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Insights, technical note, 5 min read, 2026-10-17

What changed in the 2021 WHO air quality guidelines

Of the seven guideline values that existed in both editions, WHO tightened five, left ozone unchanged and raised SO₂. A that meets national standards can still sit several times above the 2021 values.

Title
What changed in the 2021 WHO air quality guidelines
Doc no.
TN-604
Rev
P1
Discipline
AQ EA
Date
2026-10-17
Status
Published
What this means for you

If your site meets Indian air quality limits, it can still be well above WHO's health guidelines, and international lenders often ask about both.

Of seven values that existed in both editions, WHO tightened five in 2021, left ozone unchanged and raised SO₂.

Recommended guideline levels, µg/m³. Interim targets omitted.

WHO air quality guideline values, 2005 and 2021Dumbbell chart. 2005 to 2021 in micrograms per cubic metre: PM2.5 annual 10 to 5, 24-hour 25 to 15; PM10 annual 20 to 15, 24-hour 50 to 45; NO2 annual 40 to 10, 24-hour new at 25; SO2 24-hour raised from 20 to 40; O3 8-hour unchanged at 100.020406080100µg/m³PM₂.₅, annual105PM₂.₅, 24-hour2515PM₁₀, annual2015PM₁₀, 24-hour5045NO₂, annual4010NO₂, 24-hour25 new in 2021SO₂, 24-hour2040raisedO₃, 8-hour100 unchanged20052021
Data table
PollutantAveraging20052021
PM₂.₅Annual105
PM₂.₅24-hour2515
PM₁₀Annual2015
PM₁₀24-hour5045
NO₂Annual4010
NO₂24-hournone25
SO₂24-hour2040
O₃8-hour100100
WHO guideline values, 2005 and 2021.Source: WHO (2021), WHO global air quality guidelines, Table 3.26.

The 2021 guidelines set annual PM₂.₅ at 5 µg/m³, half the 2005 value, and annual NO₂ at 10 µg/m³, a quarter of it. They add a 24-hour NO₂ value of 25 µg/m³, a peak-season ozone value of 60 µg/m³ and a 24-hour CO value of 4 mg/m³. The 24-hour SO₂ value rose from 20 to 40 µg/m³. Short-term values are 99th percentiles, equivalent to three or four exceedance days a year.1

Compared with India's NAAQS

Nominal values in force as of October 2026, µg/m³. India uses 98th-percentile compliance for short-term values; WHO uses the 99th.
PollutantAveragingIndia NAAQS 2009WHO 2021Ratio
PM₂.₅Annual4058×
PM₂.₅24-hour60154×
PM₁₀Annual60154×
PM₁₀24-hour100452.2×
NO₂Annual40104×
NO₂24-hour80253.2×
SO₂24-hour80402×

India's 2009 standards set annual PM₂.₅ at 40 µg/m³, eight times the WHO 2021 guideline.2 A baseline can comply with the national standard and still sit far above the WHO value. Lenders' advisers working to IFC Performance Standard 3 often ask how far existing air quality sits from the WHO guidelines and interim targets, so we report the national standard and the WHO values side by side.

Note

Screen against both. State which one is the legal limit and which is the health-based benchmark, and keep the two from being read as the same thing.

Notes and references

  1. World Health Organization (2021). WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. Table 3.26 and notes. https://www.ncbi.nlm.nih.gov/books/NBK574591/table/ch3.tab26/ Back to text
  2. Central Pollution Control Board (2009). National Ambient Air Quality Standards, notification B-29016/20/90/PCI-L, 18 November 2009. https://cpcb.nic.in/uploads/National_Ambient_Air_Quality_Standards.pdf Back to text

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