Acid rain is rain with a pH lower than 5 that is caused by sulfur and nitrogen oxides combining with water from the atmosphere. These acid anhydrides are emitted primarily from coal burning power plants and automobile exhaust, and can accelerate structure decay, hinder plant growth, and damage aquatic ecosystems. The Clean Air Act and technologies like catalytic converters and scrubbers have helped reduce acid rain by legislating emission standards and encouraging cleaner energy use.