Acid rain forms when rainwater mixes with pollutants, primarily sulfur dioxide and nitrogen oxides, emitted from vehicles, factories, and power plants. These pollutants react with moisture in the atmosphere, creating acids that fall to Earth as precipitation.

This polluted rain can damage ecosystems by harming plants, aquatic life, and soil chemistry, as well as corroding buildings and statues made from marble or limestone. Efforts to reduce emissions from industry and transportation have significantly decreased the occurrence of acid rain in recent decades.