The WHO Global Air Quality Guidelines (2021) revised the annual mean PM2.5 standard to:

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

Explanation

WHO's 2021 Global Air Quality Guidelines tightened the annual mean PM2.5 guideline from 10 µg/m³ (2005 guideline) to 5 µg/m³, reflecting accumulating evidence of health effects at lower PM2.5 concentrations. Interim targets (IT-1 to IT-4) of 35, 25, 15, and 10 µg/m³ provide stepping-stones for countries with high baseline pollution. India's National Ambient Air Quality Standard for PM2.5 remains 40 µg/m³ (annual), significantly above the WHO guideline.

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

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