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.
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.
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.
Recommended
best match-
Industrial monitor for indoor and outdoor sites. Built-in sensors cover the essentials, and hot-swappable gas sensor cartridges and a differential-pressure option add more.
Gas 1
Aethair O₃+NO₂
Nitrogen Dioxide (NO₂)Ozone (O₃)Four-electrode electrochemical sensing measures oxidizing gases (ozone plus nitrogen dioxide) and reports one…
Gas 2
Aethair CO (electrochemical)
Carbon Monoxide (CO)An amperometric electrochemical CO cell with a solid, non-aqueous electrolyte (the long-life, low-power…
Also fits: Aethair MultiGas+, Aethair CO (ppb, four-electrode)
MultiSense
Custom MultiSense built to order
Carbon Dioxide (CO₂)Hydrogen Sulfide (H₂S)Sulfur Dioxide (SO₂)Built to order for the MultiSense socket with the Aethair CO₂ (dual NDIR) cartridge's sensor (0-5000 ppm, 1 ppm resolution), the Aethair H₂S cartridge's sensor (0-50 ppm, 5 ppb resolution), and the Aethair SO₂ cartridge's sensor (0-100 ppm, 5 ppb resolution).
Expansion
Free: differential pressure
Also measures 13 more
Dew PointHeat IndexRelative HumidityIlluminance / LightParticle Count (by size)PM1PM10PM2.5Total PM / Mass Conc.Barometric PressureEquivalent Continuous Level (Leq)Sound Pressure Level (SPL)Air Temperature -
Aethair IAQ built-in sensors
Compact plug-in monitor with every sensor built in, nothing to add. Ideal for straightforward indoor air quality monitoring.
Built-inFormaldehyde (CH₂O)Also measures 11 more
Dew PointHeat IndexRelative HumidityParticle Count (by size)PM1PM10PM2.5Total PM / Mass Conc.Barometric PressureAir TemperatureVOC (Total) -
Thiamis connects 3 instruments
A gateway that connects the third-party instruments you choose to the Aethair Platform.
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Chlorine (Cl₂)Ammonia (NH₃)Nitric Oxide (NO)RAE Systems (Honeywell) portable gas detectors.
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Defiant Technologies Specialty Air Analyzers
BTEX / speciated VOCDefiant Technologies FROG portable gas chromatograph (GC-PID).
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AMETEK Brookfield Specialty Air Analyzers
Mercury (Hg)Jerome portable single-gas analyzers by AMETEK Brookfield (formerly AMETEK Arizona Instrument).
Also measures 4 more
Methane (CH₄)Combustible (LEL)Oxygen (O₂)VOC (Total) -
Not coveredHydrogen Chloride (HCl)Hydrogen Fluoride (HF)no supported configuration measures these yet
Who measures each parameter
| Parameter | Aethair PRO | Aethair IAQ | External |
|---|---|---|---|
| Carbon Monoxide (CO) | CO (electrochemical), MultiGas+ +1 cartridges | — | 3 instruments via Thiamis |
| Carbon Dioxide (CO₂) | CO₂ (dual NDIR), CO₂ (self-calibrating NDIR) cartridges | Built in | 2 instruments via Thiamis |
| Hydrogen Sulfide (H₂S) | H₂S cartridge | — | 4 instruments via Thiamis |
| Sulfur Dioxide (SO₂) | MultiSense, SO₂ cartridges | — | 1 instrument via Thiamis |
| Nitric Oxide (NO) | NO cartridge | — | 1 instrument via Thiamis |
| Nitrogen Dioxide (NO₂) | NO₂, MultiSense +1 cartridges | — | 1 instrument via Thiamis |
| Ozone (O₃) | O₃+NO₂ cartridge | — | 1 instrument via Thiamis |
| Ammonia (NH₃) | — | — | 1 instrument via Thiamis |
| Chlorine (Cl₂) | — | — | 1 instrument via Thiamis |
| Mercury (Hg) | — | — | 1 instrument via Thiamis |
| Formaldehyde (CH₂O) | MultiSense cartridge | Built in | — |
| Hydrogen Chloride (HCl) | HCl cartridge | — | — |
| Hydrogen Fluoride (HF) | HF cartridge | — | — |
| BTEX / speciated VOC | — | — | 1 instrument 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
Covers part of it (combine on one Thiamis)
RAE Gas Detectors8 of 14: Cl₂, CO, CO₂, H₂S, NH₃, NO, NO₂, SO₂
TSI GM460 Gas Detectors2 of 14: CO, H₂S
RKI Instruments GX6000 Gas Detectors2 of 14: CO, H₂SAMETEK Brookfield Jerome Specialty Air Analyzers2 of 14: H₂S, MercuryDefiant Technologies FROG Specialty Air Analyzers1 of 14: BTEX / speciated VOC
Particles Plus Particulate & Dust Monitors1 of 14: CO₂
2B Technologies POM Specialty Air Analyzers1 of 14: O₃Solutions
Solutions Judged Against NIOSH REL
Occupational Safety & Industrial Hygiene
Worker exposure, confined space, and heat stress
Article Library
Articles on NIOSH REL
Ozone Exposure Limits and Monitoring: OSHA and NAAQS
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
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
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.
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.
Monitor Against NIOSH REL
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