--- title: "TWA / Noise Dose Sensors, Monitors & Limit Values | Aethair" description: "Noise TWA and dose express a worker's exposure over an 8-hour day. OSHA, NIOSH, and MSHA limits, exchange rates, and how dosimeters measure it." url: https://aethair.io/parameters/sound_twa/ --- Measurements · Noise # TWA / Noise Dose Also known as noise dose, noise dosimeter reading Time-weighted average / noise dose over the work shift. [Schedule a call](https://calendly.com/d/ctdd-mkr-v9v/aethair-demo-website) [Configure TWA / Noise Dose Monitoring](https://aethair.io/build/?params=sound_twa) At a glance dB Noise 1 way to measure it - **Thiamis** 1 external instrument 2 standards set limits - [OSHA OSHA / NIOSH noise](https://aethair.io/standards/occupational_noise/) - [MSHA MSHA noise](https://aethair.io/standards/msha_noise/) What it is ## What is TWA and noise dose? Time-weighted average (TWA) is a worker's noise exposure expressed as a single level normalized to a standard eight-hour working day. It takes the varying noise received across the shift and states the steady level that would give the same exposure over eight hours. **Noise dose** expresses the same exposure as a percentage of the permitted daily amount, where 100 percent equals the limit, so a dose of 50 percent means half the allowable exposure has been used. Both are computed from the average level using a defined **exchange rate** and a **criterion level** (the level that, sustained for eight hours, equals 100 percent dose). Under US rules the exchange rate is 5 dB and the average is Lavg; on the equal-energy basis used elsewhere it is 3 dB and the average is Leq. TWA and dose are two views of one exposure (a level and a percentage), and both determine compliance in hearing conservation. ### Health and operational effects Occupational noise-induced hearing loss is among the most common and most preventable workplace injuries, and TWA and dose are the metrics that decide what must be done about it. When exposure reaches the **action level**, an employer is typically required to provide hearing protection, information, and audiometric testing. At the higher **exposure limit**, exposure must be reduced by engineering or administrative controls regardless of the protection worn. Because these duties are triggered by the daily exposure, TWA and dose sit at the center of every hearing-conservation program. For employers, this is a legal obligation with real liability attached: the dose is what an inspector checks and what a hearing-loss claim turns on, so measuring it accurately is both a duty and a defense. Limits ## TWA / Noise Dose Limit Values The published values, by averaging period; each links to the standard that sets it. 8-hour TWA (OSHA PEL) [**90 dBA** OSHA / NIOSH noise](https://aethair.io/standards/occupational_noise/ "5 dB exchange rate") 8-hour TWA (OSHA action level) [**85 dBA** OSHA / NIOSH noise](https://aethair.io/standards/occupational_noise/ "5 dB exchange rate") 8-hour TWA (NIOSH REL) [**85 dBA** OSHA / NIOSH noise](https://aethair.io/standards/occupational_noise/ "3 dB exchange rate") 8-hour TWA (PEL) [**90 dBA** MSHA noise](https://aethair.io/standards/msha_noise/ "5 dB exchange rate; integrates 90 to at least 140 dBA") 8-hour TWA (action level) [**85 dBA** MSHA noise](https://aethair.io/standards/msha_noise/ "hearing conservation program; integrates 80 to at least 130 dBA") - **OSHA / NIOSH noise** 90 dBA: 5 dB exchange rate - **OSHA / NIOSH noise** 85 dBA: 5 dB exchange rate - **OSHA / NIOSH noise** 85 dBA: 3 dB exchange rate - **MSHA noise** 90 dBA: 5 dB exchange rate; integrates 90 to at least 140 dBA - **MSHA noise** 85 dBA: hearing conservation program; integrates 80 to at least 130 dBA TWA is reported in decibels, normally dB(A), and dose as a percentage, both stated with their exchange rate and criterion level. Interpret 100 percent dose as the daily limit reached, with the action level at a defined lower percentage. The applicable action and limit values appear in the table on this page. Because a short spell of very loud work can dominate a whole shift's dose, the value shows which tasks drive exposure and where controls or job rotation would help most. How it is measured ## How TWA / Noise Dose Is Measured TWA and dose are measured with a personal **noise dosimeter** (IEC 61252; ANSI S1.25 in the US) or a sound level meter with dosimetry built to **[IEC 61672](https://aethair.io/standards/iec_61672/)**, which integrates the sound level continuously through the shift, applying the configured exchange rate and criterion level to produce the TWA and the accumulating dose. Because this is a personal, configurable, occupational measurement rather than a simple ambient reading, it uses an external dosimeter logged on a Thiamis gateway, which can also report the anchor metric Leq for context. Getting the exchange rate, criterion level, and measurement duration right is essential, as each directly changes the result. An instrument on Thiamis 1 A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account. ### Placing noise dosimeters A dosimeter is worn by the worker, with the microphone mounted on the shoulder within a few centimeters of the ear, so that it samples the noise the person actually receives as they move between tasks and locations. It should be worn for a full and representative shift, since a dose measured over a partial or atypical period does not represent the real daily exposure. The worker must be briefed not to shout into or shield the microphone, as tampering distorts the reading. ### Every device and option that measures TWA / Noise Dose 1 | Device / option | Type | Base platform | Measures | | - | - | - | - | | [TSI](https://aethair.io/sensors/tsi-noise-monitors/) · SP-SE-1, SP-SE-1-1/1, SP-SE-1-1/3… | Noise Monitors | Thiamis | TWA / Noise Dose [Average Level (Lavg)](https://aethair.io/parameters/sound_lavg/) [Equivalent Continuous Level (Leq)](https://aethair.io/parameters/sound_leq/) [Max Level (Lmax)](https://aethair.io/parameters/sound_lmax/) [Min Level (Lmin)](https://aethair.io/parameters/sound_lmin/) [Peak Level (Lpeak)](https://aethair.io/parameters/sound_lpeak/) [Sound Pressure Level (SPL)](https://aethair.io/parameters/sound_spl/) | [Configure TWA / Noise Dose Monitoring](https://aethair.io/build/?params=sound_twa) Where it is monitored ## Solutions That Monitor TWA / Noise Dose Each one links to the solution page showing what that industry measures it for. ### [Cabin Air Quality](https://aethair.io/solutions/cabin-air/) Often added Respirable dust, gases, and blast monitoring ### [Noise Monitoring](https://aethair.io/solutions/noise/) Often added Environmental and occupational sound levels ### [Occupational Safety & Industrial Hygiene](https://aethair.io/solutions/occupational-safety/) Often added Worker exposure, confined space, and heat stress Related ## More in Noise [Sound Pressure Level (SPL)](https://aethair.io/parameters/sound_spl/) [Equivalent Continuous Level (Leq)](https://aethair.io/parameters/sound_leq/) [Peak Level (Lpeak)](https://aethair.io/parameters/sound_lpeak/) [Statistical Level (Ln)](https://aethair.io/parameters/sound_ln/) [Max Level (Lmax)](https://aethair.io/parameters/sound_lmax/) [Min Level (Lmin)](https://aethair.io/parameters/sound_lmin/) [Average Level (Lavg)](https://aethair.io/parameters/sound_lavg/) [Sound Exposure Level (SEL)](https://aethair.io/parameters/sound_sel/) Questions ## TWA / Noise Dose FAQ What is the difference between TWA and dose? They are two expressions of the same exposure: TWA states it as an eight-hour-equivalent level in decibels, while dose states it as a percentage of the permitted daily amount, with 100 percent being the limit. One is a level, the other a proportion, and instruments usually report both. What are the criterion level and action level? The criterion level is the sustained eight-hour level that equals 100 percent dose: the daily limit expressed as a level. The action level is a lower exposure at which duties such as providing hearing protection and audiometric testing begin. The exact figures depend on the regulation and appear in the limit table on this page. How does TWA relate to Leq? TWA is Leq (or Lavg under a 5 dB exchange rate) normalized to a standard eight-hour day. On the equal-energy 3 dB basis, an eight-hour Leq and the TWA are effectively the same quantity; the difference arises when a 5 dB exchange rate is used for US compliance. How do I monitor noise dose? Noise dose needs an external IEC 61672 sound level meter with dosimetry on a Thiamis gateway, worn for a full shift and configured with the correct exchange rate and criterion level. Use the configurator to combine noise exposure with dust and the other parameters your workplace assessment requires.