Air Pollution

Because respiratory surfaces are in constant contact with the surrounding air, any unfavourable change in air quality directly affects the respiratory system; the severity of harm depends on the concentration of the pollutant, the duration of exposure and the organism affected. Even plants suffer from air pollution, showing poor crop yield and premature death. Automobiles are now a dominant source of air pollution, and regular vehicle maintenance together with the use of lead-free petrol or diesel helps reduce pollutant emissions from exhausts.

Air pollutants fall into two broad types. Particulate pollutants are solid particles or fine liquid droplets (aerosols); particles around 10 micrometres in diameter tend to settle onto the soil, while particles of 1 micrometre or less remain suspended in the air for long periods. According to the Central Pollution Control Board (CPCB), particulate matter of 2.5 micrometres or less (PM2.5) causes the greatest harm to human health, since these fine particles can be inhaled deep into the lungs, causing irritation, inflammation, breathing disorders and even premature death; smoke, smog, pesticides, heavy metals, dust and radioactive particles are all examples of particulate pollutants.

Gaseous pollutants include carbon dioxide (CO2), carbon monoxide (CO), sulphur dioxide (SO2), nitric oxide (NO) and nitrogen dioxide (NO2). Carbon dioxide levels, once fairly stable, are rising rapidly due to the burning of fossil fuels and increasing deforestation; photosynthesis and the natural weathering of silicate rocks (which removes CO2 by forming limestone) partly offset this, but the balance is under growing strain -- a single jet crossing the Atlantic, for instance, consumes about 35 tonnes of oxygen and releases about 70 tonnes of CO2. Carbon monoxide, a poisonous gas, forms from the incomplete combustion of fuels such as coal or wood, with vehicle exhaust being its largest source; because CO binds to haemoglobin far more readily than oxygen does, it reduces the blood's ability to carry oxygen to body tissues, which is why CO exposure typically produces giddiness, exhaustion and headache, and can be fatal at high concentrations. Nitrogen dioxide and nitrogen monoxide, released by automobiles and chemical industries, are equally harmful -- NO2 combines with water vapour to form nitric acid, causing eye and lung irritation and, at high concentrations, injury to the lungs, liver and kidneys.

Several engineered control measures help curb these pollutants. Electrostatic precipitators (Fig. 15.4) are widely used to remove particulate matter -- soot and dust -- from industrial exhaust; a high voltage ionises the smokestack air, charged dust particles are drawn to oppositely charged collector plates, and vibration periodically dislodges the collected dust into a reservoir, removing up to about 99% of particulate matter from a thermal power plant's exhaust. Exhaust gas scrubbers (Fig. 15.5) clean both dust and gases such as SO2 by passing exhaust through a spray of water or lime, while catalytic converters (Fig. 15.6), fitted to vehicles that must run on unleaded petrol (since lead deactivates the catalyst), convert harmful oxides of nitrogen and hydrocarbons in the exhaust into far less harmful nitrogen, oxygen, carbon dioxide and water. …