Acid rain is caused by sulfur dioxide and nitrogen oxides from the burning of fossil fuels combining with water vapor and falling as rain with a pH lower than normal. It has harmful effects on forests as it damages trees and soil, on aquatic life by killing fish and affecting reproduction, on buildings by corroding stone and other materials, and on humans by potentially causing respiratory illness. Solutions include reducing emissions at power plants, decreasing electricity usage, switching to renewable energy sources, and improving vehicle emissions controls.