Air Pollution and Its Control

Clean air is essential for every living organism, and once it becomes contaminated with pollutants it can injure plants, slow crop growth and yield, and damage the respiratory systems of humans and animals. How harmful a given pollutant is depends on three things: its concentration in the air, how long an organism is exposed to it, and the organism itself — some species and some individuals are far more sensitive than others.

Where industrial air pollution comes from

Chimneys of thermal power stations, metal smelters and many other factories release a mixture of gases. Along with harmless gases such as nitrogen and oxygen, this exhaust also carries particulate matter (fine solid or liquid specks suspended in air) and harmful gases. Before this exhaust can be let out into the atmosphere, industries are expected to filter or trap out the harmful components.

Electrostatic precipitator. The most widely used device for removing particulate matter is the electrostatic precipitator, which can trap more than 99 per cent of the particulates present in a thermal power plant's exhaust. Inside it, electrode wires are held at a very high voltage; this creates a corona (a glow of ionised air) that releases free electrons. These electrons stick to the dust particles passing through, giving each one a negative charge. Metal collecting plates on either side of the airstream are earthed, so they attract and hold these now-charged particles, while the cleaned air moves on. For this to work well the air must move slowly enough between the plates for the charged dust to settle out.

Scrubbers. A companion device, the scrubber, is used to remove acidic gases such as sulphur dioxide. Here the exhaust stream is passed through a fine spray of water or lime, which absorbs the gas.

• cover: 16.1 Electrostatic precipitator — a two-part diagram showing a scrubber tower (dirty air enters at the base, a water/lime spray zone near the top absorbs SO2, clean air exits through a chimney, and collected particulate matter drains at the bottom) alongside an electrostatic precipitator duct (dirty air carrying dust particles passes negatively charged discharge-corona wires and is drawn to grounded collection plates, releasing clean, dust-free air).

The PM2.5 problem. Even after such filtration, very small particulates can escape capture. The Central Pollution Control Board (CPCB) has identified particulate matter of 2.5 micrometres or less in diameter — PM2.5 — as especially dangerous, because particles this small are inhaled deep into the lungs. There they cause breathing difficulty, irritation, inflammation, lasting lung damage, and can even lead to premature death.

Vehicular pollution

In India's large cities, automobiles are now a leading source of air pollution, and the problem is spreading from the metros to smaller towns as vehicle numbers rise everywhere. Regular servicing of vehicles and the use of lead-free fuel both help cut down emissions. Most modern vehicles also carry a catalytic converter, a device fitted into the exhaust system that uses costly metal catalysts — platinum, palladium and rhodium — to clean up the exhaust as it passes through: unburnt hydrocarbons are converted to carbon dioxide and water, while poisonous carbon monoxide and nitric oxide are converted to carbon dioxide and nitrogen gas. Because lead poisons (inactivates) these catalysts, any vehicle fitted with a converter must run only on unleaded petrol.

Noise as an air pollutant

India's Air (Prevention and Control of Pollution) Act, passed in 1981, was amended in 1987 to formally recognise noise as a form of air pollution. Noise is simply sound at an undesirably high level, and although we often associate loud sound with entertainment, chronic or extreme exposure causes real physiological and psychological harm. A single brief exposure to extremely high sound levels — 150 decibels or more, as produced by a jet or rocket taking off — can permanently rupture the eardrum and impair hearing. Even the comparatively lower, chronic noise levels of a large, busy city can permanently damage hearing over time, and noise is also linked to sleeplessness, a faster heartbeat and altered breathing — in short, physiological stress. Reducing noise pollution calls for both engineering fixes, such as using sound-absorbing materials or muffling noisy machinery in industry, and strict enforcement of existing rules — for instance, horn-free zones around hospitals and schools, limits on the loudness of firecrackers and loudspeakers, and restrictions on the hours loudspeakers may be used.