Air Pollution for UPSC: Pollutants, Standards and the Questions That Trip Aspirants

Air pollution in UPSC Prelims looks like a current affairs topic, but the questions rarely are. In Tootler's bank of transcribed papers, 20 General Studies questions between 2010 and 2024 deal with air pollution and toxic pollutants, spread across 12 different exam years. Most of them test three things that do not change from year to year: how India measures air quality, where the main pollutants come from, and the chemistry of acid rain, smog and carbon monoxide.

That makes this one of the more learnable areas of the Environment syllabus. This guide covers each of those three areas with the official source behind it, then the group of questions on toxic substances such as lead and mercury, and shows the questions UPSC built on each.

This guide is part of Tootler's series on environment and ecology for UPSC, which maps all 284 environment questions in Prelims by topic.

How India measures air quality

The National Air Quality Index was launched in October 2014 as "One Number, One Colour, One Description". It covers eight pollutants for which short-term national ambient air quality standards exist: PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead. A sub-index is worked out for each, and the worst sub-index becomes the overall AQI. The result is reported in six categories: Good, Satisfactory, Moderately polluted, Poor, Very Poor and Severe.

The list is the whole point of the 2016 question, which mixed AQI gases with two greenhouse gases that are not part of the index:

The policy that sits on top of the index is the National Clean Air Programme. According to the Central Pollution Control Board's submission to the National Green Tribunal, the Ministry of Environment launched it in January 2019 for 131 identified cities, with the aim of meeting national standards or cutting PM10 concentrations by 40% by 2025-26 against a 2019-20 base, through action plans prepared for each city. UPSC has not yet asked about it directly, which makes it a reasonable candidate for a future paper.

UPSC has also asked about the World Health Organization's air quality guidelines, in 2022. Read those guidelines from WHO itself rather than from secondary notes, because the question turned on exact numbers and on what PM10 and ozone do in the body.

Where the main pollutants come from

Questions on sources keep returning to one answer: burning fuel. The US Environmental Protection Agency puts it plainly for sulphur dioxide. "The largest source of SO2 in the atmosphere is the burning of fossil fuels by power plants and other industrial facilities." Smaller sources include extracting metals from ore, volcanoes, and locomotives, ships and other vehicles that burn high-sulphur fuel. UPSC used exactly that list of sources as its options in 2024:

The same theme runs through the coal questions. In 2011, UPSC asked which gases come from coal combustion at thermal power plants; in 2020 it asked about the contents of coal ash, the emissions of coal plants and the ash content of Indian coal. A 2014 question did the same for the steel industry, and a 2019 question for the burning of crop residue. Each time the options list gases such as carbon monoxide, oxides of sulphur and nitrogen, methane and ozone, and the skill being tested is whether you can connect a process to what it releases.

The chemistry UPSC keeps testing

Three pieces of atmospheric chemistry account for five questions in this set.

Acid rain has been asked three times, in 2010, 2013 and 2022. The EPA's explanation is the one to remember: acid rain results when sulphur dioxide and nitrogen oxides are released into the air, carried by wind, and react with water, oxygen and other chemicals to form sulphuric and nitric acids, which then fall as rain, snow, fog or dry particles. Carbon dioxide, carbon monoxide and ozone keep appearing in the options as distractors.

Photochemical smog (2013) is a question about ozone at ground level. Ground-level ozone is not emitted directly; it forms when nitrogen oxides and volatile organic compounds react in the presence of sunlight, and it is the main ingredient of smog. That is why the options that mention sunlight deserve attention and those set at low temperature or in the evening do not fit the chemistry.

Carbon monoxide (2010) is about the blood. When inhaled, CO forms carboxyhaemoglobin, which inhibits the body's oxygen intake. The question offered several plausible-sounding mechanisms; only the one involving haemoglobin matches.

Where air pollution meets climate change

Some pollutants are counted twice, once as air pollutants and once as climate pollutants. Black carbon, methane and hydrofluorocarbons stay in the atmosphere for a much shorter time than carbon dioxide but warm it strongly, which is why they are grouped as short-lived climate pollutants. Cutting them improves air quality and slows warming at the same time.

That overlap is the subject of the 2017 question on the Climate and Clean Air Coalition. The coalition was launched on 16 February 2012 by the United States, Sweden, Bangladesh, Canada, Mexico and Ghana, with support from the UN Environment Programme, as the first effort to reduce black carbon, hydrofluorocarbons and methane together. Who started it and which pollutants it targets are exactly the two statements UPSC tested, so those are the facts to hold on to.

If you are also preparing the climate change part of the syllabus, revise black carbon and methane in both places. A question can approach them from either side.

Toxic pollutants: lead, mercury and the newer chemicals

The remaining questions move from the air to toxic substances in everyday products and the environment, and they have become more frequent in recent papers.

Lead and mercury are the two to know well, because the World Health Organization's fact sheets answer most of what UPSC has asked. On lead, WHO states that "there is no level of exposure to lead that is known to be without harmful effects", and lists its continuing uses: pigments, paints, solder, jewellery, toys, some traditional cosmetics and some traditional medicines. That list explains the 2012 question on sources of lead poisoning after leaded petrol was banned:

On mercury, WHO identifies human activity as the main cause of releases, particularly coal-fired power stations, residential coal burning, waste incinerators and mining for mercury, gold and other metals. In the environment, bacteria convert mercury into methylmercury, which builds up in fish and shellfish. UPSC asked about gold mining and coal plants as mercury sources in 2023.

The newer chemicals follow one pattern. Brominated flame retardants (2014), industrial nanoparticles (2014), triclosan (2021) and PFAS, used in many consumer products (2024), were each asked as a set of statements about where the substance is found, whether it breaks down, and whether it builds up in living bodies. Persistence and bioaccumulation are the two words to look for in every one of them.

How to prepare air pollution for Prelims

Learn this topic from four short sources rather than from notes: the AQI launch release for the eight pollutants, the EPA pages on sulphur dioxide, acid rain and ground-level ozone for the chemistry, and WHO's fact sheets on lead and mercury for toxic pollutants. Together they cover most of the questions in this guide.

When a new pollutant appears in the news, ask the same three questions UPSC asks: where does it come from, does it persist, and does it accumulate in living things? Those three answers will handle most statement-based questions on substances you have never studied before.

Frequently asked questions