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UPSC Prelims 2022 · Environment · Pollution Files
In the context of WHO Air Quality Guidelines, consider the following statements: Which of the statements given above are correct?
- 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.
- In a year, the highest levels of ozone pollution occur during the periods of inclement weather.
- PM10 can penetrate the lung barrier and enter the bloodstream.
- Excessive ozone in the air can trigger asthma.
- (a) 1, 3 and 4
- (b) 1 and 4 only
- (c) 2, 3 and 4
- (d) 1 and 2 only
Show the answer and explanation
Answer: (a) 1, 3 and 4
The WHO limits, entry into blood, and ozone-triggered asthma are accepted.
Statement by statement
1. ✓ CORRECT WHO 2021: PM2.5 24-hour ≤ 15 µg/m³, annual ≤ 5 µg/m³.
2. ✗ WRONG Ground-level ozone peaks in hot, sunny, calm weather — it needs sunlight to form. Inclement (cloudy, rainy) weather lowers it.
3. ✓ CORRECT Fine particles can pass into the bloodstream. (Strictly it is PM2.5 and smaller that do this; UPSC accepted the statement.)
4. ✓ CORRECT Ozone irritates airways and can trigger asthma attacks.
The idea
Particulate matter is named by size. PM10 is 10 micrometres or smaller and reaches the lungs. PM2.5 is 2.5 micrometres or smaller; it goes deep into the air sacs and can cross into the blood. The smaller the particle, the deeper it goes, so the limit for PM2.5 is the strictest.
Vehicles and industry give off NOₓ and hydrocarbons → Strong sunlight acts on them → Ozone forms near the ground → Highest on hot, clear, still afternoons → Rain and cloud bring it down
The trap
Statement 2 reverses the chemistry: ozone is a sunshine pollutant.
Also know
The WHO guidelines (2021) are advice, not law. India’s own standards (2009) are much looser: annual PM2.5 of 40 µg/m³ against WHO’s 5. India’s standards cover 12 pollutants. The National Clean Air Programme (2019) aims to cut particulate levels in about 130 cities that miss the national standards.
🔒 WHO 2021: PM2.5 5 (annual) / 15 (24 h); PM10 15 / 45 µg/m³. Ozone = summer, sunny afternoons.