mid-14c., "discharge of semen other than during sex," later, "desecration, defilement" (late 14c.), from Late Latin pollutionem (nominative pollutio ) "defilement," noun of action from past participle stem of Latin polluere "to soil, defile, contaminate," from por- "before" + -luere "smear," from PIE root *leu- "dirt; make dirty" (cf. Latin lutum "mud, mire, clay," lues "filth;" Greek lyma "filth, dirt, disgrace," lymax "rubbish, refuse;" Old Irish loth "mud, dirt;" Lithuanian lutynas "pool, puddle"). Sense of "contamination of the environment" first recorded , but not common until .
They occur mainly in the form of sulphur dioxide. It is produced in large quantity during smelting of metallic ores and burning of petroleum and coal in industries, thermal plants, home and motor vehicles. In the air, SO 2 combines with water to form sulphurous acid (H 2 SO 3 ) which is the cause of acid rain. It causes chlorosis and necrosis of vegetation. Sulphur dioxide, above 1 ppm, affects human beings. It causes irritation to eyes and injury to respiratory tract. It results in discolouration and deterioration of buildings, sculptures, painted surfaces, fabrics, paper, leather, etc.