Sulphur dioxide is considered one of the most corrosive gases that contribute significantly to the process known as "stone decay" in buildings. Its corrosive nature poses substantial risks to architectural structures constructed with materials such as limestone and marble. Here are the effects of sulfur dioxide on buildings:

1. Acid Rain Formation:

- Sulphur dioxide emissions interact with water vapor and oxygen in the atmosphere to form acidic compounds.

- These acidic compounds (sulfuric acid and sulfurous acid) contribute to the formation of acid rain.

2. Reaction with Building Materials:

- The acidic nature of sulfur dioxide and acid rain causes direct chemical reactions with building materials.

- Limestone (calcium carbonate) and marble (calcium-magnesium carbonate) react with acids, leading to the dissolution of the minerals.

3. Erosion and Weathering:

- The chemical reaction results in the erosion and weathering of these materials, causing surfaces to become pitted and rough.

- Over time, this weakens the structure and leads to deterioration and decay of the building's exterior.

4. Discoloration:

- Sulphur dioxide can also cause discoloration of building surfaces.

- The reaction between sulfur dioxide and moisture forms sulfates, which appear as dark crusts or stains on building facades.

5. Structural Damage:

- In extreme cases, the continued exposure to sulfur dioxide can lead to severe structural damage.

- The weakened stone and mortar may crumble and break away, compromising the stability and safety of the building.

The impact of sulfur dioxide on buildings varies based on several factors such as the concentration of the gas, the type and composition of the construction materials, and the prevailing climate conditions in the region. Nevertheless, it remains a major concern for the preservation of historical monuments, cultural heritage sites, and architectural landmarks worldwide.