The WHO Air Quality Guidelines (AQG) 2021 set a 24-hour mean concentration limit for PM2.5 at:
- A 25 µg/m³ (unchanged from 2005 AQG)
- B 15 µg/m³ (revised downward from 25 µg/m³) ✓
- C 10 µg/m³
- D 35 µg/m³ (Indian NAAQS 2019 standard)
Correct answer: B. 15 µg/m³ (revised downward from 25 µg/m³)
Explanation
The WHO Air Quality Guidelines 2021 (updated from 2005) revised the 24-hour mean PM2.5 guideline downward from 25 µg/m³ to 15 µg/m³, reflecting new evidence that fine particulate matter causes cardiovascular and respiratory harm at lower concentrations than previously thought. The annual PM2.5 guideline was revised from 10 µg/m³ to 5 µg/m³. The Indian National Ambient Air Quality Standards (NAAQS 2009, not 2019) allow 60 µg/m³ 24-hour mean PM2.5 — considerably more permissive than WHO AQG.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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