Acid rain

These gases also cause acid rain. They form acids when they mix with water in the atmosphere. That acidic water can then fall as rain.

When the droplets fall as rain, snow or other precipitation, the result is acid rain.

Sulfur dioxide irritates the respiratory system. It can also form acids in rainwater. This acid rain can harm trees and aquatic life and wear away materials such as limestone in buildings.

Others are indirect: pollutants undergo reactions in the atmosphere, producing acid rain or contributing to smog. Scientists therefore monitor pollutant concentrations as well as developing ways to prevent or remove emissions.