Pollution in UPSC Prelims: 29 questions, and sulphur dioxide in seven of them

Most aspirants prepare pollution from the headlines: a bad AQI week, a plastic ban, a study on microplastics. The past papers ask something narrower. They name a source, such as a coal plant, a copper smelter or burning stubble, and ask what comes out of it. Or they name a chemical and ask where you would find it.

This post looks at the 29 pollution questions asked from 2016 to 2026, 15 on air and water and 14 on waste, plastics and chemicals, and turns them into a plan.

What the numbers say

Three things stand out.

It was asked every year until 2026. Every paper from 2016 to 2025 had at least one pollution question. 2026 was the first paper with none.

Year	Air, water	Waste, chemicals	Total
2016	1	0	1
2017	1	1	2
2018	1	0	1
2019	2	5	7
2020	2	1	3
2021	3	3	6
2022	2	1	3
2023	0	1	1
2024	2	2	4
2025	1	0	1
2026	0	0	0

It comes in bursts. 2019 and 2021 alone gave 13 of the 29. In 2021, three came back to back as questions 16, 17 and 18, and two more as 74 and 75. The last two papers gave just one between them. Don’t read too much into that: 2018 also had one, and 2019 followed with seven.

Most questions give you a list. 18 of the 29 asked you to judge a list of items or statements. The other 11 were single-answer questions. The Statement-I/Statement-II format arrived here in 2024 (chewing gum), with a three-statement version in 2025 (activated carbon).

What UPSC is really testing

By our count, the 29 questions fall into five groups:

• What a source releases: 9 questions. Crop residue burning, nitrogen from farms and livestock, benzene exposure, coal ash and coal plants, copper smelters, furnace oil, magnetite particles, mercury, and the largest source of sulphur dioxide.
• Chemicals and plastics in daily life: 7. Four farm pesticides, microbeads, Bisphenol A, triclosan, PET, PFAS and the plastic in chewing gum.
• Clean-up and reuse: 6. Bioremediation, pyrolysis and plasma gasification, steel slag, the R2 code for electronics recycling, membrane bioreactors and activated carbon.
• Measures and rules: 5. India’s Air Quality Index, Biological Oxygen Demand, the WHO Air Quality Guidelines, the Solid Waste Management Rules, 2016 and extended producer responsibility.
• Causes and effects: 2. The effects of sand mining in riverbeds and the gases behind acid rain.

Sulphur dioxide runs through the first group and beyond. It appeared in seven questions: India’s AQI (2016), crop burning (2019), coal plants (2020), copper smelters and furnace oil (both 2021, the second as “sulphur emissions”), acid rain (2022) and its largest source (2024). It was never the false statement or the wrong option.

The trap most questions share

The familiar pollutant is not always the right one. Carbon monoxide is the gas most aspirants name first. It was in the correct answer twice, for India’s AQI and for crop burning. It was the wrong option twice: copper smelters are a concern for sulphur dioxide and heavy metals in their slag, and acid rain comes from sulphur dioxide and nitrogen oxides.

Knowing that a gas is harmful is not enough. UPSC tests which job each one does:

Pollutant	Where it was right	Where it was wrong
Sulphur dioxide	AQI, crop burning, coal plants, copper smelters, furnace oil, acid rain	Never
Carbon monoxide	AQI, crop burning	Copper smelters, acid rain
Ozone	Forms from crop-burning gases, can trigger asthma	Causing acid rain, peaking in bad weather
Methane	Released by crop burning	Counted in India’s AQI

The single-answer questions set a second trap: a name that sounds like another field. “Gasification” suggests natural gas, but pyrolysis and plasma gasification turn waste into energy. “Bioreactor” suggests vaccines, but membrane bioreactors treat wastewater. “Biological” Oxygen Demand suggests blood or forests, but it is a water test.

The false statements usually overreached. Bioremediation cannot treat “any” heavy metal “completely” (2017). PET cannot store “any” alcoholic drink, and burning it does release greenhouse gas (2022). And “no known safe level” is WHO’s phrase for lead, not mercury (2023).

How to prepare

1. Make a source table. List the sources from the papers: crop burning, coal plants, copper smelters, furnace oil, vehicle brakes and engines, gold mining, farm soils, cattle and poultry. Next to each, write what it gives off. Power plants were a correct source in four questions: for sulphur dioxide (twice), magnetite and mercury.

2. Give each gas a job. Note whether it is in India’s AQI, causes acid rain or is a greenhouse gas. India’s AQI counts eight pollutants: PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead. Carbon dioxide and methane are greenhouse gases and are not on that list.

3. Know the particles. PM2.5 can cross from the lungs into the blood; PM10 cannot. Magnetite particles come from burning and friction, not from microwaves or phone lines.

4. Learn where each everyday chemical is found. BPA: polycarbonate plastic. Triclosan: toiletries. PFAS: non-stick pans and food packaging, and they don’t break down easily. Carbofuran, methyl parathion, phorate and triazophos: farm pesticides.

5. For every clean-up term, write what it treats. Membrane bioreactors: wastewater. R2: electronics recycling. Activated carbon: industrial effluents. Steel slag: roads, cement and acidic farm soil. Bioremediation cannot break down heavy metals.

6. Read the two waste rules for their details. The Solid Waste Management Rules, 2016 use three segregation streams, not five. Extended producer responsibility was first made an important feature by the e-Waste Rules, 2011, not the 1999 recycled plastic rules. Our post on laws and acts covers the wider area.

7. Solve the past questions before reading more. Start with air and water pollution and waste, plastics and chemicals, then see the whole pollution topic.

What not to expect here: the pollution Acts and the pollution control boards. None of the 29 questions asked about them. They do come up elsewhere, as in the 2018 question on how the NGT differs from the CPCB, so study them with environmental bodies and laws.

Two questions that show the pattern

2021 · What a source releases. Why is there a concern about copper smelting plants? Two of the three statements were right: copper slag can leach heavy metals, and smelting releases sulphur dioxide. The trap was carbon monoxide. It is a familiar pollutant, but copper ores are sulphides, so sulphur dioxide is the gas to worry about. See the question

2022 · Which job each pollutant does. Four statements on the WHO Air Quality Guidelines. Only two were correct: the PM2.5 limits of 15 and 5 µg/m³, and that excess ozone can trigger asthma. Ozone peaks in sunny weather, not bad weather, and it is PM2.5, not PM10, that enters the bloodstream. See the question

For the other high-yield areas, see the must-do list. Clean-up terms that sound like biotech are a cousin of the trap in our post on biotechnology.