Compliance · World Health Organization
WHO Global Air Quality Guidelines 2021
Ambient air: health guideline (global)
The World Health Organization's Global Air Quality Guidelines, updated in 2021, are the world's most authoritative health-based reference values for air pollution. Unlike national regulatory limits, which balance health against economic and technical feasibility, the WHO guidelines answer a narrower question: at what concentrations does the scientific evidence show air pollution harms human health? They cover the key pollutants (PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide), and the 2021 update tightened the values substantially, most notably halving the annual PM2.5 guideline to 5 µg/m³ in light of evidence of harm at ever-lower concentrations.
The standard
What the WHO 2021 Guidelines Are
They are guidelines, not law, but they set the direction of travel for regulation worldwide and are the benchmark against which national standards are judged.
Limit values
WHO 2021 Limit Values
Every value World Health Organization publishes for the 6 parameters below, with its averaging period, as the standard states it.
Who it applies to
Who Uses Them
Governments use the WHO guidelines as the scientific foundation for setting and revising their own legal limits. The EU's 2024 revision explicitly moved its limit values closer to them. Beyond regulators, healthy-building programs, multinational employers seeking a consistent global standard, NGOs, researchers, and cities benchmarking their progress all measure against the WHO values precisely because they are health-based rather than politically negotiated. For organizations that want to demonstrate best practice rather than mere legal compliance, the WHO guideline is the aspirational target.
Monitoring
How Monitoring Relates to the Guidelines
Because the WHO values cover the same pollutants as most ambient standards, a monitoring station built for regulatory purposes also reports against the WHO guidelines directly. The difference is the threshold, not the measurement. The guidelines emphasize annual and 24-hour averages for particulate matter, so continuous, long-term monitoring rather than spot checks is what allows a meaningful comparison. As with any ambient deployment, covering the full pollutant set requires combining particulate, ozone, and multi-gas instruments with meteorology.
The WHO guidelines set no separate measurement method; the same reference and calibrated lower-cost instruments used for regulatory monitoring apply.
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.
Built-inPM10PM2.5Gas 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
Aethair MultiSense
Sulfur Dioxide (SO₂)Simultaneous electrochemical detection of sulfur dioxide, nitrogen dioxide, and formaldehyde at…
Expansion
Free: differential pressure
Also measures 12 more
Dew PointHeat IndexRelative HumidityIlluminance / LightParticle Count (by size)PM1Total PM / Mass Conc.Barometric PressureEquivalent Continuous Level (Leq)Sound Pressure Level (SPL)Air TemperatureFormaldehyde (CH₂O)
Who measures each parameter
| Parameter | Aethair PRO |  Aethair IAQ |  External |
|---|---|---|---|
| PM2.5 | Built in | Built in | 3 instruments via Thiamis |
| PM10 | Built in | Built in | 3 instruments via Thiamis |
| Ozone (O₃) | O₃+NO₂ cartridge | — | 1 instrument via Thiamis |
| Nitrogen Dioxide (NO₂) | NO₂, MultiSense +1 cartridges | — | 1 instrument via Thiamis |
| Sulfur Dioxide (SO₂) | MultiSense, SO₂ cartridges | — | 1 instrument via Thiamis |
| Carbon Monoxide (CO) | CO (electrochemical), MultiGas+ +1 cartridges | — | 3 instruments via Thiamis |
Devices
Devices That Cover WHO 2021
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 Detectors3 of 6: CO, NO₂, SO₂
TSI DustTrak Particulate & Dust Monitors2 of 6: PM10, PM2.5
Met One E-BAM Particulate & Dust Monitors2 of 6: PM10, PM2.5
Trolex Particulate & Dust Monitors2 of 6: PM10, PM2.5
TSI GM460 Gas Detectors1 of 6: CO
RKI Instruments GX6000 Gas Detectors1 of 6: CO
2B Technologies POM Specialty Air Analyzers1 of 6: O₃Solutions
Solutions Judged Against WHO 2021
Indoor Air Quality
Offices, schools, homes, and public buildings
Outdoor Monitoring
Continuous outdoor air quality networks that sit alongside reference monitors
Wildfire Air Monitoring
Smoke plumes, black carbon, and community exposure
Article Library
Articles on WHO 2021
Wildfire Smoke Air Quality Monitoring: Detection & Response
Wildfire smoke monitoring for EHS and public health teams: real-time PM2.5, the AQI 151 respirator trigger, and how to alert workers before smoke arrives.
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.
Questions
WHO 2021 FAQ
How are the WHO guidelines different from legal limits like the NAAQS or EU directive?
The WHO guidelines are purely health-based (they state where evidence shows harm begins), while legal limits such as the US NAAQS and EU directive balance health against feasibility and are generally set higher. The WHO values are the scientific benchmark that regulations aim toward; meeting them represents best practice beyond legal compliance.
Why were the 2021 guidelines tightened so much?
Accumulating evidence showed health harm continuing at concentrations previously thought safe, with no clear threshold for PM2.5. The WHO responded by lowering several values sharply (the annual PM2.5 guideline was halved to 5 µg/m³) to reflect the science rather than what is currently achievable.
Are the WHO guidelines legally binding?
No. They are recommendations, not law. Their influence is indirect but powerful: national and regional regulators use them as the evidence base when setting binding limits, and organizations adopt them voluntarily as a health-based target.
How do I monitor against the WHO guidelines?
Deploy the same kind of ambient station used for regulatory monitoring (particulate, ozone, and multi-gas instruments with weather) and compare the long-term averages against the WHO values. Use the configurator to assemble a configuration covering the pollutants relevant to your site.
Monitor Against WHO 2021
Schedule a callBuild a Configuration All standards
A 30-minute call: you tell us about your application, then we demo the solution for it.