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.
Groundwater contamination may result from leakage of very small amounts of leachate. TCE is a carcinogen typically found in landfill leachate. It would take less than 4 drops of TCE mixed with the water in an average swimming pool (20,000 gallons) to render the water undrinkable. Some surveys conducted have shown that 82% of the landfills have leaks and up to 41% of the landfills had a leak area of more than one square foot. EPA sponsored research shows that burying household garbage in the ground poisoned the ground water. The EPA has stressed that, even with the double liner landfills, the probability of leaking is very high.