Noise, Thermal and Radioactive Pollution: Limits and Units

The correct answer is B, Decibel. The decibel, written dB, is the unit in which the intensity level of sound is expressed, and it is named after Alexander Graham Bell. The scale is logarithmic, so every rise of 10 dB stands for a ten-fold rise in sound intensity; zero decibel is the threshold of hearing and about 120 to 130 decibel is the threshold of pain. For legal limits the A-weighted decibel is used because it follows the response of the human ear. A is wrong because the ohm is the unit of electrical resistance. C is wrong because the joule is the unit of work and energy, and although sound carries energy, its loudness is not reported in joule. D is wrong because the ampere is the unit of electric current. Candidates who confuse these pick an electrical unit instead of the acoustic one.

The correct answer is C, 2000. The Noise Pollution (Regulation and Control) Rules were notified in the year 2000 and they were framed under the Environment (Protection) Act, 1986. These Rules create the four zones, industrial, commercial, residential and silence zone, fix a day limit and a night limit for each, define day as 6 a.m. to 10 p.m. and night as 10 p.m. to 6 a.m., and restrict loudspeakers at night. A is wrong because 1986 is the year of the parent Environment (Protection) Act, which is the enabling law and not the noise rules themselves; this is the commonest trap in the question. B is wrong because 1991 is associated with the Public Liability Insurance Act. D is wrong because 2010 is the year of the National Green Tribunal Act, which created the tribunal that later heard noise cases.

The correct answer is B, 55 dB(A). The Rules of 2000 lay down 55 decibel as the day-time limit and 45 decibel as the night-time limit for a residential area. Reading the full table across is the safest way to answer such questions: industrial 75 by day and 70 by night, commercial 65 and 55, residential 55 and 45, and silence zone 50 and 40. A is wrong because 45 decibel is the night-time limit for the same residential zone, and the swapping of day and night values is the usual trap. C is wrong because 65 decibel is the day-time limit for a commercial area. D is wrong because 75 decibel is the day-time limit for an industrial area, the highest of the four zones. Day runs from 6 a.m. to 10 p.m.

The correct answer is B, 100 metres. The Rules define a silence zone as the area up to 100 metres around a hospital, an educational institution or a court, and such an area has to be declared by the competent authority. Inside it the limits are the strictest of the four zones, 50 decibel by day and 40 by night, and horns, drums, loudspeakers and firecrackers are not allowed. A is wrong because 50 is the decibel limit of the silence zone by day, not a distance, and the number is placed in the options to mislead. C is wrong because 200 metres has no place in these Rules. D is wrong because 500 metres is far wider than the Rules provide; the limit is deliberately kept small so that the zone can actually be enforced around a single building.

The correct answer is B, a fall in the dissolved oxygen content. The solubility of a gas in water decreases as the temperature of the water rises, so hot water discharged from a power station holds less dissolved oxygen. The damage is doubled because the metabolic rate of fish and other aquatic animals roughly doubles for every ten degrees of warming, so their demand for oxygen rises just as the supply falls. A is wrong because it states the relation in reverse; only cold water is rich in dissolved oxygen, which is why cold mountain streams support trout. C is wrong because salinity is raised by evaporation or by saline effluent, not by waste heat as such. D is wrong because a sharp fall in pH is caused by acidic effluent or acid rain, not by thermal discharge.

The correct answer is A, cooling water from power stations. Thermal and nuclear power stations draw large volumes of water to condense steam and return it several degrees warmer, which makes them the single largest source of waste heat in rivers, lakes and coastal waters. Refineries, steel plants, paper mills and distilleries follow. B is wrong because municipal sewage is the chief source of organic and bacterial pollution, raising the biochemical oxygen demand, but it is not discharged hot. C is wrong because agricultural run-off carries nitrate and phosphate and causes eutrophication, a nutrient problem rather than a heat problem. D is wrong because plastic waste is a physical pollutant that chokes and entangles aquatic life and breaks down into microplastics; it adds no heat to the water at all.

The correct answer is B, cooling tower. A cooling tower releases the heat of used cooling water to the atmosphere by evaporation or by air flow, so that the water leaving the plant is close to the temperature of the receiving river or lake. Cooling ponds, spray ponds and the reuse of waste heat through cogeneration work on the same principle. A is wrong because an electrostatic precipitator removes fine dust and fly ash from flue gas by charging the particles, an air pollution device. C is wrong because a cyclone separator uses the spin of the gas stream to throw out coarse particles, again an air pollution device. D is wrong because a scrubber washes gases such as sulphur dioxide out of a flue stream with a liquid or a slurry of lime.

The correct answer is B, bones. Strontium lies just below calcium in the same group of the periodic table and behaves chemically like it, so strontium-90 taken in through milk, water or food is deposited in bone and in teeth in place of calcium. With a half-life of about twenty-nine years it then irradiates the bone marrow for decades and can cause bone cancer and leukaemia. A is wrong because the thyroid gland is the organ that concentrates iodine-131, which is why potassium iodide tablets are given after a reactor accident. C is wrong because the liver is a general site of storage for several heavy metals and for plutonium, but not the specific target of strontium. D is wrong because the retina is damaged by ultraviolet and by very high doses of radiation generally, and no isotope collects there selectively.

The correct answer is B, becquerel. The becquerel, symbol Bq, is the SI unit of activity and equals one disintegration per second. A is wrong because the curie is the older, non-SI unit of the same quantity, defined from the activity of one gram of radium, and one curie equals 3.7 multiplied by ten raised to the power ten becquerel; it is a correct unit of radioactivity but not the SI one, which makes it the strongest distractor here. C is wrong because the roentgen measures exposure, that is the ionisation produced in air, and not the activity of the source. D is wrong because the sievert measures equivalent dose, the biological effect of the radiation absorbed by tissue; the related unit gray measures absorbed dose, with one gray equal to 100 rad and one sievert equal to 100 rem.

The correct answer is B, 1986. Reactor number four of the Chernobyl plant, in Ukraine in the then Soviet Union, exploded on 26 April 1986 and scattered radioactive iodine, caesium and strontium over a wide area of Europe. A is wrong because 1979 is the year of the Three Mile Island accident in Pennsylvania in the United States, a partial meltdown with far smaller release. C is wrong because 1995 has no major nuclear accident attached to it; it is placed in the options only as filler. D is wrong because 2011 is the year of the Fukushima Daiichi accident in Japan, which followed the Tohoku earthquake and tsunami of 11 March. Chernobyl and Fukushima are the only two events rated at level 7, the highest, on the International Nuclear Event Scale.

The correct answer is B, about 8 days. Iodine-131 has a half-life of roughly eight days, which is why the danger from it after an accident is intense but short-lived, and why potassium iodide tablets are distributed at once so that the thyroid gland is saturated with ordinary iodine and absorbs less of the radioactive form. A is wrong because eight hours is far too short; an isotope of that life would decay away before it spread beyond the plant. C is wrong because eight months would keep the contamination of milk and vegetables going for years, which is not what is observed after a release. D is wrong because eight years belongs to the long-lived group; it is caesium-137 at about thirty years and strontium-90 at about twenty-nine years that make an area uninhabitable for decades.

The correct answer is B, Geiger-Muller counter. A Geiger-Muller counter contains a gas-filled tube in which an entering ray ionises the gas and sets off a pulse of current, so each pulse counts one particle; a film badge worn by a worker records the total dose instead. A is wrong because an anemometer measures wind speed. C is wrong because a hygrometer measures the humidity of air. D is wrong because an audiometer measures hearing sensitivity and is used to detect noise-induced hearing loss, which makes it a tempting choice in a chapter that also covers noise pollution, but it measures the response of the ear and not radiation. Candidates should link each pollutant to its instrument: decibel meter for noise, thermometer for thermal load and Geiger-Muller counter for radiation.