Noise monitoring instrumentation makes data collection simple. But inconsistent analysis and interpretation of exposure sample data can lead to problems preventing hearing loss. It’s easy to fall into a false sense of security that your hearing conservation program (HCP) is based on correct assumptions. You should review your decision process to justify including each individual worker in the program based on a full and complete understanding of the data you are collecting.
Noise exposure data has become relatively simple to obtain, even in extreme environments, using miniature, bodily-worn (usually shoulder-mounted) personal dosimeters. They are easy to use and calibrate, and can be operated with minimal experience or training. Dosimeters have gone from providing a very basic set of dose calculations to producing literally dozens of calculated noise exposure metrics to meet various sampling methods and regulatory standards in use around the world.
An OSHA-compliant hearing conservation program requires you to monitor noise exposure levels to accurately identify employees exposed to noise at or above 85 decibels (dB) averaged over eight working hours. There are numerous ways an experienced safety or industrial hygiene professional may arrive at a conclusion that appears to be correct and is believed to meet regulatory requirements for implementing a hearing conservation program -- but actually does not.
Projecting noise dose
Noise level samples are averaged into an overall value of exposure by dosimeters “on the fly” using a complex formula that integrates the non-linear decibel scale. Accepted models of dose-exposure and hearing loss relationships vary. For OSHA, 100 percent dose is equivalent to being in an average noise level of 90dB for a full eight hours. Dosimeters must first calculate an average decibel level and compare that result to the time of exposure in order to express the sample in “% noise dose.” Yet the dosimeter can also provide a “Projected Dose” that takes a less than full shift data set and extrapolates the known levels to represent what the outcome would be if the levels stayed the same for the duration of the exposure.
Sometimes it’s difficult to obtain a full-shift measurement. The dosimeter may need to be collected before the worker stops their day and exits the premises. In this case, using the above example, the dosimeter’s dose reading will be less than 100 percent, for the mere fact that the full eight hours was not actually measured. If you had only 7.5 hours of data (450 minutes) the dose reading would be 93.75 percent (450 divided by 480 minutes or 7.5 divided by eight hours).
This is why dosimeters have a “projected dose” function to provide you with a second set of values (usually labeled “PDose or Proj Dose”). This value would be 100 percent because the dosimeter can only extrapolate based on the information it has already collected. If the average level was 90dB after 7.5 hours, it will assume the remaining 30 minutes of exposure was also 90dB, which is often the case – but not always.
Ask, “Can I assume the last 30 minutes were ‘noise free,’ or is it more likely that the noise exposure levels were consistent with what the first 7.5 hours of the day were like.”
If you expect there was no noise, the 93.75 percent exposure would be acceptable, given professional judgement supporting observational evidence. But you could assume sound was still present, and the exposure would be as much as 100 percent or somewhere in between the two values of Dose and PDose.
A worker is included in an HCP when exposure is greater than 50 percent of the allowable OSHA dose on any given day. The 50 percent exposure level is the Action Level. This triggers inclusion of a worker in the program. A 50 percent dose equals an eight-hour decibel average level of 85dB, which is half of 90dB (remember, decibels are non-linear units and in OSHA’s regs, the 5dB Exchange Rate represents a halving or doubling of the exposure).
This is where the metric labeled “TWA” can cause confusion. The Time-Weighted Average actually starts the day at 0dB (or sometimes the lowest reading on the dosimeter display) and only increments with the time of exposure. The TWA value will always be lower than the actual measured average dB value up until exactly eight hours, at which point the two values are equal. (see fig.1)
It’s not uncommon to see these terms used interchangeably. Sometimes this leads to wrong conclusions. The wording of 1910.95(c)(1) states, “The employer shall administer a continuing, effective hearing conservation program, as described in paragraphs (c) through (o) of this section, whenever employee noise exposures equal or exceed an eight-hour time-weighted average sound level (TWA) of 85 decibels (italics mine) measured on the A scale (slow response) or, equivalently, a dose of 50 percent.”
Note the wording “eight-hour Time Weighted Average.” If the measurement time (dosimeter run time) is LESS than a full eight hours, and it cannot be assumed noise was not present during the time that was not measured, then using a TWA value of less than eight hours could prevent an eligible worker from being included in the HCP. They are working in an 85dB environment but the TWA (<8h) shows a lower value.
Monitoring longer shifts
When dealing with workers who have 10 and 12 hours shifts, the Action Level must be adjusted to account for the longer duration of exposure. Hearing loss is a function of exposure level over time – so the levels to look for would be lower than the 85dB Lavg.
Here the dosimeter’s TWA value can be very useful. This metric accounts for the additional time of exposure automatically. It does this by “compressing” the additional effect of the sound energy entering the ear into an equivalent eight-hour exposure. The “Time-Weighted” average will be higher than the actual measured noise level, which is shown in the area of figure one which lies to right of eight hours of exposure. So a worker who is in an 83.3dB environment would NOT be in an HCP for an eight-hour day. Working the same job for ten hours would be the same as being in 85 dB for eight hours, thus they WOULD be included in the program.
If you doubt the interpretation of the term TWA and how dosimeters collect and report noise exposure values, review the instrument’s documentation or consult your supplier.