• John Suter

85 dB is Not Always 85 dB – Why You Should Use the TLV for Noise

Most safety professionals are aware that the TLVs for chemicals are usually more protective than the OSHA PELs. Many even use the TLVs when possible, to provide the employees under their care with a healthier work environment. But not as many safety pros use, or are even aware of, the more protective Threshold Limit Value for noise.

But isn’t the TLV 85 dBA, just like the OSHA Action Level? Yes and no. The key is that this 85 dBA is a Time Weighted Average. If you were to stand next to a machine holding a direct-reading sound level meter, you are NOT measuring the Time Weighted Average – unless the noise level never varies, which doesn’t happen in real life – the noise level varies as the machine performs work, the employee moves around the plant, etc.

OSHA addresses this reality by requiring that “when the daily noise exposure is composed of two or more periods of noise exposure of different levels, their combined effect should be considered”. This is done using the “noise dose” – C1/T1 + C2/T2 + … + Cn/Tn. C in this formula is the amount of time the employee is actually exposed to noise at a particular decibel level, and T is the amount of time they are allowed to be exposed at that level. C doesn’t change based on whether you are using the TLV or the OSHA criteria, but T does.

Both the TLV and the OSHA Action Level allow an employee to be exposed to 85 dBA for 8 hours. So if an employee were exposed to an unvarying noise of 85 dBA for 8 hours, they would be exposed at just the limit according to both the TLV and the OSHA Action Level.

Where the TLV and OSHA part ways is the real life situation of varying noise levels. For every increase of 3 dB, the TLV reduces the allowed exposure time by half (and doubles it for every decrease of 3 dB). This 3 dB is called the “exchange rate” or “doubling rate”. OSHA, on the other hand, uses a doubling rate of 5 dB. Throughout the workday, this difference in doubling rates can produce a large difference in the Time Weighted Average noise level measured by a noise dosimeter.

Let’s look at a simple example of an employee operating a machine at 106 dBA for 9 minutes, and spending the rest of their 8 hour workday at 82 dBA. Using the OSHA criteria, this employee would be exposed at 84.9 dBA, just below the OSHA Action Level of 85 dBA. Using the TLV criteria, this same employee would be exposed at 89.6 dBA, well over the TLV of 85 dBA. Remember that decibels are on a logarithmic scale – the large difference between these numbers becomes obvious when comparing the noise doses, which are on a linear scale – 49% vs 289%.

The scientific consensus is that the 3 dB doubling rate is more representative of how human hearing is damaged by noise. When the first OSHA noise standard was adopted in 1970, both OSHA and the TLV used a 5 dB doubling rate. The TLV switched to 3 dB in 1994. The US military, most other countries and many corporations use 3 dB. Use of 3 dB is also advocated by the National Institute for Occupational Safety and Health, the American Industrial Hygiene Association, the National Safety Council, the American Academy of Audiology, and other members of the 85-3 Coalition.

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