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📌 The Question:

In the context of WHO Air Quality Guidelines, consider the following statements:

1. The 24-hour mean of PM2.5 should not exceed 15 μg/m³ and annual mean of PM2.5 should not exceed 5 μg/m³.
2. In a year, the highest levels of ozone pollution occur during the periods of inclement weather.
3. PM10 can penetrate the lung barrier and enter the bloodstream.
4. Excessive ozone in the air can trigger asthma.

Which of the statements given above are correct?

(a) 1, 3 and 4
(b) 1 and 4 only
(c) 2, 3 and 4
(d) 1 and 2 only

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✅ Correct Answer: (b) 1 and 4 only

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🧠 Classroom Explanation:

• Statement 1 – Correct
WHO (2021) Air Quality Guidelines prescribe:
– Annual mean PM2.5 ≤ 5 μg/m³
– 24-hour mean PM2.5 ≤ 15 μg/m³

• Statement 2 – Incorrect
Ground-level ozone is a photochemical pollutant.
It forms when NOx and VOCs react under sunlight, therefore highest ozone levels occur during sunny, hot weather, not during inclement (rainy/cloudy) conditions.

• Statement 3 – Incorrect
PM10 particles can reach deep into the lungs, but cannot cross the lung-blood barrier.
It is PM2.5 that can enter the bloodstream and cause systemic health effects.

• Statement 4 – Correct
Ozone is a powerful respiratory irritant and can:
– Trigger asthma attacks
– Reduce lung function
– Cause chest tightness and airway inflammation

Hence, only statements 1 and 4 are correct.

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🔍 Curiosity Raiser:
Why do cities often issue ozone alerts on sunny summer afternoons, even when there is no visible smoke?

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📚 Enrich Notes (Prelims Edge):

• PM2.5 = most dangerous particulate pollutant
• PM10 = lung deposition, not bloodstream
• Ozone = secondary pollutant (not directly emitted)
• WHO guidelines revised downwards in 2021 (important CA)
• Frequently tested statement-pair elimination logic

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🪶 IAS Monk Whisper:
When the air looks clean, ozone may still be quietly burning the lungs.