Allowable Noise Exposure Time

Occupational noise limits are built from two numbers, and neither is physics. The CRITERION LEVEL is the steady level permitted for a full eight-hour shift — 85 dBA under NIOSH and most Canadian provincial regulations, 90 dBA under the OSHA permissible exposure limit in the United States. The EXCHANGE RATE is how much extra noise halves the allowable time. Together they define the whole table: allowable time is eight hours divided by $2^{(L-L_c)/q}$.

The exchange rate is where jurisdictions genuinely disagree, and it is not a rounding difference. NIOSH, the European directive and most Canadian jurisdictions use 3 dB, the equal-energy rule: 3 dB is a doubling of sound energy, so doubling the energy halves the permitted time. It follows from the physics of dose and is supported by the hearing-loss data. OSHA's enforceable standard uses 5 dB, a historical compromise from the 1970s that is more permissive at every level above the criterion. Put a worker at 100 dBA and the two rules give 15 minutes and 2 hours respectively — a factor of eight, same worker, same machine. Canadian rules are provincial and not uniform: most now use 85 dBA with a 3 dB exchange, but the exchange rate and criterion have both varied by province and over time, so check the regulation that actually governs the workplace rather than the one you learned first.

Real days are not steady, so the calculation generalises to a DOSE: for each period at each level, divide the time spent by the time allowed at that level, and add the fractions. A dose of 1.0 (or 100 %) is a full day's permitted exposure. The equivalent continuous level $L_{ex}$ or TWA is that dose re-expressed as the single steady level that would have produced it. Note the shape of it — because the scale is logarithmic, a few minutes at a very high level can dominate an otherwise quiet day, and the loud fifteen minutes of grinding matters more than the six quiet hours around it.

Three things this arithmetic does not cover. It does not protect against IMPULSE noise: a single impact peak above 140 dB can cause immediate permanent injury regardless of what the daily average says, and peak limits are stated separately for that reason. It says nothing about hearing protection performance, which derates badly in real use — laboratory attenuation ratings are earned on carefully fitted subjects, and the usual field guidance is to take roughly half the rated attenuation for earmuffs and less for plugs unless workers have been individually fit-tested. And it is a limit, not a target. The hierarchy of controls puts quieting the source, then enclosing it, then separating people from it, all above personal protection; an exposure sitting at exactly 100 % of the permitted dose is a workplace with no margin at all, and the criterion level itself is a level at which some proportion of the exposed population still loses hearing over a working lifetime.