“Everything is poisonous, nothing is poisonous, it is all a matter of dose.”

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“Everything is poisonous, nothing is poisonous, it is all a matter of dose.”
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Everything is poisonous, nothing is poisonous, it is all a matter of dose.”

-- Claude Bernard, French physiologist

The science of hearing loss has been well documented for decades. It is based on using the concept of “noise dose” to establish an equivalent relationship between different combinations of noise levels and durations of exposure. As either the noise level or the duration of exposure goes up, so does the dose and so does the hazard.

In Hearing Conservation, nearly every occupational noise exposure standard around the world expresses the balance of noise and duration through Permissible Exposure Limit (PEL) or criterion level for noise. Such limits are set based on an analysis of specific health risks and the cost/benefit of limiting exposure to that hazard.  There are about 500 chemicals/contaminants that have established permissible exposure limits; noise is just one of them. For noise, OSHA and most other US standards set the PEL at 90 dB of noise exposure over an 8-hour time.  Worldwide the majority of PELs for noise are set at 85 dB over an 8-hour time

These concepts of PEL, exchange rate and dose can be somewhat nebulous to consider, so let’s think of them in a more concrete way. 

Imagine noise as a liquid and your ear as a cup. The cup can only hold a given amount of the liquid before it overflows (causing noise-induced hearing loss). The amount of liquid is the noise level, the rate at which it is being poured is the exchange rate, the period of time the liquid is poured is the duration of the exposure. 

If the water [noise] is poured evenly over time, it will fill the cup adequately. It can accommodate the water without overflowing.

If the water is dumped hastily into the cup, it overflows. Or if we just keep pouring and pouring for a long time, it overflows. In the case of noise, this would be too much noise – and a higher risk of noise-induced hearing loss. 

Dose
In the real world, we must strike a healthy balance between noise and dose. This balance is expressed as a percent of total noise dose, generally over an 8-hour work shift.

For example:

  • A noise dose of 75% indicates a worker was exposed to noise equivalent to 75% of the permissible exposure level for an 8-hour work shift.
  • A 200% noise dose indicates the worker was exposed to the equivalent of twice the allowable permissible exposure level for an 8-hour work shift.

The dose calculation takes two factors into consideration: Permissible Exposure Level, and the exchange rate. Under current OSHA regulations in the United States, a 100% dose is equivalent to a 90 dBA time-weighted average noise exposure using a 5 dB exchange rate. 

How Can We Protect Ourselves from Too Much Noise?
There are several actions we can take to prevent an “over dose” on noise and prevent noise-induced hearing loss:

  • Engineering controls (control the amount of liquid in the pitcher)
  • Administrative controls (only pour at limited times)
  • Use hearing protection (a cover over the cup) in our efforts to prevent noise-induced hearing loss. 

Recently, advances in intelligent hearing protection enhance everyone’s ability to protect people from noise exposures and measure one’s daily dose.

  • Earplug fit-testing systems such as VeriPRO® can measure how well the cover over the cup (earplug) prevents liquid (noise) from getting into the cup (ear). 
  • In-ear-dosimetry, such as Howard Leight’s QuietDose™ technology, measures exactly how much liquid (noise) gets into the cup!

Knowing the individual dose of noise and being able to control that noise dose helps us to avoid being “poisoned” by the hazardous noise.