Sulphur dioxide is considered an air pollutant because it readily undergoes oxidation in the atmosphere to form Sulphur trioxide which then reacts with water vapour to form sulphuric acid . Which comes down in the form of acid rain. acid rain causes deforestation which is also not good for the environment.
Even in low concentration of causes the irritation in the eyes, respiratory problem and due to this it affects the larynx to cause breathlessness.
Harmful for plants also, long exposure to can reduce the colour of the leaves. It is because the formation of chlorophyll is affected by sulphur dioxide.