Research
WHO Global Air Quality Guidelines 2021
By Jason Curtis · 4 min read · Updated 2026-05-21
On September 22, 2021, the World Health Organization released its first update to the Global Air Quality Guidelines (AQGs) in 16 years. The headline change: the annual PM2.5 guideline was cut in half, from 10 to 5 micrograms per cubic meter. The WHO estimated that meeting the new level worldwide could prevent millions of premature deaths each year.
Background
WHO has issued air quality guidelines since 1987, with major revisions in 2000 and 2005. The guidelines are non-binding. They synthesize the best available epidemiological evidence and recommend ambient air concentrations below which health risk is judged to be acceptable. Countries decide whether and how to adopt them as legal standards.
The 2021 update was led by the WHO European Centre for Environment and Health and reviewed by an international Guideline Development Group. It drew on roughly 500 new studies published since 2005, with systematic reviews commissioned for PM2.5, PM10, ozone, NO2, sulfur dioxide, and carbon monoxide.
What changed
The 2021 numbers are stricter across the board.
- PM2.5 annual mean: 5 micrograms per cubic meter (down from 10 in 2005).
- PM2.5 24-hour mean: 15 micrograms per cubic meter (down from 25).
- PM10 annual mean: 15 micrograms per cubic meter (down from 20).
- PM10 24-hour mean: 45 micrograms per cubic meter (down from 50).
- NO2 annual mean: 10 micrograms per cubic meter (down from 40).
- NO2 24-hour mean: 25 micrograms per cubic meter (new).
- Ozone 8-hour mean: 100 micrograms per cubic meter (unchanged).
- Ozone peak-season mean: 60 micrograms per cubic meter (new).
- SO2 24-hour mean: 40 micrograms per cubic meter (down from 20 to 500 depending on metric).
- CO 24-hour mean: 4 milligrams per cubic meter (new).
WHO also kept four interim targets for PM2.5 (35, 25, 15, 10 micrograms per cubic meter) so countries with very polluted air can track progress in stages.
Why the cuts
Studies since 2005 kept finding health effects at concentrations well below the old guidelines. Cohort studies in North America and Europe, including extended follow-ups of the Harvard Six Cities and ACS cohorts, the ELAPSE consortium across Europe, and Medicare-based work by Joel Schwartz and Francesca Dominici, showed mortality risk rising at PM2.5 levels in the single digits. The Global Burden of Disease project linked ambient PM2.5 to roughly 4 million deaths a year globally, with a large share occurring in countries that already met the old 10 microgram target.
How it compares to U.S. and EU standards
The U.S. EPA tightened the annual PM2.5 NAAQS from 12 to 9 micrograms per cubic meter in February 2024, still nearly double the WHO guideline. The European Union adopted a 2030 target of 10 micrograms per cubic meter for PM2.5 in 2024, also roughly twice the WHO number. As of 2024, only a handful of countries (including Australia, New Zealand, and several Nordic nations) have legal standards close to the WHO guideline.
Why it matters
The guidelines reset what counts as "clean" air. A city with an annual PM2.5 of 8 micrograms per cubic meter would have looked compliant under the 2005 guidelines; under the 2021 guidelines it is 60 percent over the limit. State of Global Air analyses suggest that more than 99 percent of the world's population breathes air exceeding the 2021 PM2.5 guideline.
Open questions
WHO did not set a guideline for ultrafine particles, black carbon, or specific PM2.5 chemical components, citing thinner evidence. Future updates will likely address these. The 2021 document is also explicit that no safe threshold has been demonstrated for PM2.5: health effects appear to continue all the way down to background levels.
Sources
- WHO global air quality guidelines (full document, 2021)
- Updated WHO Air Quality Guidelines 2021 (PMC review)
- WHO Air Quality Guidelines 2021: Joint Statement (PMC)
- California ARB summary of WHO 2021 AQGs (presentation)
- EPA final rule: Reconsideration of the National Ambient Air Quality Standards for Particulate Matter (2024)
- State of Global Air 2024 (Health Effects Institute and IHME)
This article is for educational purposes only. Canairy does not provide medical advice, diagnosis, or treatment. Talk to a qualified health professional about your specific situation.