Personal noise dosimetry: measuring a full shift of exposure

This page explains what the rules and common practice call for. Every site is different, and what a survey or a filing needs at your facility is something we work out with you after we see it.

This is a service, not a rental. We bring the dosimeters, run your shift and tell you what the results mean.

Personal noise dosimetry measures the noise one worker receives over a whole shift, using a noise dosimeter that the worker wears.

Buying a dosimeter is not hard, but what does not come in the box is the part that makes the number defensible:

• Calibration records that show the dosimeter was checked for accuracy before and after every use.
• Careful, correct work in the field.
• A documented reason for which workers were chosen to represent which group.

A dosimeter and a sound level meter answer different questions

A sound level meter is the hand-held instrument, and it reads the noise at one location. The table shows how the two compare.

As an example, a sound level meter tells you the press is at 98 decibels (dBA), but dosimetry tells you more:

• The operator spends about an hour a day at that press, and the operator's 8-hour time-weighted average (TWA) is 83 dBA, a dose near 38%.
• The material handler walks past the press all day, and the handler's 8-hour TWA is 87.

Only one of those two workers is in your hearing conservation program, and the sound level meter reading cannot tell you which.

OSHA's action level for noise is a 50% dose, or an 8-hour TWA of 85 dBA, and it determines who goes in the program.

(For those who want to follow the math: OSHA allows 8 hours at 90 dBA, and each additional 5 decibels cuts the allowed time in half. At 98 dBA, Table G-16, OSHA's table of permissible noise exposures, therefore allows 2.6 hours. One hour at the press is 1 of those 2.6 hours, which is a dose of about 38%. A standard formula converts that dose into an 8-hour TWA of 83 dBA, and both formulas are shown below.)

Allowed time at 98 decibels = 8 ÷ 2^((98 − 90) ÷ 5) = 2.6 hours
Dose = 100 × 1 ÷ 2.6 = about 38%
8-hour TWA = 16.61 × log10(dose ÷ 100) + 90 = 83 decibels

You need both instruments, because one does not replace the other.

What OSHA requires the dosimeter to measure

OSHA's noise standard says the measurement must include all continuous, intermittent and impulsive (sudden, sharp) sound from 80 decibels to 130 decibels.

Two things follow.

The 80 dBA threshold is not optional. A dosimeter with its threshold (the level at which it starts counting) set at 90 dBA will consistently read too low for the action level, because it discards all the sound between 80 and 90 dBA. That is the very range that decides whether someone lands above or below an 8-hour TWA of 85 dBA.

This is the most important setting on the dosimeter, and getting it wrong produces a clean-looking report that understates what you must do.

Impulsive noise counts. Stamping, drop hammers, pneumatic tools and riveting all produce it, and the standard says plainly that this kind of sound must be included in the measurement. In practice, that means the dosimeter has to be capable of capturing it, although the standard does not say so in those words.

When area monitoring is not good enough. Sometimes sound level readings taken at fixed locations do not fit the job, and OSHA's noise standard names three causes:

• Workers are highly mobile.
• The sound level varies significantly.
• A significant part of the noise is impulsive.

In those cases, the standard requires representative personal sampling, which means dosimeters worn by workers who fairly represent the group. The one exception is where the employer can show that area monitoring gives equivalent results. In a plant where people move, dosimetry is needed most of the time.

Similar exposure groups: the same idea as air monitoring

A dosimeter does not go on everybody, only on enough people in each similar exposure group (SEG) to show what the whole group receives. How the exposure groups are drawn therefore determines both your cost and how defensible the results are.

What separates exposure groups for noise:

• Where the worker is, and for how long. This is the biggest factor and the one that intuition gets wrong, because time near the source matters more than how loud the source is.
• How much the worker moves. A worker at a fixed station and a roaming maintenance technician are different groups, even in the same department.
• Task mix. Someone who runs one machine is not the same as someone who covers four.
• Shift. Different production, different machines running, and sometimes very different noise.
• Overtime. A 12-hour shift is not an 8-hour dose. The 8-hour TWA calculation has to use the actual shift length, and a survey that assumed 8 hours will understate a 12-hour shift.

→ How many workers do you need to sample?

How we run the dosimetry

• Each dosimeter is checked before and after every use. Each check is made against a sound calibrator, and both records belong in the report. A reading with no post-shift check is a number you cannot stand behind. If the dosimeter drifted during the shift the reading is invalid, and without that check you cannot show there was no drift.
• Full shift. That includes breaks and lunch as workers actually take them. Removing the dosimeter for breaks changes the dose, and if the break room is quiet, removing it makes the result too high.
• Microphone on the shoulder, near the ear. This is standard practice, and being off by a few inches matters more than people expect.
• Field notes. They record what ran, what did not and what was out of the ordinary, which is what makes the number mean something 6 months later.
• Area sound level readings at the same time. These tie the dosimetry results back to the sources.

What the report includes

A complete report gives you, for each worker:

• The 8-hour TWA
• The dose percentage
• The maximum and peak levels
• The shift length the 8-hour TWA was calculated against

For each exposure group, the report shows whether it is over the action level, over the PEL or clear. It also states what each result requires you to do, and by when.

→ What belongs in a noise survey report · What drives the cost of a noise survey