Pariksha SaathiLesson 6 of 10 · Environment and Ecology
Pollution: Air, Water, Soil, Noise, Radioactive and Solid Waste
Drafted with AI assistance and fact-checked on 28 October 2026; not yet read line by line by a human editor. Spotted a mistake? Tell us. Editorial standards & corrections
Pollutants and their kinds
Pollution is any undesirable change in the physical, chemical or biological character of air, water or soil that harms living things. Primary pollutants are released directly, such as sulphur dioxide, carbon monoxide and soot, while secondary pollutants form in the air through reactions, such as ground-level ozone and peroxyacetyl nitrate (PAN). Biodegradable pollutants such as sewage are broken down by microbes; non-biodegradable ones such as plastics, DDT and heavy metals persist.
Air pollutants and the AQI
Carbon monoxide from incomplete burning in vehicles binds to haemoglobin far more readily than oxygen. Sulphur dioxide comes mainly from coal-fired power plants and smelters, and nitrogen oxides from vehicles and power plants. Particulate matter is graded by size: PM10 particles are smaller than 10 micrometres and PM2.5 smaller than 2.5 micrometres, small enough to reach deep into the lungs. Electrostatic precipitators remove over 99 per cent of particulate matter from power-plant exhaust, scrubbers remove gases such as sulphur dioxide, and catalytic converters in cars, which need unleaded petrol, turn carbon monoxide and hydrocarbons into carbon dioxide and water. India's National Air Quality Index has six bands: Good (0-50), Satisfactory (51-100), Moderately Polluted (101-200), Poor (201-300), Very Poor (301-400) and Severe (401-500).
Smog and acid rain
Smog is smoke mixed with fog. Classical or sulphurous smog, like the London smog of December 1952, forms in cold, damp weather from coal smoke and sulphur dioxide. Photochemical smog forms on warm sunny days when nitrogen oxides and hydrocarbons from vehicles react in sunlight to make ozone and PAN, irritating the eyes and lungs. Smog worsens under a temperature inversion, when warm air sits over cooler air near the ground and traps pollutants. Normal rain is slightly acidic, with a pH of about 5.6, because carbon dioxide dissolves in it; acid rain is more acidic because sulphur dioxide and nitrogen oxides form sulphuric and nitric acids. It harms forests, lakes, crops and buildings, and the yellowing of the Taj Mahal's marble led to the Taj Trapezium Zone, where polluting industries are restricted.
Water pollution, BOD and eutrophication
The biggest source of water pollution in India is untreated domestic sewage, followed by industrial effluents and the run-off of fertilisers and pesticides from farms. Biochemical oxygen demand (BOD) is the oxygen that micro-organisms need to break down the organic matter in a water sample, usually measured over 5 days at 20 degrees Celsius; the higher the BOD, the more polluted the water. When nitrates and phosphates from fertilisers, detergents and sewage enrich a lake, algae multiply in a bloom; when they die, their decay uses up dissolved oxygen, fish die and the lake ages fast. This is eutrophication, called cultural or accelerated eutrophication when humans cause it. Water hyacinth, known as the terror of Bengal, chokes such nutrient-rich waters.
Soil, noise and radioactive pollution
Soil is polluted by excess fertilisers and pesticides, industrial and mining waste, and heavy metals. Noise is measured in decibels (dB), and a 1987 amendment to the Air Act brought noise within the meaning of air pollutant. The Noise Pollution (Regulation and Control) Rules 2000 set day and night limits of 75 and 70 dB for industrial areas, 65 and 55 for commercial areas, 55 and 45 for residential areas and 50 and 40 for silence zones (at least 100 metres around hospitals, schools and courts), with day counted from 6 am to 10 pm. A sound of 150 dB or more, as from a jet or rocket take-off, can permanently damage the eardrum. Radioactive pollution comes from nuclear tests, accidents, mining and nuclear waste. The Chernobyl disaster of 26 April 1986 in Ukraine, then in the Soviet Union, and the Fukushima accident in Japan after the 2011 earthquake and tsunami are the major examples.
Solid waste, plastics and e-waste
The Solid Waste Management Rules 2016 require waste to be segregated at source into wet (biodegradable), dry and domestic hazardous waste. Wet waste can be composted or turned into biogas and dry waste recycled; the rest should go to scientific sanitary landfills rather than open dumps. Plastics do not biodegrade; they break into microplastics, pieces smaller than 5 millimetres, now found in oceans, fish and drinking water. E-waste, from discarded computers, phones and appliances, contains lead, mercury and cadmium; India's e-waste rules, first notified in 2011, make producers responsible for collecting their products under Extended Producer Responsibility (EPR).
Famous disasters
In the Bhopal gas tragedy on the night of 2-3 December 1984, methyl isocyanate (MIC) gas leaked from the Union Carbide pesticide plant and killed thousands; it led to the Environment (Protection) Act 1986 and the Public Liability Insurance Act 1991. Minamata disease, identified in Japan in 1956, was caused by methylmercury that a chemical factory discharged into Minamata Bay and that people ate in fish; the Minamata Convention on mercury (2013) is named after it. Itai-itai disease in Japan came from cadmium in river water used for rice, and the Seveso accident in Italy in 1976 released dioxin.
Key facts
- Primary pollutants are emitted directly (SO2, CO, soot); secondary pollutants form in the air (ground-level ozone, PAN, acid in acid rain).
- PM10 = particles below 10 micrometres; PM2.5 = below 2.5 micrometres, which reach deep into the lungs.
- India's AQI bands: Good 0-50, Satisfactory 51-100, Moderately Polluted 101-200, Poor 201-300, Very Poor 301-400, Severe 401-500.
- Control devices: electrostatic precipitator removes over 99 per cent of particulates; scrubbers remove SO2; catalytic converters need unleaded petrol.
- London smog (1952) = sulphurous smog from coal; photochemical smog = NOx plus hydrocarbons in sunlight, producing ozone and PAN; normal rain pH is about 5.6.
- BOD is measured over 5 days at 20 degrees Celsius; high BOD means heavy organic pollution; nitrates and phosphates cause eutrophication.
- Noise limits (day/night, dB): industrial 75/70, commercial 65/55, residential 55/45, silence zone 50/40; noise added to the Air Act in 1987.
- Microplastics are plastic pieces smaller than 5 mm; e-waste rules (first 2011) use Extended Producer Responsibility; Solid Waste Management Rules 2016 require segregation at source.
- Bhopal 1984: methyl isocyanate from Union Carbide, which led to the Environment (Protection) Act 1986 and the Public Liability Insurance Act 1991.
- Disaster pairs: Minamata (1956, Japan) = mercury; itai-itai (Japan) = cadmium; Seveso (1976, Italy) = dioxin; Chernobyl (1986) and Fukushima (2011) = radiation.
Quick revision
Primary vs secondary pollutants; PM10, PM2.5; AQI six bands, 0-500; London smog 1952 sulphurous; photochemical smog gives ozone and PAN; normal rain pH 5.6; BOD over 5 days at 20 C; eutrophication by nitrates and phosphates; noise rules 2000, silence zone 50/40 dB; SWM Rules 2016; microplastics below 5 mm; Bhopal 1984 MIC; Minamata mercury; itai-itai cadmium; Chernobyl 1986.