When sulphur containing fossil fuel is burnt, oxides of sulphur are produced.

The most common species sulphur dioxide, is a gas that is poisonous to both animals and plants. A low concentration of sulphur dioxide causes respiratory disease. e.g., asthma,

bronchitis, emphysema in human beings. Sulphur dioxide causes irritation to the eyes, resulting in tears and redness.

High concentration of $\mathrm{SO}_{2}$ leads to stiffness of flower buds which eventually fall off from plants. Uncatalysed oxidation of sulphur dioxide is slow. The presence of particulate matter in polluted air catalyses the oxidation of sulphur dioxide to sulphur trioxide.

$2 \mathrm{SO}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})} \rightarrow 2 \mathrm{SO}_{3(\mathrm{~g})}$

The reaction can also be promoted by ozone and hydrogen peroxide.

$\mathrm{SO}_{2(\mathrm{~g})}+\mathrm{O}_{3(\mathrm{~g})} \rightarrow \mathrm{SO}_{3(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})}$

$\mathrm{SO}_{2(\mathrm{~g})}+\mathrm{H}_{2} \mathrm{O}_{2(\mathrm{~g})} \rightarrow \mathrm{H}_{2} \mathrm{SO}_{4(\mathrm{aq})}$