UPSC Prelims 2022 Q44: WHO Air Quality Guidelines — Explained
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?
- 1, 3 and 4
- 1 and 4 only
- 2, 3 and 4
- 1 and 2 only
Answer: (b) 1 and 4 only
Statement-wise explanation
- 1: correct — WHO revised its Global Air Quality Guidelines on 22 September 2021, the first update since 2005. For PM2.5 the 24-hour mean guideline is 15 µg/m³, down from 25, and the annual mean is 5 µg/m³, down from 10. Both figures in the statement match the 2021 guidelines.
- 2: incorrect — Ground-level ozone is a secondary pollutant. It forms when nitrogen oxides and volatile organic compounds react in sunlight. So ozone peaks in hot, sunny, still weather, usually summer afternoons. Cloudy, rainy or windy (inclement) weather cuts the sunlight and disperses the pollutants, so ozone levels fall.
- 3: incorrect — WHO states that both PM2.5 and PM10 can penetrate deep into the lungs, but only PM2.5 can enter the bloodstream. PM10 particles (10 microns or less) are too coarse to cross the lung barrier. The statement gives PM10 a property that belongs to PM2.5.
- 4: correct — Ozone is a strong oxidant. It inflames and damages the airways. Excess ozone can cause breathing problems, trigger asthma, reduce lung function and cause lung disease. Children, older people and people with asthma or COPD are most at risk.
How to eliminate
Statement 3 is the easiest to remove: the standard fact is that PM2.5, not PM10, reaches the blood. Smaller particles go deeper, and only the finest cross into the bloodstream. Removing 3 knocks out two of the four options and leaves '1 and 4 only' or '1 and 2 only'. Both contain statement 1, so you can choose without knowing the exact WHO numbers. The decision rests on statement 2, and basic chemistry settles it: ground-level ozone is photochemical smog, made by sunlight acting on NOx and VOCs. It builds up on bright, hot days, not in inclement weather. Statement 4 is plainly true: ozone is a well-known respiratory irritant. So the question can be solved from two concepts, PM size versus depth of penetration and ozone needing sunlight, without memorising the 15/5 µg/m³ figures.
Know more
WHO Global Air Quality Guidelines (AQG) 2021
WHO released updated AQG on 22 September 2021, replacing the 2005 guidelines. The guidelines are not legally binding. They are evidence-based reference values that countries can use to set their own standards. They cover six 'classical' pollutants: PM2.5, PM10, ozone (O₃), nitrogen dioxide (NO₂), sulphur dioxide (SO₂) and carbon monoxide (CO). WHO also gives interim targets (for annual PM2.5: 35, 25, 15 and 10 µg/m³) as steps for heavily polluted countries.
| Pollutant | Averaging time | 2005 AQG | 2021 AQG |
|---|---|---|---|
| PM2.5 | Annual | 10 µg/m³ | 5 |
| PM2.5 | 24-hour | 25 | 15 |
| PM10 | Annual | 20 | 15 |
| PM10 | 24-hour | 50 | 45 |
| O₃ | Peak season | – | 60 |
| O₃ | 8-hour | 100 | 100 |
| NO₂ | Annual | 40 | 10 |
| NO₂ | 24-hour | – | 25 |
| SO₂ | 24-hour | 20 | 40 (relaxed) |
| CO | 24-hour | – | 4 mg/m³ |
Nearly every value was tightened. SO₂ is the one exception: its 24-hour guideline was raised.
India's comparison. India's National Ambient Air Quality Standards (NAAQS, 2009) cover 12 pollutants. They set PM2.5 at 40 µg/m³ annual and 60 µg/m³ for 24 hours, and ozone at 100 µg/m³ over 8 hours. In Parliament the government has said the WHO values are advisory.
Two ozones. Stratospheric ozone (the ozone layer) is beneficial and protected under the Montreal Protocol. Tropospheric (ground-level) ozone is a harmful secondary pollutant and a short-lived greenhouse gas.
In the news
WHO released its revised Global Air Quality Guidelines on 22 September 2021, the first revision in 16 years. It sharply cut the PM2.5 limits to 5 µg/m³ annual and 15 µg/m³ for 24 hours. That made the guidelines a natural topic for Prelims 2022.
NCERT Concepts to Revise
- Particulate matter: Statements 1 and 3 depend on how particulate matter is sorted by size, and on what PM2.5 and PM10 mean.
- Tropospheric ozone: Statements 2 and 4 are about ground-level ozone: how it forms and how it affects health.
- Air pollution: Defines air pollution as harmful gases plus particulate matter, the two groups of pollutants this question tests.