Noise is one of the most common occupational hazards in any industry. While the manufacturing sector is 13% of the American workforce this sector has 72% of all recordable hearing loss cases. Not only does the employee suffer from reduced or lost hearing our economy loses an estimated $242 Million/year in worker’s compensation for the disability. (OSHA Regional Emphasis Program)

The first attempt to regulate noise hazards in American workplaces was in 1969 in the Walsh-Healey Public Contract Act. At the same time the Bureau of Labor Standards added a construction occupational noise standard to the Construction Safety Act, which became the OSHA Construction standards, CFR 40 1926, in 1971.

The exposure limit was set at 90dBA 8-hour Time Weighted Average with a 5dBA exchange rate. This limit was based on the American Governmental Industrial Hygienists Association, ACGIH, Threshold Limit Value, TLV. The Current TLV is 85dBA with a 3dBA exchange rate.

How does noise work?  You don’t have to take a physics class to know the phrase ‘sound waves’.  This is a correct term to use when talking about noise.

You know when that car drives up beside you with the radio blaring and you can feel your car vibrate?  It’s because noise creates pressure and pressure travels outward in a wave. 

That sound pressure is funneled into your ears and down into that snail looking cochlea where there are tiny little cilia that are called ‘hair cells’ because that’s what they look like.

The cochlea is full of liquid and the fibers.  When the pressure waves go down the ear canal the pressure is transferred to the cochlea which cause the hair cells to move, like seaweed under a wave. 

So, here’s the interesting thing, not all the hair cells move!  The frequency of the sound determines how far up the spiral of the cochlea the sound wave will go and which hair cells are moved.  That information is transmitted via the auditory nerve as frequency and volume.

Have you ever noticed that when you turn your car on in the morning the radio sounds really loud?  This is because, by the time you headed for home you had tired your cilia out.  Seriously, it’s called Auditory Fatigue, and overnight they rested up, hopefully, in the quiet of your house or while you were sleeping. 

This is also called a Temporary Threshold Shift, meaning, the threshold at which you can hear a sound has to be louder just to get the cilia to react.  Now, when the radio sounds just fine, the loss may be permanent, a Standard Threshold Shift. 

The easiest way to define the difference between the two is to think about the grass in your yard.  If you walk across the grass and then turn to look behind you, you can see your feet have pushed down the blades of grass (like noise pressing down the cilia in your ears), but after a while the grass straightens up.  This is a temporary threshold shift.

If your yard is on the corner and you don’t have a fence, people will ‘cut the corner’ over and over again until they have worn a path in your yard where grass no longer grows.  This is a Standard, or permanent, threshold shift.  It is also called Noise Induced Hearing Loss.