HEARING CONSERVATION – BACK TO BASICS
1. Introduction
Of all the possible safety topics, hearing conservation is the most difficult. The
reason for this is that there is no pain associated with hearing loss. In fact, we
actually enjoy loud noise that damages our hearing!
It is by no means an easy task to convince people to wear, what is often perceived as,
uncomfortable PPE, especially when they cannot immediately feel or see the benefits.
There is only one way to do this so that a willingness to comply is created, and that is
to ensure that the employee fully understands the working of the ear, how the ear is
damaged, that noise induced damage is irreversible and totally isolates one from the
world.
2. The workings of the ear
2.1 The Pinna
The pinna is part of the outer ear, it has all sorts of interesting folds and convolutions
and is there in order to funnel sound from our environment into and down the ear
channel. The pinna actually accounts for 11% of our hearing and grows in size as we
age. It may be interesting to note that each person has an individual ear print, pretty
much like their own individual finger print.
2.2 The ear channel
Once the sound has been trapped by the pinna it is sent down the ear channel. The ear
channel is made up of very delicate skin which can easily be damaged. In fact, often
the cause of ear infections is from people cleaning in their ear channels with all sorts
of objects and damaging the delicate internal skin.
Many PPE wearers claim that their ear plugs give them infections, this I am willing to
wager is on the whole untrue. In many cases the problem is damage to the ear
channel by cleaning and then the introduction of a dirty plug. A dirty ear plug in a
healthy ear will do absolutely not damage. The reason I can say this is because of the
ears fantastic natural defence mechanism to germs and bacteria, ear wax. Ear wax is
so deadly that a tablespoon of it will kill the eater in three hours. Any bugs or germs,
including those on plugs, are killed very quickly by the bitter wax.
The ear channel is also a self cleansing organ and should be left to do its job unaided.
Any dirt will in time grow out and appear in the outer ear where it can be removed
without damaging the ear channel.
Also, of additional interest is that male ear channels are longer than female ear
channels and that African and Asian ear channels are generally narrower.
2.3 The ear drum and middle ear ossicles
The eardrum is a tiny piece of the ear and is the part that converts sound energy into
motion. Sound travelling down the ear channel causes the eardrum to vibrate. This
kind of phenomenon can be seen if a jet plane takes a turn close to a building. You
will notice that the windows vibrate.
The vibrations of the eardrum are picked up by the first of the middle ear ossicles.
The middle ear ossicles are three little bones that work as the amplification system of
the ear. They use a lever principle in order to achieve this. As I said, the first bone
picks up the vibrations of the eardrum, it amplifies those vibrations and passes them
onto the second bone which in tern increases the vibrations from the first bone. The
third bone picks up the vibrations of the second and also increases them, it then taps
against the part of the ear called the cochlea. Once the vibations reach the end of the
third bone they are twenty times larger than the original vibrations of the eardrum.
2.3 The cochlea
The cochlea, this is where it all happens! The cochlea is a fluid filled organ with
about 30 000 tiny hairs protruding out of its internal walls. The tapping of the third
bone of the middle ear ossicles causes the fluid in the cochlea to move pretty much
like the ocean surges backward and forward and this motion in tern causes the little
hairs to move backward and forward too. The movement of the hairs generates an
electrical impulse which is sent down the auditory nerve and this impulse is
interpreted in the brain as sound. It is damage to the hairs in the cochlea that leads to
hearing impairment caused by noise.
3. What does noise actually do to the ear?
3.1 Temporary threshold shift
This phenomenon is experienced by all of us almost everyday. In layman’s terms it
means going temporarily, slightly deaf through the course of each day. Throughout
the day all of us are exposed to different levels of noise, whether it is, office chatter,
machinery working, traffic or music and television. With each exposure our ear goes
through the process of enabling us to hear, causing the hairs in the cochlea to move.
Now, as the day progresses the hairs, for want of a better description, get tired. They
no longer move as much as they did early in the morning. The decrease in movement
causes us to hear less well. When you rest at night in a quiet environment these hairs
recover and are fully functional the next day.
I am sure you can recall a time when you got into your car in the morning and
quickly turned the volume on your radio down from the setting it was on the night
before. I am sure you wondered if you were really listening to the radio at such a loud
level the night before. Yes, you were, but it did not sound as loud to you because you
had gone slightly deaf through the course of the day, experienced a threshold shift.
3.2 Noise induced hearing loss
When a person is continually exposed to noise above 85db and wears no protection
or is not adequately protected then the pressure placed on the hairs in the cochlea is
just far too much for themt to recover. What happens is that they no longer recover to
become fully functional, even with rest and quiet and so you develop permanent
deafness. The scary thing is that once these hairs have been damaged they can not
repair themselves. You will never be able to regain your hearing once you have lost it
in this manner.
In further illustration, think of a patch of grass. Compare the blades of grass standing
up to the healthy hairs in a cochlea. If you stand on the grass, the blades lie flat, but
given time they will stand up again. However, if you continue to stand of the grass,
eventually they will die.
3.3 Impact sound and the nerve cells in the cochlea
Impact sound is very dangerous for the hairs of the cochlea. Examples of impact
sound would be things like gun shots, blasts or hammering on metal. This kind of
sound causes the impact of the third bone of the middle ear ossicles to be significantly
bigger as it taps against the cochlea. This then causes the movement of the fluid to
be more violent, in fact it can actually cause the hairs in the cochlea to snap off. As
above, damage to these hairs is irreparable. They will never regenerate and you will
never regain the hearing you have lost.
4. Common objections to wearing hearing protection
4.1 I will wear a hearing aid if I start to lose my hearing
It is a misconception that hearing aids can correct noise induced hearing loss. Noise
damage is due to the destruction of the nerve cells (hairs) in the cochlea that enable us
to hear. Hearing aids can restore the ability to detect and discriminate sounds to a
certain extent, but when insufficient hair cells are present to receive the amplified
sounds that the aid provides, the results are not fully satisfactory.
If wearing a hearing protection device for 8 hours a day, five days a week is not a
pleasant thought, imagine how uncomfortable wearing a hearing aid 24 hours a day
for 7 days a weeks is.
4.2 Earplugs give me ear infections
The likelihood of this happening is minimal. The reason for this is discussed earlier
in the section dealing with the ear channel and the ear wax. Ear wax is a potent germ
and bacteria killer and should eliminate most problems of this sort. Instead attention
should be given to eradicating incorrect cleaning methods (such as poking foreign
objects into the ear canal – such as ear buds, keys, etc which damage the sensitive
lining of the ear canal) and cleanliness should be stressed. Certain individuals such as
diabetics who are prone to infection should be carefully monitored.
4.3 My machine sounds different when I wear hearing protection
Yes it will, but all it takes is a little time in order for you to learn the new sound of
your machine. Also by protecting your hearing you will always be able to hear it and
you will not be experiencing temporary deafness throughout the day, so the sound will
be the same all day long, unlike the operator without hearing protection.
4.4 I have already lost some of my hearing so what is the point
The fact that you may already have noise induced hearing loss does not protect one
from further loss. Hearing loss normally starts out in the higher frequencies and with
continual exposure moves to damage the lower frequencies too. It can affect the areas
of the cochlea needed for understanding and hearing speech. Yes it is true that
hearing protectors cannot restore loss, but they can prevent further loss.
5. The decibel scale and maximum exposure times
The decibel scale
Getting to grips with the decibel scale is not an easy thing and is widely
misunderstood. Most scales or measuring systems are progressive and so this is the
way we naturally understand most things to be. The decibel scale, however, is a
logarithmic scale and works according to formulas.
The best way to explain the problem is to ask you to consider the following: 85db is
approximately so loud that if you stand one metre away from another person, you will
have to raise your voice to be heard. How loud then would you consider the hardly
audible sound of the ticking of a watch to be? Because, as I said earlier, we think
progressively, so most people will say, 1db or maybe 5db. The ticking of a wrist
watch is in fact 30db. 1bd is so quiet that it can only be achieved in special isolation
rooms, and it is so quiet that you can literally hear your body functioning! But then,
you may ask how do we get from 30bd to 85db and there is such a huge change in
actual noise, because 30 and 85 are not so far apart? When working with the decibel
scale we double the amount of noise with each three decibels that we go up. So, 33db
is double the amount of noise of a ticking watch.
What is frightening is that this is often not understood by even safety personnel and
they may think that noise of 88bd is not such a big problem as it is “only 3 decibels”
more than 85. In reality it is double the amount of noise!
5.2 Exposure times
The magic number of 85db being the level at which hearing protection must be worn
is not just some randomly made up number. It is the maximum amount of noise we
can withstand for 40 hours a week before our hearing starts to be damaged. It is 40
hours a week as the normal working week is made up of 8 hours a day with 5 working
days in a week.
If a worker is exposed to a noise level of 88db then he or she can only endure 20
hours a week until they start to damage their hearing. At 91db the human ear can only
withstand 10 hours and so on. With each three db increase in noise the maximum
exposure time halves. Take a jackhammer drill, it makes noise of 110db, a worker
exposed to this noise level can only withstand 7.6 minutes per week. Have a look at
some road works crews around South Africa and note how few of these operators
wear hearing protection.
6. Finding solutions
Engineer the problem out
By far the best solution to any noisy problem is to attempt to get rid of the source of
noise. This may entail enclosing a particularly noisy machine or purchasing new,
quieter machines. Often, however this is not as easily done as said. New machines
cost a lot of money and sometimes isolating a machine is not at all practical and
severely counter productive.
Provide training for the exposed workforce
When it comes to hearing protection we are faced with the fundamental problem of no
pain. Take most other hazards, the eye for example; people have no problems
understanding why they need to wear safety glasses. There is a lot of discomfort from
a simple speck of dust in the eye, it takes a tiny leap of the imagination to grasp the
pain that will result from a metal particle or a chemical splash. Sadly not the case
when it comes to hearing protection.
Part of any successful hearing conservation programme is the proper and detailed
training of each person. It is not good enough to simply show an employee how to
correctly wear their hearing protection. You need to go through the whole process
and explain the workings of the ear, the effect of deafness and deal comprehensively
with any excuses that may come up.
Also it is essential to keep the training top of mind by having posters in plain view.
Change the posters, print the results of tests, run competitions. Do whatever it takes
to keep hearing protection in focus.
Personal protective Equipment
When faced with problem areas and engineering the noise out is not an option, then
we need to look at training as discussed above and also personal protective
equipment. The next section offers some tips in the choice of suitable equipment and
deals with some common misconceptions.
7. Making the correct choice when it comes to protectors
First considerations
Before looking at the kind of protection you need you will have to know the noise
levels in all different areas of the plant. Once you know this you need to make a
choice of the best possible products available to you.
Each hearing protector is marked with a NRR, or Noise Reduction Rating. It may
have a NRR of 21db or 33db. A product’s NRR is the amount of decibels that it
claims to block out. A common mistake is to assume that the best protection is the
highest possible protection. This is not true, you may in fact be endangering a
person’s life by over protecting them and not enabling them to hear an on coming
vehicle or such. All you want to do is bring the noise level down to below 82db, a
figure somewhere in the 70’s is also perfectly acceptable. A case of over protection
would be giving a person working in an area of 91db a plug that blocks out 33db.
You effectively bring their level down to 59db and that is a bit too low.
Disposables, Re-usable, semi-aurals, muffs, dual protection, custom fit?
The choice is vast, so what is best. Again, as with just about everything in hearing
conservation, the answer is not easy. Hearing protection product choice, we have
found, is a very personal matter, and if you make a mistake here, you can destroy your
effort at conservation very easily.
7.2.1 Disposables
Disposables are made to be used for a short period of time, maybe up to a week
depending on conditions and then thrown away. Disposable plugs need to be rolled
down in to a tight tube and then inserted into the ear channel. Once in the ear channel
they expand and take on the shape of the channel and block sound from entering.
When inserting ear plugs of any kind it is best to reach over your head and pull up the
pinna of your ear and while doing so insert the plug. The reason for this is so that you
can help to straighten out the ear channel, which has a big kink in it, in order to get a
better fit.
Generally there are two kinds of materials used to make disposable plugs; foam plugs
and polyurethane plugs. Polyurethane plugs generally look softer and more
comfortable, but in reality foam plugs feel exactly the same once in the ear channel.
Foam plugs feel harder, but it is this hardness that gives them an advantage. They
recover slower then polyurethane plugs and so are easier to get into the ear as they
remain rolled down for longer before they start to expand. Foam plugs also do not
absorb water, where the polyurethane ones do. This can cause discomfort for workers
in hot areas as the sweat from the ear channel can be absorbed causing the plug to
expand. An advantage of polyurethane is that it is generally a cheaper material and so
ideal for the budget conscious.
7.2.2 Re-usables
Re-usable plugs are fantastic for companies who are looking to cost of ownership.
They are made from materials that can be washed and re-used. Some re-usable plugs
can be used for up to 8 months. So, your reusable plugs may be 9 times more
expensive that the disposables, but their life span more than justifies the cost.
Another point in favour of re-usable plugs is that they are generally, depending on the
brand easier to insert than you roll down disposables. I am particularly in favour of
this as you will only get the protection claimed on the package “if inserted correctly”,
it is easy to make a mistake with a disposable plug, but less so with re-usable. Reusable plugs therefore can give more piece of mind.
7.2.3 Semi Aurals or banded types
Semi Aurals are great for people who move in and out of noise environments. They
are easy to store around the neck while in quiet places and easy to don when entering
a noise zone. They can also be worn in a number or ways, under the chin, behind the
head or even over the head like an alice band. They have however got two draw
backs and that is that any scraping, such as rubbing against the collar, made on the
band reverberates in the ears and some of them can tend to be a little uncomfortable
when worn for long periods because of the pressure exerted by the band.
7.3.4 Ear muffs
Earmuffs again are fantastic for places where bone conduction is experienced. They
are easy to put on correctly and can be attached to hardhats, worn over the head or
behind the neck. Also, from a conservation point of view it is easy at a glance to see
if people are complying. Some of them are even fitted with fm radios and are ideal
for workers on production lines. The down side of muffs is that they can be very hot
and this can decrease the comfort factor.
There are many muffs available on the market ranging from your cheap ones to more
expensive muffs. When going the muff route beware of the following things. Take a
look at the band, if it is plastic, the muff will generally be cheaper, which often makes
it an attractive choice. Do not forget though that a muffs protection comes from the
amount of pressure that it exerts on the side of the head and this pressure is controlled
by the band. Plastic bands tend to weaken quickly and the protection is then no
longer existent. So, if you do choose muffs like this ensure that they are replaced
every three months or when the pressure changes. If you decide on a muff with a
sprung steel headband you will be paying more, but can expect about three years of
use from it.
Another thing to look out for is the way in which the band is attached to the cup. I
would recommend on that attaches to the cup on both sides instead of the middle.
The pressure is better distributed in such products and makes for a more comfortable
fit.
7.3.5 Dual protection
By dual protection I mean a situation where a plug is worn under a muff. This kind of
product would be used in areas with really high noise levels and also bone
conduction. Remember that you cannot work out the new NRR offered by a
combination simply by adding the protection offered by the one to the other. It has to
be calculated by a special formula. The best NRR offered by such a combination, that
I am aware of, is 36db. Issues here would be ensuring that the plug is correctly fitted
and that the muff has all the desirable features discussed above.
7.3.6 Custom fit products
Custom fit products are made to fit the individual’s ear channel and so remove the
correct fitment issue and can provide a comfortable solution. Many of them can have
attenuation levels set for the individual’s environment too which eliminates the risk of
bring the decibel level too low. Also, they are expensive products and can create a
feeling of pride in the wearer. There are, however, as will all products in this arena, a
couple of issues that need to be considered. Price is an issue and it is a large
investment for a company to make considering that hearing protectors of any sort
seldom break, but most often get lost. The cost of replacement for lost has to be of
concern. Keeping costs in mind, some custom made plugs also need to be calibrated
each year and this will be at an extra cost.
Other concerns for me would be that the moulding is done when the person is sitting
still. In real life situations ear channels move with chewing and talking and one may
experience leaks. Also a change in ear channel size can be experienced with each 9
kg change in weight. Some companies may find the time taken to set moulds
prohibitive too.
The crux of the matter when it comes to choice
Knowing the pros and cons of all the products available is very important, but there is
an even more important fact to remember. Personal protective equipment when it
comes to hearing is, just that, personal. Everyone has their own preference and
oddities regarding this issue. Some people for instance will love muffs because the
hate putting things in their ears while others can stand the idea of them as they find
them too hot. If you force a group of people to all wear muffs, you will get some
resistance and your compliance will suffer. If, however, you give a group of users a
choice of three products, you will find that not only does your compliance go up, but
your users will have a sense of ownership and buy in.
8. Conclusion
A challenging subject, fraught with curve balls, emotions and objections and that is
what makes it so exciting.