Maarg Manthan · Topic 13.2

Classification of Pollutants and Pollution Indicator Species

Environment & Ecology › Air, Marine, Noise and Other Pollution · Topic 13.2

A pollutant is any substance, energy or organism added to the environment in amounts that harm living things or ecosystems. Pollutants are classified in several ways. By origin they are natural (volcanic ash, dust storms) or man-made. By form they are solid, liquid, gaseous, or forms of energy such as heat, noise and radiation. By formation they are primary pollutants, emitted directly (carbon monoxide, sulphur dioxide, soot), or secondary pollutants, formed by reactions in the environment (ground-level ozone, peroxyacetyl nitrate, acid rain). By degradability they are biodegradable (sewage, food waste) or non-biodegradable (plastics, DDT, heavy metals); by persistence they may be persistent or short-lived; and by source they are point or non-point. Pollution indicator species show the condition of an environment: lichens indicate sulphur dioxide air pollution, tubifex worms and midge larvae of Chironomus indicate organic water pollution, coliform bacteria indicate sewage contamination, and mayfly and stonefly nymphs indicate clean water.

5 min read · UPSC Prelims · Environment & Ecology · Air, Marine, Noise and Other Pollution

At a Glance

Primary Emitted directly
Secondary Formed in air or water
Persistent Non-biodegradable
Air indicator Lichens
Water indicator Tubifex, coliform
Exam link UPSC Prelims, Environment & Ecology: Air, Marine, Noise and Other Pollution

Where Does It Fit?

This topic follows the survey of sector impacts and gives the vocabulary used in the rest of the macro: how pollutants are classed and how living organisms show pollution. Previous topic: Effects of Modern Agriculture, Housing, Power Generation, River Valley Projects, Mining, Transportation and Tourism on the Environment. Next topic: Mining and Its Environmental Impact: Direct and Indirect.

How Are Pollutants Classified?

Basis	Classes and examples
Origin	Natural (volcanic ash, forest fire smoke, dust) and anthropogenic (vehicle exhaust, industrial effluent, plastics)
Formation	Primary pollutants such as CO, SO2, NO and soot; secondary pollutants such as ozone, PAN and sulphuric acid mist
Degradability	Biodegradable (sewage, paper) and non-biodegradable (plastics, DDT, mercury)
Persistence	Persistent organic pollutants stay long; some are short-lived
Form	Solid, liquid, gas, and energy forms such as heat, noise and radiation
Medium	Air, water, soil, noise, thermal and radioactive pollution
Source type	Point sources (chimney, outfall) and non-point sources (farm runoff, urban runoff)

Which Species Indicate Pollution?

Lichens: sensitive to sulphur dioxide; their absence in cities signals polluted air
Mosses and bryophytes: absorb metals and show air pollution
Tubifex worms: thrive in oxygen-poor, organically polluted mud
Chironomus larvae: bloodworms found in polluted water with low oxygen
Coliform bacteria: show faecal contamination of water
Mayfly and stonefly nymphs: need clean, well-oxygenated water, so their presence shows good quality
Amphibians: sensitive skin shows environmental stress
Plants like petunia and tobacco Bel-W3: show leaf damage with ozone

What Else Should You Remember?

Pollutant: a substance or form of energy in the wrong place or amount
Contaminant: a substance that is present but may not be harmful
Primary pollutants: emitted directly from a source
Secondary pollutants: formed by reactions between primary pollutants and sunlight, water or other substances
Ozone: a secondary pollutant at ground level formed from NOx and volatile organic compounds
Peroxyacetyl nitrate: an eye-irritating secondary pollutant in photochemical smog
Acid rain: a secondary effect of SO2 and NOx
Biodegradable: broken down by microbes in a reasonable time
Non-biodegradable: persist in nature and accumulate
Persistent organic pollutants: such as DDT and PCBs, which are toxic, persist and travel long distances
Stockholm Convention: 2001 treaty on persistent organic pollutants
Quantitative pollutants: harmful only above certain levels, such as CO2
Qualitative pollutants: harmful in any amount, such as man-made radioactive substances
Stock pollutants: accumulate because the environment cannot absorb them quickly, such as plastics
Fund pollutants: can be absorbed up to a limit, such as sewage in a river
Biomonitoring: use of living organisms to assess pollution
Bioindicator: a species whose presence, absence or condition shows environmental quality
Sensitive indicators: disappear as pollution rises
Tolerant indicators: increase with pollution
Saprobic index: system that rates water quality using organisms
Biological Oxygen Demand: a chemical measure that complements indicator species
Lichen decline: lichens are slow-growing and absorb air pollutants directly
Link to previous: see Effects of Modern Agriculture, Housing, Power Generation, River Valley Projects, Mining, Transportation and Tourism on the Environment
Link to next: see Mining and Its Environmental Impact: Direct and Indirect
Link to indicators: see Endemic Species, Keystone Species, Indicator Species, Invasive Species
Caution: an indicator species gives a rough signal and should be combined with chemical tests
Exam hint: learn indicator species with the pollutant each shows
Heavy metals: lead, mercury and cadmium are non-degradable and build up in food chains
Particulate matter: tiny solid or liquid particles in the air, classed as PM10 and PM2.5 by size
Nutrient pollutants: nitrogen and phosphorus cause eutrophication in water
Energy pollutants: heat, noise and radiation are forms of pollution that leave no material waste
Emerging pollutants: pharmaceuticals, microplastics and personal care chemicals are of growing concern
Air Act 1981 definition: the law defines an air pollutant as any solid, liquid or gaseous substance in the atmosphere in such concentration as may be harmful

Why Does It Matter for the Exam?

Air indicator: Lichens
Water indicator: Tubifex
Secondary example: Ozone
Current affairs link: Persistent pollutants
Common errors: calling ozone a primary pollutant

As an analytical point, how a pollutant behaves depends on its class: persistent pollutants accumulate, secondary pollutants are formed away from the source and non-point pollutants are hard to trace. Strategies must match the type of pollutant.

Why Is This Topic Important?

Class Primary, secondary
Fate Degradable or not
Sign Indicators

What Came Next?

The next topic covers mining and its environmental impact, direct and indirect. Next topic: Mining and Its Environmental Impact: Direct and Indirect.

Key People and Terms

Primary pollutant Directly emitted
Secondary pollutant Formed later
POP Persistent organic pollutant
Bioindicator Pollution signal species
Tubifex Sludge worm
Lichen Alga-fungus partnership

Exam Corner

Points to Remember

Ozone is a secondary pollutant.
Lichens show SO2 pollution.
Tubifex show organic water pollution.
Plastics are non-biodegradable.
POPs: Stockholm Convention 2001.

Do Not Confuse With

Primary and secondary pollutants Emitted versus formed
Biodegradable and non-biodegradable Broken down versus persistent
Sensitive and tolerant indicators Fall with pollution versus rise with it

Memory Hook

Direct is primary, formed is secondary; lichens fade where air is foul.

Mains Angle

Questions ask for classes and indicators. These points help.

Classes Many bases
Indicators Species
Use Policy

A Question You May Face

An original practice question, not a past paper question.

“Classify pollutants and explain the use of indicator species in monitoring pollution.”

How to Answer

Define Pollutant.
Classes Origin, form.
Indicators Species.
Use Monitor.

GS Relevance

Prelims: types of pollutants, indicator species. GS Paper 3: pollution classification and bio-monitoring.

Frequently Asked Questions

What is the difference between primary and secondary pollutants?

Primary pollutants are emitted directly from a source, such as carbon monoxide, sulphur dioxide and soot from burning fuels. Secondary pollutants are not emitted directly but form in the environment through reactions of primary pollutants, for example ground-level ozone, peroxyacetyl nitrate and acid rain, which are formed with the help of sunlight, water or other substances.

What is the difference between biodegradable and non-biodegradable pollutants?

Biodegradable pollutants, such as sewage, food waste and paper, are broken down by micro-organisms and can be absorbed by nature if not in excess. Non-biodegradable pollutants such as plastics, DDT, heavy metals and radioactive wastes do not break down quickly, so they persist and accumulate in the environment and food chains.

What are pollution indicator species?

Indicator species are organisms whose presence, absence or condition shows the quality of the environment. For example, lichens are sensitive to sulphur dioxide, so they indicate clean air; tubifex worms and Chironomus larvae indicate organic pollution of water; coliform bacteria indicate sewage contamination; and mayfly nymphs indicate clean water.

Why are lichens good indicators of air pollution?

Lichens are partnerships of a fungus and an alga that absorb water and nutrients directly from the air through their surface, with no protective cuticle or roots. Sulphur dioxide and other pollutants therefore damage them quickly, and they disappear from polluted areas, so their abundance and species mix are used as measures of air quality.

What are persistent organic pollutants?

Persistent organic pollutants are toxic organic chemicals, such as DDT, PCBs and dioxins, that resist breakdown, accumulate in living tissue, move over long distances through air and water and harm health and wildlife. The Stockholm Convention, adopted in 2001, aims to eliminate or restrict their production and use.

PYQ Practice — Statement Analysis

1 Ozone at ground level is a secondary pollutant.

True

Formed by reactions.

2 Lichens indicate high levels of sulphur dioxide.

False

They indicate clean air.

3 Tubifex worms occur in organically polluted water.