Hearing Loss: Noise-Induced

 


Before you crank up the volume to “feel” the music through those ear-buds or rev your screaming new power saw or fire that brand new AR-15 with a 100-round drum magazine, read this post.  You might save yourself from permanent hearing loss.

Welcome to the world of noise-induced hearing loss (NIHL), a not so uncommon problem in our present society.

As a kid, my little bro and I picked up spent brass casings at the gun range, while Dad practiced or competed in pistol-shooting.  Being 4 or 5 years old, I don’t recall much about those days, and when I asked Mom if Dad provided us with hearing protection, she said, “No, I don’t thinks so.  Well, come to think of it, sometimes he put cigarette butts in your ears.”  Ah, parenting back in ‘60s.

I think that’s one reason my ears ring constantly, 24/7 to this day (also known as Tinnitus.  See Tinnitus: Ringing in the Ears and Tinnitus: Causes and Remedies)--another manifestation of the permanent damage from loud noise exposure, and often associated with hearing loss.  Lighting-up firecrackers by the truckload during my youth also didn’t help.

Your ears can take a lot of punishment.  They can pick up the slightest sounds, so we don’t need super-loud volumes to hear.  Yet in today’s society we like to ramp things up.  Over-the-top and super-size are the products of our desire.  We want it all, from head-pounding music to palpable theatrical acoustic effects. 

Your ears can tolerate loud noise, but to a point.  Damage occurs with loud noise, to those little “hair cells” we spoke about in a Hearing and the Inner Workings of the Cochlea.  When damaged, the number of functioning cells is diminished, and the reduction in your ability to hear results.  Fortunately by the graces of the miraculous human body, these hair cells often recuperate and repair themselves and hearing loss often is temporary.  Bash your poor ears with loud noise for extended periods of time or expose them to unnatural noise volumes for shorter moments, and the damage can be permanent, and your hearing is lost forever.  I’ll emphasize that last word:  FOREVER.

FOREVER is good for some things such as not taking ER-call or avoiding crazy people who drive you nuts.  But for other things both pleasurable and essential, FOREVER is very, very bad.  Losing your hearing FOREVER is VERY, VERY BAD.

This falls under the concept of COMMON SENSE.  Yet common sense eludes many of us, most often the male sex of our species.

Loud noise delivers increased pressure waves to the inner ear (cochlea).  Typically the higher frequencies are damaged first.  One reason is related to the frequency-specific, manner in which the cochlea picks up sound, where higher frequencies are picked up by hair cells closer to the oval window (see Figure 3 of Hearing and the Inner Workings of the Cochlea).  Most sounds for speech intelligibility are found in the higher frequencies.  Female and children’s voices are also in this spectrum, which is the reason a lot of guys with high-frequency hearing loss are blamed for ignoring their wives or children (or grandchildren) when in fact they really aren’t hearing them well.  No joke: I’ll occasionally see couples where the male spouse points to his wife saying, “I don’t have a problem, she does,” and she responds, “He can’t hear or he’s ignoring me!”  And when we get the audiogram (hearing test) and high frequency loss is noted, the husband smiles, “See!  I’m not ignoring you!” thus giving legitimacy to his behavior, only to find it will end when she demands he get hearing aids.

Below is a short list of activities and noise exposure (from a variety references, including the American Academy of Audiology), measured in decibels (dB).  A decibel is a logarithmic measure of sound pressure.  This means for every increase of 10dB, there is a 10-fold increase in sound pressure to the ear.  0 dB is the threshold where sound is barely noticed by a normal person with normal hearing.  50-60dB are normal conversation levels.


Sound level (dB)

Activity

150

Jet engine at take off (at 25meters)àear drum rupture

140

Fireworks, gun shots, custom car stereos at full volume, aircraft carrier deck

130

Jackhammer, ambulances

120

Jet planes, chain saw, oxygen torch (121dB)

110

Concerts (any music genre), car horns (at 1 meter), sporting events, steel mill.  Live rock concert (108-114dB), many wind/brass musical instruments (92-110dB)

100

Snowmobiles, MP3 players at full volume, motorcycle, farm tractor

90

Lawn mowers (96-107dB), power tools, hair dryers, blenders

80

Alarm clocks, garbage disposal, dishwasher

70

Traffic, vacuum cleaners

60

Conversation in a restaurant

50

Conversation at home. Moderate rainfall

40

Quiet library

30

Whisper, quiet rural area

20

Leaves rustling

10

Breathing

 

 

           

 Hearing loss can occur at sustained levels of 80-90dB.  One can potentially tolerate up to 90dB of noise continuously for up to 8 hours.  Human ears vary in tolerance however, and some folks may suffer hearing loss with shorter durations.  In the United States, OSHA (Occupational Safety and Health Administration) states any worker cannot be exposed to 90dB of noise in the workplace for more than 8 hours in a day.  The higher the levels, the shorter the duration of exposure.   For every 5 dB increase in noise level, the exposure time is cut in half, due to the logarithmic nature of dB scale.  This is called the decibel exchange rate

Table 1

OSHA Standards

Sound Level (dB)

Duration (hours)

90

8 hours

95

4

100

2

105

1

110

0.5

115

0.25

120

0.125

125

0.0625 (3 min, 45 sec)

130

0. 0313 (1 min, 53 sec)

 However, NIOSH (National Institute for Occupational Safety and Health) found hearing loss can occur at lower levels, and recommends exposures at 85dB to no more than 8 hours, and set a 3 dB exchange rate.  OSHA does require employers to implement a hearing conservation program when noise exposure is at or above 85 decibels averaged over 8 hours per day (for instance, identifying hazardous noisy areas in the workplace, controlling noise levels, providing hearing protection, and implementing regular hearing testing).

 

Table 2

NIOSH recommendations

Sound Level (dB)

Duration (hours: minutes: seconds)

82

16 hours

85

8

88

4

91

2

94

1

97

0:30:00 (30 min)

100

0:15:00

103

0:07:30 (7 min, 30 sec)

106

0:3:45

109

0:01:53

112

0:00:56 (56 sec)

115

0:00:28

118

0:00:14

121

0:00:07

124

0:00:03

127

0:00:01

 

Thus, any person entering into areas or engaging in activities where noise levels are 85dB or higher must wear hearing protection.

Hearing protective devices (HPDs) come in two major forms: ear plugs and ear muffs.  Ear plugs come in a variety of styles: foam, soft rubber or plastic.  However, this protection is variable; it depends on how well one inserts and properly fits the plug into the ear canal.  Custom made ear plugs are even better, fashioned by an audiologist and molded to fit snugly within the unique contours of your ear.

Ear muffs provide similar levels of protection, and are more consistent, given the muffs simply cover the entire ear.  Better yet is dual protection--a combination of both ear plugs and ear muffs.

Hunters also should wear HPDs.  Noise cancelling ear muffs are a great and relatively inexpensive means (compared to the cost of the gun, scope, rifle and all the other gear); one can hear environmental noise and also be protected during firing.  If you can spare the expense and the wait for federal approval, a sound suppressor fitted to your rifle or shotgun is a great alternative. 

Printed on the package of the HPD is usually is a number called the NRR (Noise Reduction Rating) labelled in decibels.  For instance, a set of earplugs might have an NRR of 28 dB while ear muffs might carry a 26 dB NRR rating (the range varies from product to product).  Common sense says noise is reduced by 28 or 26dB respectively, and if you use both, you’ll get 54dB of protection (28 + 26 =54).  But lo and behold—this ain’t the case! 

The NRR is determined by testing in a controlled lab setting, where an experimenter properly applies the HPD, and the result is averaged over a group of test subjects.  Thus, it does not take into account fitting variability or consistency of use.  Thus, the NRR is variable between users and can overestimate the level of protection.  The formula to determine more real-world hearing protection is to subtract 7 from the NRR and divide that result by 2:

Actual hearing protection = (NRR – 7)/2

Thus, with the NRR 28 ear plugs, (28-7)/2 = 8.5dB, and with the 26 dB ear muffs, (26-7)/2 = 6.5dB of noise reduction results.

If you use both, you add 5dB to the device with the higher NRR.  In this case, 28 + 5 = 33 NRR, which is still better than either device alone.

The key to preventing hearing loss from noise is a combination of ear protection and limiting exposure to noise, and for some of us, an exercise in a little bit of common sense.

 

©Randall S. Fong, M.D.

www.randallfong.com

For more topics on medicine, health and the weirdness of life in general, check out the rest of the blog site at  randallfong.blogspot.com


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