WHO Global Air Quality Guidelines: Particulate Matter (PM2.5 and PM10), Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide
About this book
WHO Global Air Quality Guidelines: Particulate Matter (PM2.5 and PM10), Ozone, Nitrogen Dioxide, Sulphur Dioxide and Carbon Monoxide, published by the World Health Organization in 2021, represents the first comprehensive update of the WHO's air quality guideline values since 2005. Based on a rigorous systematic review of the epidemiological literature commissioned by WHO and undertaken by an international panel of experts, the 2021 guidelines significantly tighten recommended exposure limits, reflecting the substantial body of research accumulated over the preceding 15 years demonstrating health effects at lower concentrations than previously recognised. The most significant and widely discussed update is the reduction of the annual mean PM2.5 guideline value from 10 μg/m³ (2005) to 5 μg/m³ (2021) — a halving of the recommended limit.
This revision is based on strong evidence that long-term PM2.5 exposure at concentrations well below 10 μg/m³ is associated with increased all-cause mortality, cardiovascular disease, and respiratory outcomes. The new guideline is presented as a health-based target rather than a regulatory standard, acknowledging that compliance will require decades of emission reduction effort in most urban environments globally. Annual mean NO₂ is tightened from 40 μg/m³ to 10 μg/m³, reflecting evidence of cardiovascular effects at concentrations below the earlier threshold.
This is particularly significant for urban environments in Europe, Asia, and North America, where roadside NO₂ concentrations regularly exceed the new guideline value by a factor of 3-6 in traffic-intensive corridors. The 2021 guidelines introduce interim targets — IT-1 through IT-4 — for each pollutant, providing countries with a stepwise framework for progressive improvement that acknowledges the difficulty of achieving full compliance immediately. Countries are encouraged to use the interim targets as regulatory planning benchmarks while working toward the final guideline values.
A chapter on indoor air quality briefly addresses domestic combustion sources (wood and coal burning, kerosene lamps, biomass cookstoves), which are the primary source of PM2.5 exposure for billions of people in South Asia, sub-Saharan Africa, and other regions without access to clean cooking fuels. The 2021 guidelines argue for integrating outdoor and indoor air pollution control strategies, particularly through promotion of clean cooking transitions.