Environmental Pollution and Control II: Air Pollution Control — Settling Chambers, Cyclones, Wet Collectors, Fabric Filters and Electrostatic Precipitators, Control of Gaseous Pollutants, Indoor Air Pollution, Vehicular Emissions and Urban Air Quality — and Noise Pollution: Weighting Networks, Noise Indices, Noise Dose, Standards, Control and Vibration

Nitrogen oxides are controlled first by combustion modification (low-NOx burners, staged air and fuel, flue-gas recirculation) and then by flue-gas treatment: selective catalytic reduction injects ammonia or urea over a vanadia-titania catalyst at about 300 to 400 °C, reducing NO to nitrogen and water (4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O), while selective non-catalytic reduction injects them at about 900 to 1,100 °C without a catalyst, with lower efficiency. In December 2015 India notified emission standards for coal-fired thermal power plants covering particulate matter, sulphur dioxide, nitrogen oxides and mercury, which require flue-gas desulphurisation at many plants; the deadlines for compliance have been extended more than once.

Indoor air pollution comes above all from burning solid fuels, wood, dung, crop residues, coal and kerosene, in open fires and simple stoves, which exposes women and young children to smoke laden with fine particles, carbon monoxide and carcinogens; the WHO attributes millions of deaths a year worldwide to household air pollution. Other indoor sources are tobacco smoke, formaldehyde from pressed-wood furniture and adhesives, VOCs from paints and cleaning products, radon from soil and building materials, nitrogen dioxide and carbon monoxide from gas heaters, and moulds and dust mites in damp buildings. Sick building syndrome describes headaches, eye and throat irritation and fatigue among occupants of poorly ventilated buildings that ease when they leave. Remedies are cleaner fuels such as LPG, promoted in India under the Pradhan Mantri Ujjwala Yojana (2016), improved stoves and chimneys, source control and better ventilation.

Vehicles emit carbon monoxide and unburnt hydrocarbons, mostly from petrol engines, and nitrogen oxides and particulate matter, mostly from diesel engines, together with benzene and other air toxics. Petrol cars use a three-way catalytic converter, whose platinum, palladium and rhodium oxidise CO and hydrocarbons and reduce NOx simultaneously; it works only near the stoichiometric air-fuel ratio of about 14.7:1, held by an oxygen sensor, and it is poisoned by lead, which is why unleaded petrol was a precondition. Diesel vehicles use an oxidation catalyst, a diesel particulate filter and selective catalytic reduction with urea. India's Bharat Stage norms, modelled on the European ones, were tightened in steps: Bharat Stage IV applied nationwide from April 2017, and the country then leapfrogged Stage V to apply Bharat Stage VI nationwide from 1 April 2020, with fuel of 10 ppm sulphur, cutting permitted NOx from diesel cars by about two-thirds and requiring particulate filters. Other measures are CNG for public transport, adopted in Delhi on the Supreme Court's directions, pollution-under-control certificates, fuel-quality improvement and incentives for electric vehicles.

Urban air quality results from the combination of emissions, from vehicles, industry, construction dust, road dust, waste burning and domestic fuel, with meteorology and topography. Most Indian cities exceed the national PM₁₀ standard. The National Clean Air Programme, launched in January 2019, set city-specific action plans for non-attainment cities, those that persistently fail the standards, with an initial goal of a 20 to 30% cut in particulate concentrations by 2024 against 2017, later revised to up to 40% by 2025–26, and links funding to measured improvement. Source apportionment studies, using emission inventories and receptor models that match the chemical fingerprints of particles to their sources, guide which measures to take in each city.

Noise is unwanted sound. Its main sources are road, rail and air traffic, industry and construction, loudspeakers and firecrackers, and domestic appliances. Because the ear responds to an enormous range of pressures, sound is measured on the logarithmic decibel scale: sound pressure level Lp = 20 log₁₀(p ÷ p₀), with p₀ = 20 µPa, the threshold of hearing, equivalent to 10 log₁₀(I ÷ I₀) for intensity. Levels therefore add logarithmically: L = 10 log₁₀ Σ 10^(Lᵢ/10). Two equal sources give 3 dB more than one (80 dB + 80 dB = 83 dB); three equal sources of 85 dB give 85 + 10 log₁₀ 3 ≈ 89.8 dB; and a source 10 dB quieter than another adds only about 0.4 dB. In the open, the level from a point source falls by about 6 dB for each doubling of distance and from a line source such as a busy road by about 3 dB.

Weighting networks filter a sound level meter's response to match human hearing, which is least sensitive at low and very high frequencies. The A-weighting approximates the ear at moderate levels and is used for almost all environmental and occupational standards, levels being written dB(A); C-weighting is nearly flat and is used for high levels and peak measurements; D-weighting was designed for aircraft noise; and Z (zero) weighting is unweighted. Because noise varies over time, it is summarised by statistical and energy-averaged indices.

Two worked cases. A worker is exposed to 90 dB(A) for 2 hours and 70 dB(A) for 6 hours: Leq(8 h) = 10 log₁₀[(2 × 10⁹ + 6 × 10⁷) ÷ 8] = 10 log₁₀(2.575 × 10⁸) ≈ 84.1 dB(A), far above the arithmetic mean of 75 dB, because the loud period dominates the energy. At a roadside with L10 = 75 dB(A) and L90 = 60 dB(A), TNI = 4 × 15 + 60 − 30 = 90.

Noise dose expresses occupational exposure against the permissible time at each level: D = Σ Cᵢ ÷ Tᵢ, where Cᵢ is the actual time spent at a level and Tᵢ the time permitted at it; a dose above 1 (100%) exceeds the limit. Under the common criterion of 90 dB(A) for 8 hours with a 5 dB exchange rate, the permitted time halves for each 5 dB increase, to 4 hours at 95 dB(A) and 2 hours at 100 dB(A). A worker spending 4 hours at 95 dB(A) and 2 hours at 90 dB(A) receives D = 4/4 + 2/8 = 1.25, which exceeds the limit. Ambient noise in India is governed by the Noise Pollution (Regulation and Control) Rules 2000, framed under the Environment (Protection) Act 1986.

The Rules also forbid loudspeakers and public address systems at night, from 10 p.m. to 6 a.m., except in closed premises, subject to limited relaxation by the state on a few days a year. Noise is controlled at the source, along the path and at the receiver. At the source: quieter machines, maintenance, vibration isolation mounts, enclosures and silencers. Along the path: distance, barriers and berms, absorptive linings and, to a small extent, dense green belts. At the receiver: ear plugs and muffs, soundproofed rooms and limiting exposure time by job rotation. Passive methods use materials and structures to absorb, block or isolate sound and work best at high frequencies; active noise control generates an anti-phase signal that cancels the noise by destructive interference and works best at low frequencies, as in noise-cancelling headphones and ducts.

Vibration is oscillatory motion transmitted through solids, measured with accelerometers as displacement, velocity or acceleration against frequency; for blasting and structural damage the peak particle velocity is the standard measure. Whole-body vibration, from vehicles and heavy machinery, is linked to back disorders, and hand-arm vibration from power tools causes hand-arm vibration syndrome, including vibration white finger, a vascular and nerve disorder. The effects of noise on health are auditory and non-auditory. Prolonged exposure above about 85 dB(A) causes noise-induced hearing loss, a permanent threshold shift that typically appears first as a dip in hearing near 4,000 Hz, preceded by temporary threshold shifts and often accompanied by tinnitus. Non-auditory effects include annoyance, interference with speech and learning, sleep disturbance, stress and raised blood pressure and cardiovascular risk.