Noise Dose & TWA Calculator

What's an Exchange Rate?

The exchange rate (also called the time-intensity tradeoff or Q factor) defines how much additional sound energy is considered equivalent to doubling the permitted exposure time. Put simply: if you add Q decibels, you must halve the allowed duration.

With a 3 dB exchange rate (NIOSH and Quebec RSST): 88 dBA × 4 h is equivalent in energy to 85 dBA × 8 h. This follows the equal-energy principle, where sound intensity is halved for every 3 dB decrease — doubling the permissible duration. The 3 dB rate is considered scientifically correct according to acoustics physics.

With a 5 dB exchange rate (OSHA): 95 dBA × 4 h is considered equivalent to 90 dBA × 8 h. This is a deliberate policy choice, it is less conservative than the 3 dB rule, meaning the same exposure in dBA × time produces a smaller calculated dose under OSHA than under NIOSH or Quebec. The 5 dB rate was adopted by OSHA in 1971 based on the measurement technology and economic considerations of the time. NIOSH (1998) recommends 3 dB as better aligned with actual hearing loss risk.

Quebec's June 2023 Change

Before June 16, 2023, Quebec's RSST (Regulation Respecting Occupational Health and Safety) used the same parameters as OSHA: a criterion level of 90 dBA and a 5 dB exchange rate. On June 16, 2023, Décret 781-2021 came into force, fundamentally aligning Quebec with the NIOSH REL framework.

The new Quebec RSST standard uses a criterion level of 85 dBA and a 3 dB exchange rate (equal-energy rule). This is a significant tightening: at 90 dBA, the permitted daily exposure drops from 8 hours (OSHA) to 2 hours and 31 minutes under the new Quebec/NIOSH framework. The same noise environment that was compliant under the old RSST may now exceed the daily limit.

The modification also affects articles 2.10.7.1 to 2.10.7.9 of the Construction Safety Code (Code de sécurité pour les travaux de construction, CSTC), which references the same regulatory changes. For the most current legal text, always consult the official Quebec legislation (legisquebec.gouv.qc.ca) and the IRSST Infolabo (irsst.qc.ca/laboratoires/infolabo) for measurement methodology guidance.

When Dose Exceeds 100% — What Does It Mean?

A noise dose of 100% represents the full daily noise allowance under the chosen referential: an 8-hour exposure at the criterion level (85 dBA for NIOSH/Quebec, 90 dBA for OSHA). A dose above 100% means the worker has been exposed to more noise energy than the referential considers acceptable for a full workday.

For example, a dose of 200% means the worker received twice the daily noise energy limit. The equivalent 8-hour TWA in this case would be 3 dB above the criterion level (88 dBA with a 3 dB exchange rate, or 95 dBA with a 5 dB exchange rate). Doses above 100% are associated with increased risk of noise-induced hearing loss (NIHL), particularly with repeated over-exposures.

This tool intentionally reports a factual percentage rather than a compliance verdict. Whether a dose above 100% requires intervention depends on the applicable regulatory framework, the nature of the work, the use of hearing protection, audiometric surveillance, and professional judgment. Consult a certified industrial hygienist for workplace hearing conservation program implementation.

Frequently Asked Questions

Why doesn't OSHA equal NIOSH for the same exposure?

OSHA uses a 5 dB exchange rate and a 90 dBA criterion level, while NIOSH uses a 3 dB exchange rate and an 85 dBA criterion level. These two parameters compound: a worker exposed to 95 dBA for 4 hours has a dose of 100% under OSHA (the allowed duration at 95 dBA is 4 hours), but under NIOSH that same exposure produces a dose of approximately 800% (NIOSH permits only about 30 minutes at 95 dBA). For the same noise environment, OSHA will almost always show a lower dose than NIOSH or Quebec.

What if my noise data is below 80 dBA?

Exposures below 80 dBA contribute nothing to the cumulative dose under all three referentials. NIOSH 98-126 defines 80 dBA as the lower boundary below which no dose contribution is counted. OSHA 29 CFR 1910.95 also ignores levels below 80 dBA when computing dose (although some OSHA methods use 85 dBA as the threshold, the 80 dBA value applies to dose calculation). In this calculator, any row with a level below 80 dBA is flagged and its contribution is set to zero.

Can I sum doses from a noise dosimeter directly?

It depends on what exchange rate your dosimeter is configured for. Noise dosimeters can be programmed for either 3 dB or 5 dB exchange rates. If your dosimeter is set to the OSHA 5 dB / 90 dBA criterion, its dose readout corresponds to OSHA. If it is set to NIOSH 3 dB / 85 dBA, it corresponds to NIOSH or Quebec (since June 2023). Always check and document your dosimeter's configuration before comparing results to a specific standard. This calculator lets you compute dose from individual level-duration pairs — useful when you have dosimetry data or sound level measurements from different tasks.

What's a 'criterion level'?

The criterion level is the sound pressure level in dBA at which the permitted exposure duration is exactly 8 hours (equivalent to a 100% dose). For NIOSH and Quebec RSST (since June 2023), the criterion level is 85 dBA. For OSHA, it is 90 dBA. The criterion level is also called the permissible exposure level (PEL) for OSHA, or the recommended exposure limit (REL) for NIOSH. Combined with the exchange rate, it defines the entire permitted duration curve. ACGIH TLVs use 85 dBA as the criterion level with a 3 dB exchange rate, matching the NIOSH and Quebec approach.

Why does Quebec match NIOSH and not OSHA after June 2023?

Quebec's update via Décret 781-2021 (in force since June 16, 2023) was driven by scientific evidence. Since NIOSH's 1998 criteria document (NIOSH 98-126), the scientific consensus has aligned around the equal-energy principle (3 dB exchange rate) as the biologically correct model for predicting noise-induced hearing loss risk. The 5 dB OSHA rate underestimates risk at high levels. The ACGIH TLV for noise has also used 85 dBA / 3 dB for decades. By adopting 85 dBA and 3 dB, Quebec aligned its regulatory standard with the scientific and preventive consensus, making it more protective of worker hearing health. OSHA has not updated its 1971 noise standard despite decades of scientific recommendations to do so.

What is the difference between noise dose and TWA?

Noise dose is expressed as a percentage of the allowable daily exposure (100% = at the limit). TWA (Time-Weighted Average) is expressed in dB and represents the constant noise level that would produce the same dose over an 8-hour shift. They convey the same information: 100% dose = 90 dB TWA under OSHA. Dose is useful for comparing across different exposure durations; TWA is easier to compare against published limits.

At what noise level is hearing protection mandatory?

Under OSHA, hearing protection must be made available when the TWA reaches 85 dB (the action level). It becomes mandatory when the TWA reaches 90 dB (the PEL) or when the dose reaches 100%. Engineering controls (enclosures, barriers, quieter equipment) are preferred over PPE and must be used first when feasible.

When to Use This Noise Dose Calculator

Use this tool for OSHA compliance monitoring in high-noise industries (construction, manufacturing, agriculture), documenting worker noise exposure over a shift with variable levels, determining whether hearing protection is required (≥85 dB TWA), and generating records for occupational health programs. It applies the OSHA 5-dB exchange rate (doubling of dose for every 5 dB increase) as required by 29 CFR 1910.95.

OSHA vs. NIOSH Noise Limits

OSHA (29 CFR 1910.95): PEL = 90 dB TWA, action level = 85 dB, exchange rate = 5 dB. NIOSH: REL = 85 dB TWA, exchange rate = 3 dB (more protective). A worker exposed to 95 dB for 8 hours has 100% OSHA dose (at the limit) but 400% NIOSH dose (four times over the limit).

Using a 3-dB exchange rate means every 3 dB doubling of sound level halves the safe exposure time — a more conservative but scientifically supported approach. The OSHA 5-dB rate was set in 1971 and has not been updated despite decades of scientific recommendations. NIOSH, ACGIH, and Quebec RSST (since June 2023) all align on the 3-dB equal-energy principle.