The WHO Air Quality Guidelines (2021) set the annual mean PM2.5 standard at:

• A 10 μg/m³
• B 15 μg/m³
• C 25 μg/m³
• D 5 μg/m³ ✓

Explanation

The WHO Global Air Quality Guidelines updated in September 2021 revised the annual mean PM2.5 standard downward from 10 μg/m³ (2005 guideline) to 5 μg/m³, reflecting new evidence linking lower PM2.5 concentrations to cardiovascular and respiratory morbidity. The 24-hour mean guideline is 15 μg/m³. India's National Ambient Air Quality Standards remain at 40 μg/m³ (annual mean) for PM2.5, reflecting feasibility constraints — this is 8 times the WHO 2021 guideline.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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