--- title: "WHO 2021 Limits: PM2.5, PM10, O₃, NO₂ | Aethair" description: "WHO Global Air Quality Guidelines 2021 limit values for PM2.5, PM10, Ozone, Nitrogen Dioxide, and more: every value and averaging period, who the standard…" url: https://aethair.io/standards/who_aqg_2021/ --- 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. [Schedule a call](https://calendly.com/d/ctdd-mkr-v9v/aethair-demo-website) [Configure WHO 2021 Monitoring](https://aethair.io/build/?params=pm2_5,pm10,o3,no2,so2,co\&setting=outdoor) At a glance **World Health Organization** Ambient air: health guideline (global) 6 parameters · 10 limit values [PM2.5](https://aethair.io/parameters/pm2_5/) [PM10](https://aethair.io/parameters/pm10/) [Ozone (O₃)](https://aethair.io/parameters/o3/) [Nitrogen Dioxide (NO₂)](https://aethair.io/parameters/no2/) [Sulfur Dioxide (SO₂)](https://aethair.io/parameters/so2/) [Carbon Monoxide (CO)](https://aethair.io/parameters/co/) 1 device measures them all - **Aethair PRO** Aethair O₃+NO₂ · Aethair CO (electrochemical) · Aethair MultiSense [Official text from World Health Organization](https://www.who.int/publications/i/item/9789240034228) 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. [**PM2.5** Particulate Matter](https://aethair.io/parameters/pm2_5/) **5 µg/m³** *Annual* **15 µg/m³** *24-hour* [**PM10** Particulate Matter](https://aethair.io/parameters/pm10/) **15 µg/m³** *Annual* **45 µg/m³** *24-hour* [**Ozone (O₃)** Gases & VOCs](https://aethair.io/parameters/o3/) **100 µg/m³** *8-hour daily maximum* 99th percentile (3–4 exceedance days a year) **60 µg/m³** *8-hour peak-season* mean of the daily maximum 8-hour values over the six consecutive months with the highest ozone [**Nitrogen Dioxide (NO₂)** Gases & VOCs](https://aethair.io/parameters/no2/) **10 µg/m³** *Annual* **25 µg/m³** *24-hour* [**Sulfur Dioxide (SO₂)** Gases & VOCs](https://aethair.io/parameters/so2/) **40 µg/m³** *24-hour* [**Carbon Monoxide (CO)** Gases & VOCs](https://aethair.io/parameters/co/) **4 mg/m³** *24-hour* 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. [Customize this configuration](https://aethair.io/build/?params=pm2_5,pm10,o3,no2,so2,co\&setting=outdoor) ### Who measures each parameter | Parameter | Aethair PRO | Aethair IAQ | External | | - | - | - | - | | [PM2.5](https://aethair.io/parameters/pm2_5/) | Built in | Built in | [3 instruments](https://aethair.io/parameters/pm2_5/) via Thiamis | | [PM10](https://aethair.io/parameters/pm10/) | Built in | Built in | [3 instruments](https://aethair.io/parameters/pm10/) via Thiamis | | [Ozone (O₃)](https://aethair.io/parameters/o3/) | O₃+NO₂ cartridge | — | [1 instrument](https://aethair.io/parameters/o3/) via Thiamis | | [Nitrogen Dioxide (NO₂)](https://aethair.io/parameters/no2/) | NO₂, MultiSense +1 cartridges | — | [1 instrument](https://aethair.io/parameters/no2/) via Thiamis | | [Sulfur Dioxide (SO₂)](https://aethair.io/parameters/so2/) | MultiSense, SO₂ cartridges | — | [1 instrument](https://aethair.io/parameters/so2/) via Thiamis | | [Carbon Monoxide (CO)](https://aethair.io/parameters/co/) | CO (electrochemical), MultiGas+ +1 cartridges | — | [3 instruments](https://aethair.io/parameters/co/) 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 [**Aethair PRO** Built in + Aethair O₃+NO₂, Aethair CO (electrochemical), Aethair MultiSense Covers all 6](https://aethair.io/products/pro/) Covers part of it (combine on one Thiamis) [**RAE** Gas Detectors 3 of 6: CO, NO₂, SO₂](https://aethair.io/sensors/rae-gas-detectors/) [**TSI DustTrak** Particulate & Dust Monitors 2 of 6: PM10, PM2.5](https://aethair.io/sensors/tsi-particulate-dust-monitors/) [**Met One E-BAM** Particulate & Dust Monitors 2 of 6: PM10, PM2.5](https://aethair.io/sensors/met-one-particulate-dust-monitors/) [**Trolex** Particulate & Dust Monitors 2 of 6: PM10, PM2.5](https://aethair.io/sensors/trolex-particulate-dust-monitors/) [**TSI GM460** Gas Detectors 1 of 6: CO](https://aethair.io/sensors/tsi-gas-detectors/) [**RKI Instruments GX6000** Gas Detectors 1 of 6: CO](https://aethair.io/sensors/rki-instruments-gas-detectors/) [**2B Technologies POM** Specialty Air Analyzers 1 of 6: O₃](https://aethair.io/sensors/2b-technologies-specialty-air-analyzers/) Solutions ## Solutions Judged Against WHO 2021 ### [Indoor Air Quality](https://aethair.io/solutions/iaq-monitoring/) Offices, schools, homes, and public buildings ### [Outdoor Monitoring](https://aethair.io/solutions/outdoor-monitoring/) Continuous outdoor air quality networks that sit alongside reference monitors ### [Wildfire Air Monitoring](https://aethair.io/solutions/wildfires/) Smoke plumes, black carbon, and community exposure [All solutions](https://aethair.io/solutions/) Article Library ## Articles on WHO 2021 ### [Wildfire Smoke Air Quality Monitoring: Detection & Response](https://aethair.io/resources/wildfire-smoke-air-quality-monitoring/) 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](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. [All articles](https://aethair.io/resources/) 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](https://aethair.io/standards/epa_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.