing loss in the higher frequency ranges, and with time will impact the lower frequencies. Studies showed that cisplatin use and the presence of noise concurrently increased the degree of hearing loss for a patient.

There are many drug types that can cause issues in the hearing and balance systems. Knowing the way these drugs interact with noise exposure is beneficial in the world we live in. Noise occurs in so many aspects of our daily lives, and medications are used more often. Knowing how they interact can help us understand the cause of an individual’s hearing loss.

References