How much noise can workers be subject to?

In Brief

An average of 90 decibels over 8 hours is the most noise to which a worker may be exposed. It is important to keep the 8-hour average in mind. If an employee works longer than 8 hours, the allowable exposure is less than 90 decibels on average. Conversely, if an employee works less than 8 hours, the allowable exposure is more than 90 decibels on average. No worker may be exposed to continuous noise over 115 decibels nor to impulse noise over 140 decibels.

Important Ideas

Since the decibel scale is logarithmic, values cannot be averaged arithmetically.

Manually computing an exposure average for an employee can be complicated. The easiest and most accurate way to determine a person’s exposure is to use a sound measuring device called a dosimeter.

What is a decibel?

A decibel (abbreviated dB) is a unit of measure of sound intensity. The decibel scale is designed to express the range of human hearing, and since that range is so great, the scale is logarithmic. This means that 30 decibels is 10 times louder than 20 decibels. Interestingly, zero decibels is not the absence of sound, but the equivalent of an arbitrary reference sound (20 micropascals of sound pressure, actually). The idea is that zero decibels represents the weakest sound an average human can hear.

More Detail

Decibel measurements for hearing testing are generally “A-weighted”, abbreviated dBA. “A” weighting filters out some very high and low pitch sounds.

The average exposure over 8 hours is referred to as a “time weighted average” (TWA).

The percentage of allowable noise to which a worker is exposed daily is termed “noise dose”. Therefore, a worker exposed to 90 dBA for 8 hours has a noise dose of 100%.

The maximum amount of noise to which an employee may be exposed over 8 hours is referred as the “permissible exposure limit” (or PEL).

Both OSHA and MSHA use the concept of “exchange rate” to determine allowable noise exposure. The exchange rate can be thought of as a way of trading intensity of noise for exposure duration. For each multiple of the exchange rate you must decrease a worker’s exposure by half. Given OSHA and MSHA’s 5 dB exchange rate and the PEL of 90 dBA over 8 hours, a worker may be exposed to 95 dBA for a maximum of 4 hours, or 100 dBA for 2 hours.