--- title: "NIOSH REL Limits: CO, CO₂, H₂S, SO₂ | Aethair" description: "NIOSH Recommended Exposure Limits (RELs) limit values for Carbon Monoxide, Carbon Dioxide, Hydrogen Sulfide, Sulfur Dioxide, and more: every value and…" url: https://aethair.io/standards/niosh_rel/ --- Compliance · US NIOSH (CDC) # NIOSH Recommended Exposure Limits (RELs) Occupational: recommended guideline (USA), up to 10-hour TWA unless noted The US National Institute for Occupational Safety and Health (NIOSH), part of the CDC, publishes **Recommended Exposure Limits (RELs)** in its Pocket Guide to Chemical Hazards: time-weighted averages for up to a 10-hour workday, short-term limits (STEL, usually 15 minutes), and ceilings. They are health-based recommendations rather than enforceable law, and they are often stricter than the OSHA PELs set decades ago. Examples include **carbon monoxide at 35 ppm TWA with a 200 ppm ceiling** (OSHA: 50 ppm), **hydrogen sulfide at a 10 ppm ceiling over 10 minutes**, **nitrogen dioxide at 1 ppm STEL**, **ozone at a 0.1 ppm ceiling**, and the carcinogens formaldehyde and benzene at **0.016 ppm and 0.1 ppm TWA**. [Schedule a call](https://calendly.com/d/ctdd-mkr-v9v/aethair-demo-website) [Configure NIOSH REL Monitoring](https://aethair.io/build/?params=co,co2,h2s,so2,no,no2,o3,nh3,cl2,hg,ch2o,hcl,hf,btex) At a glance **US NIOSH (CDC)** Occupational: recommended guideline (USA), up to 10-hour TWA unless noted 14 parameters · 21 limit values [Carbon Monoxide (CO)](https://aethair.io/parameters/co/) [Carbon Dioxide (CO₂)](https://aethair.io/parameters/co2/) [Hydrogen Sulfide (H₂S)](https://aethair.io/parameters/h2s/) [Sulfur Dioxide (SO₂)](https://aethair.io/parameters/so2/) [Nitric Oxide (NO)](https://aethair.io/parameters/no/) [Nitrogen Dioxide (NO₂)](https://aethair.io/parameters/no2/) [Ozone (O₃)](https://aethair.io/parameters/o3/) [Ammonia (NH₃)](https://aethair.io/parameters/nh3/) [Chlorine (Cl₂)](https://aethair.io/parameters/cl2/) [Mercury (Hg)](https://aethair.io/parameters/hg/) [Formaldehyde (CH₂O)](https://aethair.io/parameters/ch2o/) [Hydrogen Chloride (HCl)](https://aethair.io/parameters/hcl/) [Hydrogen Fluoride (HF)](https://aethair.io/parameters/hf/) [BTEX / speciated VOC](https://aethair.io/parameters/btex/) 3 devices measure them all - **Aethair PRO** Aethair O₃+NO₂ · Aethair CO (electrochemical) · Custom MultiSense - **Aethair IAQ** - **Thiamis** RAE Gas Detectors · Defiant Technologies Specialty Air Analyzers · AMETEK Brookfield Specialty Air Analyzers [Official text from US NIOSH (CDC)](https://www.cdc.gov/niosh/npg/) The standard ## What the NIOSH Recommended Exposure Limits Are The Pocket Guide also lists IDLH values (concentrations immediately dangerous to life or health, such as 1,200 ppm for carbon monoxide), which are respirator-selection criteria, not exposure limits. Limit values ## NIOSH REL Limit Values Every value US NIOSH (CDC) publishes for the 14 parameters below, with its averaging period, as the standard states it. [**Carbon Monoxide (CO)** Gases & VOCs](https://aethair.io/parameters/co/) **35 ppm** *TWA* **200 ppm** *Ceiling* [**Carbon Dioxide (CO₂)** Gases & VOCs](https://aethair.io/parameters/co2/) **5000 ppm** *TWA* **30000 ppm** *STEL* [**Hydrogen Sulfide (H₂S)** Gases & VOCs](https://aethair.io/parameters/h2s/) **10 ppm** *Ceiling* 10-minute [**Sulfur Dioxide (SO₂)** Gases & VOCs](https://aethair.io/parameters/so2/) **2 ppm** *TWA* **5 ppm** *STEL* [**Nitric Oxide (NO)** Gases & VOCs](https://aethair.io/parameters/no/) **25 ppm** *TWA* [**Nitrogen Dioxide (NO₂)** Gases & VOCs](https://aethair.io/parameters/no2/) **1 ppm** *STEL* [**Ozone (O₃)** Gases & VOCs](https://aethair.io/parameters/o3/) **0.1 ppm** *Ceiling* [**Ammonia (NH₃)** Gases & VOCs](https://aethair.io/parameters/nh3/) **25 ppm** *TWA* **35 ppm** *STEL* [**Chlorine (Cl₂)** Gases & VOCs](https://aethair.io/parameters/cl2/) **0.5 ppm** *Ceiling* 15-minute [**Mercury (Hg)** Gases & VOCs](https://aethair.io/parameters/hg/) **0.05 mg/m³** *TWA* mercury vapor; skin [**Formaldehyde (CH₂O)** Gases & VOCs](https://aethair.io/parameters/ch2o/) **0.016 ppm** *TWA* carcinogen **0.1 ppm** *Ceiling* 15-minute [**Hydrogen Chloride (HCl)** Gases & VOCs](https://aethair.io/parameters/hcl/) **5 ppm** *Ceiling* [**Hydrogen Fluoride (HF)** Gases & VOCs](https://aethair.io/parameters/hf/) **3 ppm** *TWA* **6 ppm** *Ceiling* 15-minute [**BTEX / speciated VOC** Gases & VOCs](https://aethair.io/parameters/btex/) **0.1 ppm** *TWA* benzene; carcinogen **1 ppm** *STEL* benzene Who it applies to ## Who Uses Them Industrial hygienists and employers who want a more protective benchmark than the legal minimum, and anyone comparing OSHA's limits with current science. OSHA's annotated PEL tables list them beside the enforceable limits, and many company exposure programs and gas-detector alarm setpoints are built on them, such as the 35 ppm carbon monoxide alarm common on portable detectors. Monitoring ## How Monitoring Supports Them A REL is compared with a measured exposure over its own period: a shift TWA, a 15-minute STEL, or, for a ceiling, any moment. Continuous monitoring is what catches ceilings and short-term peaks that a shift-average sample smooths away: an ozone ceiling breached during each weld, a hydrogen sulfide spike when a hatch opens. Personal sampling in the breathing zone remains the reference for individual exposure; fixed monitors document the area. [Customize this configuration](https://aethair.io/build/?params=co,co2,h2s,so2,no,no2,o3,nh3,cl2,hg,ch2o,hcl,hf,btex) ### Who measures each parameter | Parameter | Aethair PRO | Aethair IAQ | External | | - | - | - | - | | [Carbon Monoxide (CO)](https://aethair.io/parameters/co/) | CO (electrochemical), MultiGas+ +1 cartridges | — | [3 instruments](https://aethair.io/parameters/co/) via Thiamis | | [Carbon Dioxide (CO₂)](https://aethair.io/parameters/co2/) | CO₂ (dual NDIR), CO₂ (self-calibrating NDIR) cartridges | Built in | [2 instruments](https://aethair.io/parameters/co2/) via Thiamis | | [Hydrogen Sulfide (H₂S)](https://aethair.io/parameters/h2s/) | H₂S cartridge | — | [4 instruments](https://aethair.io/parameters/h2s/) via Thiamis | | [Sulfur Dioxide (SO₂)](https://aethair.io/parameters/so2/) | MultiSense, SO₂ cartridges | — | [1 instrument](https://aethair.io/parameters/so2/) via Thiamis | | [Nitric Oxide (NO)](https://aethair.io/parameters/no/) | NO cartridge | — | [1 instrument](https://aethair.io/parameters/no/) via Thiamis | | [Nitrogen Dioxide (NO₂)](https://aethair.io/parameters/no2/) | NO₂, MultiSense +1 cartridges | — | [1 instrument](https://aethair.io/parameters/no2/) via Thiamis | | [Ozone (O₃)](https://aethair.io/parameters/o3/) | O₃+NO₂ cartridge | — | [1 instrument](https://aethair.io/parameters/o3/) via Thiamis | | [Ammonia (NH₃)](https://aethair.io/parameters/nh3/) | — | — | [1 instrument](https://aethair.io/parameters/nh3/) via Thiamis | | [Chlorine (Cl₂)](https://aethair.io/parameters/cl2/) | — | — | [1 instrument](https://aethair.io/parameters/cl2/) via Thiamis | | [Mercury (Hg)](https://aethair.io/parameters/hg/) | — | — | [1 instrument](https://aethair.io/parameters/hg/) via Thiamis | | [Formaldehyde (CH₂O)](https://aethair.io/parameters/ch2o/) | MultiSense cartridge | Built in | — | | [Hydrogen Chloride (HCl)](https://aethair.io/parameters/hcl/) | HCl cartridge | — | — | | [Hydrogen Fluoride (HF)](https://aethair.io/parameters/hf/) | HF cartridge | — | — | | [BTEX / speciated VOC](https://aethair.io/parameters/btex/) | — | — | [1 instrument](https://aethair.io/parameters/btex/) via Thiamis | Devices ## Devices That Cover NIOSH REL How much of this standard one Aethair monitor covers, followed by the third-party instruments in the sensor catalog that measure its parameters. Each connects to Environet through a Thiamis gateway, which adds whatever the monitor leaves out. Aethair monitors [**Aethair PRO** Built in + Aethair O₃+NO₂, Aethair CO (electrochemical), Custom MultiSense (CO₂, H₂S, SO₂) 6 of 14: CO, CO₂, H₂S, SO₂, NO₂, O₃](https://aethair.io/products/pro/) Covers part of it (combine on one Thiamis) [**RAE** Gas Detectors 8 of 14: Cl₂, CO, CO₂, H₂S, NH₃, NO, NO₂, SO₂](https://aethair.io/sensors/rae-gas-detectors/) [**TSI GM460** Gas Detectors 2 of 14: CO, H₂S](https://aethair.io/sensors/tsi-gas-detectors/) [**RKI Instruments GX6000** Gas Detectors 2 of 14: CO, H₂S](https://aethair.io/sensors/rki-instruments-gas-detectors/) [**AMETEK Brookfield Jerome** Specialty Air Analyzers 2 of 14: H₂S, Mercury](https://aethair.io/sensors/ametek-brookfield-specialty-air-analyzers/) [**Defiant Technologies FROG** Specialty Air Analyzers 1 of 14: BTEX / speciated VOC](https://aethair.io/sensors/defiant-technologies-specialty-air-analyzers/) [**Particles Plus** Particulate & Dust Monitors 1 of 14: CO₂](https://aethair.io/sensors/particles-plus-particulate-dust-monitors/) [**2B Technologies POM** Specialty Air Analyzers 1 of 14: O₃](https://aethair.io/sensors/2b-technologies-specialty-air-analyzers/) Solutions ## Solutions Judged Against NIOSH REL ### [Occupational Safety & Industrial Hygiene](https://aethair.io/solutions/occupational-safety/) Worker exposure, confined space, and heat stress [All solutions](https://aethair.io/solutions/) Article Library ## Articles on NIOSH REL ### [Ozone Exposure Limits and Monitoring: OSHA and NAAQS](https://aethair.io/resources/ozone-monitoring-exposure-limits/) OSHA's ozone PEL is 0.1 ppm as an 8-hour TWA. How NIOSH, ACGIH, and the 70 ppb NAAQS compare, and why ozone needs short-interval monitoring. ### [How to Choose an Air Quality Monitor: PRO, IAQ, or Thiamis](https://aethair.io/resources/choosing-aethair-monitoring-device/) Compare Aethair PRO, IAQ, and Thiamis to match the right device to your site, from commercial interiors to industrial and fenceline monitoring. ### [OSHA Air Quality Standards 2026: PELs & 29 CFR 1910.1000](https://aethair.io/resources/osha-air-quality-standards-compliance/) OSHA air quality standards for indoor and outdoor work: permissible exposure limits, Table Z-1, 29 CFR 1910.1000, and the heat and wildfire smoke triggers. [All articles](https://aethair.io/resources/) Questions ## NIOSH REL FAQ What is the NIOSH REL for carbon monoxide? 35 ppm as a time-weighted average, with a ceiling of 200 ppm, compared with OSHA's permissible exposure limit of 50 ppm. Are NIOSH RELs enforceable? No. They are health-based recommendations. OSHA's permissible exposure limits are the enforceable federal limits; the RELs are often stricter and are widely used as internal limits and alarm setpoints. What is the difference between a REL and an IDLH value? A REL is an exposure limit for workers over a stated period. An IDLH value is the concentration immediately dangerous to life or health, used to select respirators, not as an exposure limit.