How is SO₂ an air pollutant ?

SO₂ acts as an air pollutant because of the following reasons : 

(i) SO₂ is strongly irritating to the tract. SO₂ at a concentration of 5 ppm causes throat and eye irritation (resulting in cough, tears and redness in eyes). It causes breathlessness and affects larynx, i.e., voice box. 

(ii) Even at a low concentration of 0.03 ppm, it has a very damaging effect on the plants. If exposed for a long time, i.e., a few days or weeks, it shows down the formation of chlorophyll resulting in injury to the leaves including loss of green colour. This is called chlorosis. 

(iii) SO₂ dissolves in rain water and produces acid rain which damages building materials especially marble (CaCO₃).

It corrodes metals particularly iron and steel. It also brings about and deterioration of fabrics, leather, paper, etc, and affecting the colour of paints.