Assistant Provident Fund Commissioner (APFC) 2016 recruitment test, advertisement 170. Transcribed from the Commission's own booklet (series A, B-CRHE-O-PAPA); answer as per the official UPSC final answer key. This question is in Part B. Answer last reviewed 24 August 2026.

In the study of pollution, SPM refers to

(a)Sulphur Phosphorus Matter
(b)Sulphur Particulate Matter
(c)Solid Particulate Matter
(d)Suspended Particulate Matter

Answer

Why

Correct — D, (d) Suspended Particulate Matter.

SPM stands for SUSPENDED PARTICULATE MATTER: the total mass of tiny solid particles and liquid droplets held in suspension in the air. Every word of the expansion does work.

SUSPENDED — this is the defining property. The particles are small enough and light enough that air movement keeps them aloft instead of letting gravity settle them out, so they remain available to be breathed. A particle heavy enough to fall to the ground is no longer an air pollutant in this sense. PARTICULATE — matter present as discrete particles rather than as a gas. This is what separates the parameter from the gaseous pollutants that are measured alongside it, such as sulphur dioxide, oxides of nitrogen, carbon monoxide and ozone. MATTER — the category is defined by physical state and size, NOT by chemical composition. Particulate matter can be carbon from incomplete combustion, mineral dust from roads and construction, sulphates and nitrates formed in the atmosphere from gaseous precursors, metals, or organic compounds. The parameter measures how much is in the air, not what it is made of.

That last point is the key to the question. Every wrong option replaces one of these general words with something narrower — a named element in two cases, a single physical state in the third — and the whole idea of the parameter is that it is general.

HOW THE MEASURE HAS DEVELOPED is worth knowing, because modern air-quality reporting uses its successors. SPM was later refined into RESPIRABLE Suspended Particulate Matter, the fraction small enough to be drawn into the respiratory tract, which corresponds to PM10 — particles of aerodynamic diameter up to 10 micrometres. Finer still is PM2.5, up to 2.5 micrometres, which penetrates deep into the alveoli and can pass into the bloodstream, and which is therefore the fraction most closely associated with harm to health. India's National Ambient Air Quality Standards and the National Air Quality Index both use PM10 and PM2.5 rather than total SPM.

All four expansions are printed with each word capitalised, and the stem ends without punctuation.

Why the others are wrong

(a)Sulphur Phosphorus Matter — This is not a term used in air-quality measurement at all, and it fails on both elements it names. Sulphur is monitored in the atmosphere, but as the gas SULPHUR DIOXIDE, a separate parameter from particulate matter, and as the sulphate aerosols that form from it — which are counted within particulate matter along with everything else, not as a category of their own. Phosphorus is not a routinely monitored ambient air pollutant in India. The option is on the page as filler, giving the option set a fourth member built from two chemical names that look scientific together.
(b)Sulphur Particulate Matter — The second half is right and the first half is wrong, which makes this more tempting than option (a). It has a real fact behind it: sulphur dioxide emitted by burning coal and heavy fuel oil is converted in the atmosphere into sulphate particles, and those SECONDARY AEROSOLS are a substantial component of fine particulate pollution in many places. But they are a component, not the definition. Particulate matter is classified by SIZE, because size determines how deep into the lungs a particle travels, and a measure confined to sulphur compounds would omit carbon, dust, nitrates, metals and organics. The S in SPM is Suspended.
(c)Solid Particulate Matter — The closest of the three and the one worth understanding properly, because it is wrong by exactly one word. Suspended particulate matter includes LIQUID droplets and aerosols as well as solid particles — mists, acid droplets, condensed organic compounds — and confining the term to solids would exclude a real part of what is measured and breathed. More fundamentally, the defining property of the category is being SUSPENDED in air, not being solid: a solid particle that has settled on the ground is not what an air-quality standard is about, and a liquid droplet held aloft is. The option rewards a candidate who knows that particulate matter consists of particles and punishes one who has not asked what keeps them in the air.

Concept

PARTICULATE MATTER is the air pollutant most consistently associated with harm to health, and it is classified by SIZE rather than by chemistry, because size determines how far into the body a particle can travel.

SPM — Suspended Particulate Matter, the total mass of solid and liquid particles suspended in air. The older, broadest measure. RSPM or PM10 — the respirable fraction, up to 10 micrometres in aerodynamic diameter. Coarse particles that deposit largely in the upper airways; typical sources are road and construction dust and mechanical processes. PM2.5 — the fine fraction, up to 2.5 micrometres. These reach the alveoli and can pass into the bloodstream, and they are the fraction linked most strongly to respiratory and cardiovascular disease. They come mainly from COMBUSTION — vehicles, thermal power, industry, biomass and crop-residue burning — and from secondary formation in the atmosphere out of gaseous precursors.

PRIMARY AND SECONDARY PARTICLES is the other distinction worth carrying. Primary particles are emitted directly as particles. Secondary particles form in the atmosphere when gases such as sulphur dioxide, oxides of nitrogen and ammonia react and condense; this is why controlling a gaseous emission can reduce particulate pollution, and why the sulphate fraction that option (b) alludes to exists at all.

THE INDIAN MEASUREMENT FRAMEWORK. Ambient air quality is regulated under the Air (Prevention and Control of Pollution) Act, 1981 and the Environment (Protection) Act, 1986, with standards notified by the Central Pollution Control Board and monitoring carried out with the State Pollution Control Boards. The NATIONAL AIR QUALITY INDEX reports a single number derived from eight pollutants — PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead — in six categories running from Good through Satisfactory, Moderate, Poor and Very Poor to Severe. The index is set by the worst-performing pollutant, which in most Indian cities for most of the year is particulate matter.

The reason this parameter dominates Indian air-quality discussion is that particulate pollution comes from a very wide set of sources — transport, industry, power, construction, waste burning, agricultural residue and domestic cooking — so no single control measure addresses it, and any serious response has to work on all of them at once.

The geography and environment strand of this paper is small, and it is asked in the plainest possible form: expand an acronym, name a cause, define a term. This item is an acronym expansion with four candidates that all end in the same two words, so the entire question is which word the S stands for.

That is a common construction for abbreviation items, and it has a reliable defence. Take each option and ask what the resulting term would MEAN, not whether it sounds like something you have read. 'Solid Particulate Matter' would exclude droplets, which are certainly in the air and certainly breathed. 'Sulphur Particulate Matter' would exclude carbon and dust, which are the bulk of the mass. Only one expansion produces a category broad enough to be the thing that air-quality standards actually measure.

The paper asks about acid rain a few questions later, and the two items belong together. Acid rain comes from sulphur dioxide and oxides of nitrogen; those same gases are the precursors of the secondary sulphate and nitrate particles that make up part of the particulate load. One piece of atmospheric chemistry answers both.

Environmental vocabulary is worth preparing as vocabulary. The terms recur across papers — carbon footprint, acid rain, particulate matter, ozone depletion, eutrophication, biomagnification — and each is usually tested once, briefly, with the discrimination resting on a single word of the definition.

Key facts

SPM stands for Suspended Particulate Matter: the total mass of solid particles AND liquid droplets held in suspension in air.
The category is defined by physical state and size, not by chemical composition — carbon, mineral dust, sulphates, nitrates, metals and organic compounds all count.
RSPM, or PM10, is the respirable fraction up to 10 micrometres in aerodynamic diameter; PM2.5 is the fine fraction up to 2.5 micrometres.
PM2.5 penetrates to the alveoli and can enter the bloodstream, which is why it is the fraction most strongly linked to respiratory and cardiovascular harm.
Primary particles are emitted directly; secondary particles form in the atmosphere from gases such as sulphur dioxide, oxides of nitrogen and ammonia.
India's National Air Quality Index reports eight pollutants — PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead — in six categories from Good to Severe.
Ambient air quality in India is regulated under the Air (Prevention and Control of Pollution) Act, 1981 and the Environment (Protection) Act, 1986, with standards notified by the Central Pollution Control Board.

Study next

Particulate matter by size class — SPM, PM10 and PM2.5 — and why size rather than composition governs the health effect.Primary and secondary particulate matter, and why controlling gaseous emissions reduces particle concentrations.The National Air Quality Index: its eight pollutants, its six categories and the worst-pollutant rule for computing it.Major air pollutants and their principal sources: sulphur dioxide, oxides of nitrogen, carbon monoxide, ground-level ozone, ammonia and lead.Air pollution law and institutions in India: the Air Act of 1981, the Environment (Protection) Act of 1986, and the Central and State Pollution Control Boards.

Common traps

Reading the S as Sulphur. It stands for Suspended, and particulate matter is classified by size rather than by chemical composition.
Reading the S as Solid. Suspended particulate matter includes liquid droplets and aerosols, and the defining property is remaining aloft rather than being solid.
Confusing particulate matter with the gaseous pollutants measured alongside it. Sulphur dioxide, oxides of nitrogen and ozone are separate parameters.
Assuming a more chemically specific expansion must be the more technical one. The parameter is deliberately general because the health effect depends on size.

Environment items on EPFO papers are short and definitional: expand an acronym, identify the cause of a named phenomenon, or say what a term means. The option sets for acronym questions almost always share most of their words, so the discrimination is on one word and the reliable method is to ask what each candidate expansion would MEAN rather than which sounds familiar. Expect SPM, AQI, BOD, COD, CFC, ODS and GHG, and expect them alongside cause-and-effect items on acid rain, the greenhouse effect, ozone depletion and eutrophication. Since the same atmospheric chemistry underlies several of these, learning the precursor gases once answers questions in more than one place.

Related PYQs

EPFO_APFC_2016_Q63

Acid rain is due to

(a) Sulphur dioxide pollution
(b) Carbon monoxide pollution
(c) Pesticide pollution
(d) Dust particles in the atmosphere

Answer(a) Sulphur dioxide pollution

The item on the cause of acid rain — the same precursor gases, sulphur dioxide and oxides of nitrogen, that also form the secondary particles counted within suspended particulate matter.

EPFO_APFC_2016_Q27

The term 'Carbon footprint' means

(a) A region which is rich in coal mines
(b) The amount of reduction in the emission of CO2 by a country
(c) The use of Carbon in manufacturing industries
(d) The amount of greenhouse gases produced by our day-to-day activities

Answer(d) The amount of greenhouse gases produced by our day-to-day activities

The meaning of 'carbon footprint' — the other environmental vocabulary item on this paper, and answered by the same technique of testing what each candidate definition would actually mean.

Practice

practice — not a real PYQ
PM2.5 is considered more harmful to human health than PM10 mainly because it
- (a)is chemically more reactive
- (b)penetrates deeper into the lungs and can enter the bloodstream
- (c)is present in far greater mass in the atmosphere
- (d)remains suspended for a shorter time
Answer(b) penetrates deeper into the lungs and can enter the bloodstream — particles up to 2.5 micrometres reach the alveoli, where gas exchange occurs, while coarser PM10 deposits largely in the upper airways. Fine particles also remain suspended LONGER, not for a shorter time, and travel further from their source.
practice — not a real PYQ
Which of the following is NOT among the pollutants covered by India's National Air Quality Index ?
- (a)Ammonia
- (b)Ground-level ozone
- (c)Carbon dioxide
- (d)Lead
Answer(c) Carbon dioxide — the index covers PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead. Carbon dioxide is a greenhouse gas rather than a local air pollutant at ambient concentrations, and is not part of an air-quality index, which is built around effects on health.

Why