Acid rain is formed when pollution gases like sulfur dioxide and nitrogen oxides from cars, power plants, and factories mix with water in the air. When the polluted water falls as rain, it harms trees and plants and pollutes rivers, damaging ecosystems. Sulfur and nitrogen are primarily responsible for acid rain's harmful effects. While sulfur dioxide emissions have decreased since 1970, nitrogen emissions have increased due to rising vehicle use. Acid rain acidifies lakes and streams, damages forests by killing large groups of trees, and deteriorates materials like paint, stone, and buildings through erosion.