Effects on Materials - Air pollutants

Most air pollutants are reactive chemicals, so they react with most of the substances around. You may recall from your chemistry lessons in school that SO₂ oxidises to form SO₃ and when dissolved in water it forms sulphuric acid which is highly caustic. Sulphuric acid reacts with metals and their weak salts and forms corresponding sulphate. In this way, in air polluted with SO₂, aluminium metal can form aluminium sulphate and lime stone and marble can form calcium sulphate called gypsum. Such reactions have caused damage to buildings, sculptures and other historical monuments made up of stone, plaster, marble or metal painted glass works. Some of them are from middle ages or even older. The stone in the Parthenon in Athens, has deteriorated more in the past 50 years from air pollution than it had in the previous 2000 years. Similarly, the statue of liberty is corroded from SO₂ and NO₂ and TajMahal from SO₂ emitted by Mathura refineries. The sand stone statues get covered with black coating that contains large amounts of gypsum. When gypsum is formed in cracks, the expansion of its crystals causes the limestone to crumble away. Such damages are indeed costly as well as a great loss to society because many of them are irreplaceable works of art.

Since SO₂, NO₂ and O₃ are strong oxidants; they soil, fade and deteriorate fabrics. Sulphur dioxide affects leather and plastic also. Ozone cracks wind shield wipers, tyres and other rubber products. Hydrogen sulphide reacts with metallic paints and tarnishes them by forming metallic sulphides. It tarnishes silverware and jewellery also. Particulate matter sullies and erodes the buildings. Soot and tar sticks to the building stones, painted surface and is difficult to remove. Such damages require a large sum of money for repair and maintenance.