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We have some 10- and 12-hour work shifts. How do we account for the longer noise exposure times under OSHA reporting requirements, and should we be adjusting our criteria for determining which employees should be included in our hearing conservation program?
Responding is Rob Brauch, business unit manager, Casella CEL Inc., a division of IDEAL Industries Inc., Buffalo, NY.
Under OSHA 1910.95, there are two noise exposure levels to consider: the permissible exposure limit (or PEL) and the action level, which triggers the inclusion of a worker in a hearing conservation program.
It’s important to understand the fundamental relationship between exposure level and exposure time – and because quantifying noise requires using non-linear decibel units, the numbers can sometimes get confusing.
Noise exposure is often expressed as a dose, in percentages, but that result is based on an average decibel level measured over just the work time that was possible to measure to obtain a representative sample. For an 8-hour shift, you should measure at least 7 hours or more.
OSHA’s PEL states that a worker cannot be exposed to more than a 90 dB (a weighted) average for an 8-hour work shift – that equals 100 percent of the worker’s allowable noise dose. The 90 dB limit is called the “criterion level.”
For every 5 dB increase in noise level, the allowable exposure time is cut in half, so a worker could technically be exposed to four hours of noise at 95 dB(A), but the remainder of the work shift would have to be “quiet” – below the threshold of concern, which for the PEL is 90 dB. That is why noise dosimeters have a “threshold” setting defaulting of 90 dB. If the average level is below 90 dB, the employee could work longer than 8 hours without receiving the equivalent exposure exceeding the 8-hour PEL. For example, if the average decibel level was 88.3 dB, then the worker was exposed to the same amount of acoustic energy in 10 hours as he or she would have had in 8 hours at 90 dB. If the measured noise level is 87.1 dB, the equivalent allowable exposure time to reach 100 percent dose would be 12 hours. If the level is 85 dB, the worker could work twice as long as a normal shift, or 16 hours, before reaching the 100 percent dose limit.
For determining inclusion in a hearing conservation program, the average level of concern is 85 dB, which is 50 percent of the allowable dose, and in longer shifts that level also would be adjusted to a lower level, same as with the PEL. To measure these levels below 90 dB accurately, the dosimeter has to utilize a second, lower threshold setting level – which OSHA sets at 80 dB. Noise below that level is not included in the averaging function.
Often, a difficult challenge is knowing which of the answers produced by dosimeters should be compared to the regulatory limits – especially if the time of the measurement is less than the full shift length. The user sees “average,” “time-weighted average,” “dose” and “projected dose values” (all with 80- and 90-dB thresholds) on the report, which can be confusing.
Most dosimeters will program a dose calculation based on a shift length of 10 or 12 hours, but do not allow you to set the adjusted PEL criterion level to exactly 88.3 or 87.1 dB, as would be needed to precisely calculate the result as a dose percentage, and thus would slightly overestimate the noise dose. It is usually better to leave the dose criterion length set at 8 hours and the criterion set at 90, and then compare the average noise level in decibels to the adjusted allowable limits (88.3 and 87.1).
Using the LAVG – the average noise level during the instrument’s run time – as captured over the elapsed time of the measurement assumes the remaining part of the shift was as noisy as the part you measured. When in doubt, it is best to set the meter to run for the entire duration of the work shift even if you cannot be present, and use the actual dose value in percentages.
Not all manufacturers use the exact same data labels when presenting averages and TWAs on the instrument or its reports. If in doubt, contact the manufacturer.
Editor's note: This article represents the independent views of the author and should not be construed as a National Safety Council endorsement.