Pollution, environmental laws and green technology: previous-year questions
73 real UPSC Prelims questions (2011–2026) with the official answer and a clear explanation. 25 of them came in the last five papers.
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Which of the following statements with regard to Green Hydrogen is/are correct ?
- 1. It is decarbonized hydrogen obtained from natural gas reforming combined with carbon capture and storage (CCS).
- 2. It is produced using electrolysis of water with electricity generated by renewable energy.
- 3. National Green Hydrogen Mission of India aims for abatement of nearly 50 MMT of annual greenhouse gas emissions by 2030.
Select the answer using the code given below :
Answer (b) & explanation
Statements 2 and 3 are correct (b). Statement 2 is right: green hydrogen comes from splitting water by electrolysis, using renewable power. Statement 3 is right: the National Green Hydrogen Mission (2023) targets 5 MMT of green hydrogen a year by 2030 and about 50 MMT of avoided CO₂-equivalent emissions each year. Statement 1 is wrong: hydrogen from natural gas with carbon capture is blue hydrogen. So options (a), (c) and (d) fail.
Notice the colours: grey hydrogen from gas without capture (most hydrogen today), brown from coal, and pink from nuclear power. The mission's SIGHT programme supports electrolyser making. Uses include fertilisers, refineries, steel and shipping fuel.
1 says "decarbonized", which sounds green. Gas with CCS is blue.
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Answer as per the official UPSC answer key.
Which of the following statements in relation to NIRANTAR (National Institute for Research and Application of Natural Resources to Transform, Adapt and Build Resilience), a platform of institutions under the Ministry of Environment, Forest and Climate Change, is/are correct ?
- 1. Ecosystem Survey and Analysis is a vertical under this platform, the lead institute of which is Botanical Survey of India, Kolkata.
- 2. Research and Management of Ecosystem Service is a vertical under this platform, the lead institute of which is Central Zoo Authority, New Delhi.
- 3. Capacity Development Support is a vertical under this platform, the lead institute of which is Indian Institute of Forest Management, Bhopal.
Select the answer using the code given below :
Answer (b) & explanation
Statements 1 and 3 are correct (b). NIRANTAR groups the Environment Ministry's institutes into verticals. Statement 1 is right: Ecosystem Survey and Analysis is led by the Botanical Survey of India, Kolkata, with bodies like the FSI and ZSI. Statement 3 is right: Capacity Development Support is led by the Indian Institute of Forest Management, Bhopal. Statement 2 is wrong: Research and Management of Ecosystem Service is led by ICFRE, Dehradun; the Central Zoo Authority is only a member. So options (a), (c) and (d) fail.
Notice the purpose: pooling institutes avoids duplicate work and links research to climate adaptation and resilience. Other Ministry bodies include the Wildlife Institute of India (Dehradun) and GBPNIHE for the Himalaya.
the Central Zoo Authority does sit in that vertical, but as a member, not the lead.
Answer as per the official UPSC answer key.
Consider the following statements :
- Statement I : Studies indicate that carbon dioxide emissions from cement industry account for more than 5% of global carbon emissions.
- Statement II : Silica-bearing clay is mixed with limestone while manufacturing cement.
- Statement III : Limestone is converted into lime during clinker production for cement manufacturing.
Which one of the following is correct in respect of the above statements?
Answer (b) & explanation
Both II and III are correct, but only III explains I (b). Statement I is right: cement gives about 7–8% of the world's CO₂. Statement II is right: silica-bearing clay is mixed with limestone to make cement, but that step releases no carbon. Statement III is right and is the reason: heating limestone (CaCO₃) in a kiln gives lime (CaO) and CO₂. This calcination step, plus the fuel burnt for the high heat, causes most of the emissions. So options (a), (c) and (d) fail.
Notice the fixes: blending in fly ash or slag, LC3 (limestone calcined clay cement), and carbon capture. India is the world's second-largest cement producer after China, and its carbon market covers cement.
Answer as per the official UPSC answer key.
With reference to ‘Direct Air Capture’, an emerging technology, which of the following statements is/are correct?
- I. It can be used as a way of carbon sequestration.
- II. It can be a valuable approach for plastic production and in food processing.
- III. In aviation, it can be a source of carbon for combining with hydrogen to create synthetic low-carbon fuel.
Select the correct answer using the code given below.
Answer (c) & explanation
All three statements are correct (c). Direct Air Capture (DAC) pulls CO₂ straight from the open air using chemical filters or liquids. Statement I is right: the gas can be stored underground, a way of carbon sequestration. Statement II is right: it can be used as a raw material for plastics, and in food processing and fizzy drinks. Statement III is right: joined with green hydrogen, it can make synthetic low-carbon aviation fuel. So options (a), (b) and (d) fail.
Notice the difference from carbon capture and storage (CCS) at a factory chimney, where CO₂ is far more concentrated. DAC is still very expensive and energy-hungry. The world's largest plant, Mammoth, runs in Iceland.
Answer as per the official UPSC answer key.
Consider the following :
- I. Cigarette butts
- II. Eyeglass lenses
- III. Car tyres
How many of them contain plastic?
Answer (c) & explanation
All three contain plastic (c). Item I is right: cigarette filters are made of cellulose acetate, a plastic, and are among the most common litter on beaches. Item II is right: most spectacle lenses are polycarbonate or other plastics, not glass. Item III is right: tyres contain synthetic rubber and polymer fibres, and tyre wear is a big source of microplastics in air and water. So options (a), (b) and (d) fail.
Notice that microplastics, pieces under 5 mm, are now found in human blood, lungs and even the placenta. India banned many single-use plastic items from July 2022. Talks on a global plastics treaty have not yet reached agreement.
Answer as per the official UPSC answer key.
Consider the following statements :
- Statement I : Activated carbon is a good and an attractive tool to remove pollutants from effluent streams and to remediate contaminants from various industries.
- Statement II : Activated carbon exhibits a large surface area and a strong potential for adsorbing heavy metals.
- Statement III : Activated carbon can be easily synthesized from environmental wastes with high carbon content.
Which one of the following is correct in respect of the above statements?
Answer (a) & explanation
Both II and III are correct and both explain Statement I (a). Statement II is right: activated carbon is full of tiny pores, so one gram has a huge surface area, often over 1,000 m², on which pollutants and heavy metals stick (adsorption). That makes it work well. Statement III is right: it can be made cheaply from coconut shells, rice husk and other carbon-rich waste, which makes it attractive. Both explain Statement I, so options (b), (c) and (d) fail.
Notice the spelling: adsorption means sticking to a surface; absorption means soaking in. Activated carbon is used in water filters, gas masks, and treating poisoning in hospitals. India's coconut-growing south is a big producer.
Answer as per the official UPSC answer key.
Artificial way of causing rainfall to reduce air pollution makes use of
Answer (a) & explanation
Silver iodide and potassium iodide are used (a). Cloud seeding sprays particles into suitable clouds so that water droplets or ice crystals form around them. Silver iodide is the standard agent, as its crystal shape is close to ice, and potassium iodide is used with it. Rain then washes dust and smoke out of the air. Options (b), (c) and (d) are wrong: silver nitrate, potassium nitrate and potassium chloride are not the usual agents, though common salt is used in some warm-cloud seeding.
Notice the catch: seeding needs clouds with enough moisture; it cannot make rain from a clear sky. Delhi tried cloud seeding against smog with IIT Kanpur in 2025, with little success because the air was too dry.
Answer as per the official UPSC answer key.
Which one of the following is the exhaust pipe emission from Fuel Cell Electric Vehicles, powered by hydrogen ?
Answer (d) & explanation
The exhaust emission is water vapour (d). A fuel cell combines hydrogen with oxygen from the air to make electricity, which runs the vehicle's motor. The only by-product is water, which leaves the exhaust as vapour. There is no carbon dioxide or smoke. Option (a) is wrong: hydrogen peroxide is not produced. Option (b) is wrong: hydronium ions exist only in water solutions, not as an exhaust. Option (c) is wrong: oxygen is used up, not released.
Notice India's trials: hydrogen fuel cell buses run in Leh and Delhi, and Toyota's Mirai car was tested here. The catch is cost: making green hydrogen and building filling stations is expensive. Indian Railways is also testing a hydrogen train on the Jind–Sonipat route.
Answer as per the official UPSC answer key.
“Membrane Bioreactors” are often discussed in the context of :
Answer (d) & explanation
Membrane bioreactors are a wastewater treatment technology (d). A membrane bioreactor treats sewage in two steps in one unit. First, microbes break down the dirt and organic matter, as in normal treatment. Then a fine membrane filters out solids, bacteria and even some viruses. The clean water can be reused for gardens, industry or toilet flushing. It is compact, so it is popular where land is short, such as cities and apartment blocks. Options (a), (b) and (c), about IVF, nanomedicine and vaccines, do not fit.
Notice India's need: cities treat only part of their sewage, and the Namami Gange and AMRUT schemes fund new plants. Reusing treated water is key in water-stressed cities like Chennai and Bengaluru.
Answer as per the official UPSC answer key.
Consider the following materials :
- 1. Agricultural residues
- 2. Corn grain
- 3. Wastewater treatment sludge
- 4. Wood mill waste
Which of the above can be used as feedstock for producing Sustainable Aviation Fuel ?
Answer (c) & explanation
All four can be used to make Sustainable Aviation Fuel (c). Sustainable Aviation Fuel (SAF) is jet fuel made from non-fossil carbon, so it can cut lifecycle emissions by up to 80%. Item 2 is right: corn grain gives ethanol, which can be turned into jet fuel by the alcohol-to-jet route. Items 1, 3 and 4 are right: crop residues, sewage sludge and wood mill waste can all be converted, through processes like gasification and pyrolysis. Used cooking oil is another common feedstock.
Notice India's targets: 1% SAF blending in international flights by 2027 and 2% by 2028. India's first SAF-blended commercial flight flew in 2023. The Global Biofuels Alliance, launched at the G-20 in New Delhi, supports such fuels.
corn is a food crop, which makes it look "not sustainable". It is still a valid feedstock.
Answer as per the official UPSC answer key.
With reference to perfluoroalkyl and polyfluoroalkyl substances (PFAS) that are used in making many consumer products, consider the following statements :
- 1. PFAS are found to be widespread in drinking water, food and food packaging materials.
- 2. PFAS are not easily degraded in the environment.
- 3. Persistent exposure to PFAS can lead to bioaccumulation in animal bodies.
Which of the statements given above are correct ?
Answer (d) & explanation
All three statements are correct (d). PFAS, or per- and polyfluoroalkyl substances, are used in non-stick pans, stain- and water-proof fabrics, firefighting foams and grease-proof food wrappers. Statement 1 is right: they now turn up in drinking water, food and food packaging almost everywhere. Statement 2 is right: their carbon-fluorine bond is one of the strongest in chemistry, so they hardly break down: hence the name "forever chemicals". Statement 3 is right: they build up in the bodies of animals and people over time, called bioaccumulation.
Notice the health risks: links to some cancers, thyroid problems and weakened immunity. The USA set its first legal limits for PFAS in drinking water in 2024, and the Stockholm Convention has listed several PFAS for elimination.
Answer as per the official UPSC answer key.
Consider the following statements :
- Statement-I : Many chewing gums found in the market are considered a source of environmental pollution.
- Statement-II : Many chewing gums contain plastic as gum base.
Which one of the following is correct in respect of the above statements ?
Answer (a) & explanation
Both statements are correct and II explains I (a). Statement I is right: discarded chewing gum is a major source of litter and pollution: spat-out gum sticks to streets and pavements for years, and is costly to clean. Statement II is right and explains it: most modern gum bases are synthetic polymers such as polyvinyl acetate and polyisobutylene, which are plastics, in place of the old natural chicle from the sapodilla tree. These do not break down, so gum stays in the environment.
Notice that Singapore has banned the sale of most chewing gum since 1992. Gum also adds to microplastics, and studies have found that chewing it can release microplastic particles into saliva. Some makers are going back to natural, biodegradable gum bases.
Answer as per the official UPSC answer key.
Consider the following statements regarding World Toilet Organization :
- 1. It is one of the agencies of the United Nations.
- 2. World Toilet Summit, World Toilet Day and World Toilet College are the initiatives of this organization, to inspire action to tackle the global sanitation crisis.
- 3. The main focus of its function is to grant funds to the least developed countries and developing countries to achieve the end of open defecation.
Which of the statements given above is/are correct ?
Answer (a) & explanation
Only statement 2 is correct (a). Statement 1 is wrong: the World Toilet Organization is a non-profit founded in Singapore in 2001 by Jack Sim, not a UN agency. Statement 2 is right: it started the World Toilet Summit, the World Toilet College and World Toilet Day on 19 November, which the UN adopted as an official day in 2013. Statement 3 is wrong: it campaigns, trains and raises awareness; it is not a funding body that gives grants to countries.
Notice India's Swachh Bharat Mission (2014), which built over 10 crore toilets and declared rural India open defecation free in 2019. Its second phase, ODF Plus, focuses on waste management. SDG 6 aims at sanitation for all by 2030.
the UN does observe World Toilet Day, which makes 1 look true.
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Answer as per the official UPSC answer key.
Consider the following :
- 1. Aerosols
- 2. Foam agents
- 3. Fire retardants
- 4. Lubricants
In the making of how many of the above are hydrofluorocarbons used?
Answer (c) & explanation
HFCs are used in three of the four (c). Hydrofluorocarbons (HFCs) replaced the ozone-depleting CFCs. Besides refrigeration and air-conditioning, they are used as aerosol propellants, as foam-blowing agents for insulation, and as fire suppressants that put out fires without water damage. Item 4 is wrong: HFCs are not used in making lubricants. So options (a), (b) and (d) fall.
Notice the twist: HFCs do not harm the ozone layer, but they are powerful greenhouse gases, often thousands of times stronger than CO₂. The Kigali Amendment (2016) to the Montreal Protocol phases them down. India, in the third group, will freeze use in 2028 and cut 85% by 2047. Its India Cooling Action Plan (2019) supports this.
Answer as per the official UPSC answer key.
With reference to the role of biofilters in Recirculating Aquaculture System, consider the following statements :
- 1. Biofilters provide waste treatment by removing uneaten fish feed.
- 2. Biofilters convert ammonia present in fish waste to nitrate.
- 3. Biofilters increase phosphorus as nutrient for fish in water.
How many of the statements given above are correct?
Answer (b) & explanation
Two statements are correct (b). A Recirculating Aquaculture System cleans and reuses the same water, so it needs far less water and land. Statement 1 is right: the biofilter treats wastes from fish droppings and uneaten feed, after solids are removed. Statement 2 is right: its bacteria turn toxic ammonia from fish waste into nitrite and then into much less harmful nitrate; this is nitrification. Statement 3 is wrong: biofilters do not raise phosphorus; fish need clean water, and extra phosphorus only feeds unwanted algae.
Notice the link with aquaponics, where the nitrate-rich water feeds plants grown alongside the fish. India's PM Matsya Sampada Yojana (2020) funds RAS and biofloc units. India is the world's second-largest fish producer.
Answer as per the official UPSC answer key.
Consider the following statements regarding mercury pollution :
- 1. Gold mining activity is a source of mercury pollution in the world.
- 2. Coal-based thermal power plants cause mercury pollution.
- 3. There is no known ‘safe level’ of exposure to mercury.
How many of the above statements are correct?
Answer (b) & explanation
Two statements are correct (b). Statement 1 is right: small-scale gold mining, which uses mercury to bind gold particles, is the largest source of mercury released by people worldwide. Statement 2 is right: coal burning, mainly in thermal power plants, comes next, as coal holds traces of mercury. Statement 3 is wrong under UPSC's key: "no known safe level" is what the WHO says about lead. For mercury, health agencies set tolerable intake limits, though they treat it as highly toxic.
Notice the Minamata Convention (2013, in force 2017), named after Japan's mercury poisoning disaster from factory waste. India has ratified it. Mercury turns into methylmercury in water and builds up in fish, harming the brain.
the phrase "no known safe level" is borrowed from lead.
Answer as per the official UPSC answer key.
With reference to green hydrogen, consider the following statements :
- 1. It can be used directly as a fuel for internal combustion.
- 2. It can be blended with natural gas and used as fuel for heat or power generation.
- 3. It can be used in the hydrogen fuel cell to run vehicles.
How many of the above statements are correct?
Answer (c) & explanation
All three statements are correct (c). Green hydrogen is made by splitting water using renewable electricity. Statement 1 is right: it can be burnt directly in a modified internal combustion engine, and trucks and buses running this way are being tested. Statement 2 is right: it can be blended with natural gas, often up to about 20%, for heating or power; India has tested this in city gas networks. Statement 3 is right: it can be used in a fuel cell, which combines hydrogen with oxygen to make electricity, emitting only water, to run vehicles.
Notice India's National Green Hydrogen Mission (2023), which aims at 5 million tonnes a year by 2030. Its main early users will be fertiliser plants, refineries and steel.
Answer as per the official UPSC answer key.
With reference to coal-based thermal power plants in India, consider the following statements :
- 1. None of them uses seawater.
- 2. None of them is set up in water-stressed district.
- 3. None of them is privately owned.
How many of the above statements are correct?
Answer (d) & explanation
None of the statements is correct (d). Statement 1 is wrong: coastal plants such as those at Mundra in Gujarat and others on the Tamil Nadu and Andhra coasts use seawater for cooling. Statement 2 is wrong: studies have found many thermal plants in water-stressed districts, and some have shut for days in droughts for lack of water. Statement 3 is wrong: large coal plants are run by private companies such as Adani Power, Tata Power and JSW, besides public firms like NTPC. All three "none" statements fail.
Notice the hint: UPSC's absolute words like "none", "all" and "only" are often wrong. Coal still gives about 70–75% of India's power, and plants must meet stricter water-use and emission norms.
"none" statements are easy to sink with one example each.
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Answer as per the official UPSC answer key.
Consider the following statements :
- 1. In India, the Biodiversity Management Committees are key to the realization of the objectives of the Nagoya Protocol.
- 2. The Biodiversity Management Committees have important functions in determining access and benefit sharing, including the power to levy collection fees on the access of biological resources within its jurisdiction.
Which of the statements given above is/are correct?
Answer (c) & explanation
Both statements are correct (c). Statement 1 is right: under the Biological Diversity Act, 2002, every local body, such as a panchayat or municipality, sets up a Biodiversity Management Committee. BMCs are key to the Nagoya Protocol's aim of fair and equitable sharing of benefits from biological resources and traditional knowledge, because they bring it down to villages. Statement 2 is right: BMCs keep the People's Biodiversity Register, are consulted on access to local resources, and may levy collection fees on those who take resources from their area.
Notice the three-tier structure: the National Biodiversity Authority in Chennai, State Biodiversity Boards and BMCs. The Act was amended in 2023 to ease rules for AYUSH practitioners and research.
Answer as per the official UPSC answer key.
Consider the following heavy industries :
- 1. Fertilizer plants
- 2. Oil refineries
- 3. Steel plants
Green hydrogen is expected to play a significant role in decarbonizing how many of the above industries?
Answer (c) & explanation
Green hydrogen can help decarbonise all three (c). Item 1 is right: fertiliser plants use hydrogen to make ammonia, the base of urea. Item 2 is right: oil refineries use hydrogen to remove sulphur from fuels and to crack heavy oils. Both now mostly use grey hydrogen made from natural gas, which releases CO₂. Switching to green hydrogen, made from water using renewable power, cuts those emissions. Item 3 is right: steel plants can use hydrogen in place of coking coal to remove oxygen from iron ore, called direct reduced iron, leaving water instead of CO₂.
Notice India's National Green Hydrogen Mission (2023) with its SIGHT incentive programme. Sweden's HYBRIT project made the first fossil-free steel, and Indian steelmakers are running pilots.
Answer as per the official UPSC answer key.