Noise Exposures and Exchange Rates

4/5/2021

The risk of noise that may impair our hearing is based upon two factors: noise intensity level (loudness) and the duration of exposure. From an occupational perspective, persons working around loud noise sources, such as heavy equipment and machinery during a typical 8-hour day, are most susceptible to hearing loss.  In addition to regular 8-hour work shift exposures, impulse noise – those quick bursts of high-intensity noises coming from such equipment as pile drivers, direct-push core-boring machines, as well manual hammering – can also cause significant risks to hearing loss. Thus, workplace sources of repeated noise exposures need to be identified, assessed and – based upon these results – controlled. 

Identifying sources of high noise can be relatively easy; however, such operations need to be quantified. That’s when a sound level meter (SLM) is necessary. To identify a loud noise source, the instruments’ microphone is placed in the immediate direction of the source. Once identified, many SLM manufacturers require placing the microphone perpendicular to the noise source, mimicking the human ears (being located in a 90-degree direction from a noise source when facing the source). According to OSHA, any work task that causes employees to be exposed to 90 decibels (based on the A-weighted scale dBA– refer to the earlier Safety Message dated 1/25/2021 that describes the A-weighted scale) for an 8-hour time-weighted average period, is considered to be excessive (and is referred to as the permissible exposure limit – PEL). When employee noise exposures reach this limit (that is, a 90-dBA time-weighted average measurement for an 8-hour work shift), employers are required to institute controls to reduce employee exposures to less than 90 dBA.  In addition, noise levels that are measured to be 85 dBA (for an 8-hour time-weighted average), require exposed personnel to be placed in the company’s Hearing Conservation Program, as required by OSHA’s Hearing Conservation Program: CFR 1910.95. In addition, the employer must provide hearing protectors (i.e. ear plugs or muffs) to its workers (however, it is NOT a requirement for these items to be worn- only available for employees to wear).

Meanwhile the American Conference of Governmental Industrial Hygienists (ACGIH); an organization that provides worker exposure limit guidelines (aka Threshold Limit Values – TLVs), has established 85 as the 8-hour time-weighted average exposure, which also requires the employer to institute exposure reduction controls (per ACGIH guidelines, employee exposures at 85 dBA – based on an 8-hour time-weighted average – need to be controlled). 

So, it appears that US companies are in a bit of a quandary, as OSHA and ACGIH are requiring worker noise exposures to be controlled at 2 different levels: OSHA at 90 dBA and ACGIH, at 85 dBA.  As it turns out, OSHA is the law and ACGIH merely provides guidance for worker safety and health. However, if work is being performed at a DOE site, through the Worker Safety and Health Program – 10 CFR 851.23; Safety and Health Standards – the more stringent (protective) of the two occupational exposure limits must be used. In addition, there are many non-DOE worksites where, through legal contract, requires noise assessment to be based on ACGIH TLVs. Aside from these scenarios, OSHA PELs are the required limits for which workers need to be protected in typically workplace settings.

While a SLM can help us identify elevated noise levels, they do not have the ability to measure employee exposures based on an 8-hour time period.  That’s where the noise dosimeter comes in. This instrument is placed about a worker’s hearing zone; typically, the collar, simulating the noise that enters the employees’ ears.  The dosimeter remains on the collar for the duration of the activity- preferably for the entire day or 8-hour shift. Throughout the shift, noise is continuously collected and integrated to calculate the average value; typically referred to as the 8-hour time weighted average. And depending on whether the field activity is being monitored where the OSHA PEL of 90 dBA or the ACGIH TLV of 85 dBA is the exposure criterion, the noise dosimeter needs to be set at that level so it will record the noise data as a percentage of that exposure limit. For instance, if the dosimeter averages the time-weighted average to be exactly 90 dBA where the OSHA PEL is the benchmark, the instrument will read 100%. If using the ACGIH TLV 85 dBA is the criterion and exposure is measured to be exactly 85 dBA, the instrument will also read 100%.  The tricky part happens when the measured exposures are NOT exactly equal to its corresponding occupational exposure criterion of 90 dBA (for OSHA compliance) or 85 dBA (meeting the ACGIH TLV).

This is where the term, exchange rate, enters. The exchange rate is the amount by which a sound level is changed when the exposure time is halved or doubled. There are two types of exchange rates typically used in use in workplace settings: 3 dBA exchange rate or the “3 dB rule,” (used by ACGIH) and 5 dBA exchange rate or the “5 dB rule” (used by OSHA).

This means that 50% of 90 dBA, using the 5-dB exchange rate, would yield 85 dBA. Or if doubled, the dosimeter measurement would be 95 dBA. Meanwhile, while 50% of 85 dBA, using the 3-dB exchange rate, would yield 82 dBA and when the noise is doubled, would yield 88 dBA. Easy? Yes, it is a little confusing. So, it’s nice to know that dosimeters can be set up, internally, to provide the correct measurements based whether field assessments need to be compared to the OSHA PEL of 90 dBA or the ACGIH TLV of 85 dBA; and thus alleviating us from complicating everything by using our fingers and toes to do the field math.

Because noise is based on a logarithmic scale, we cannot simply add or subtract noise levels. For example, 80 dB plus 80 dB does not equal 160 dB.  And while there is a mathematical formula that uses logarithmic functions, there is a rule of thumb that can be used for adding two noise sources for which employees may be simultaneously exposed.   

• To add two noise sources together where the difference between these two operations is 0–1 dB, add 3 dB to the higher level. So, 80 plus 80 would equal 83 dB.
• When the difference is 2–3 dB, then add 2 dB to the higher level. So, 83 plus 80 = 85 dB.
• When the difference is 4–9 dB then add 1 dB to the higher level. So, 85 plus 80 = 86 dB
• When the difference is 10 or more dB, then the additional noise from the lower source will not influence the overall noise level! So, 90 dB plus 80 dB would still be 90 dB.

Remember, this is just a “rule-of-thumb” field estimate method. A noise dosimeter, when placed in the proper setting, will provide an accurate and precise time-weighted average measurement based on the instruments’ internal electronics.

Remember that if you are not a S&H professional or a cross-trained person to perform simple industrial assessment techniques, it is best to let you ES&H professional conduct noise evaluations!

The coming of the wireless era will make war impossible, because it will make war ridiculous

Guglielmo Marconi (credited with the invention of the radio)