Similar to smog, acid rain is caused by chemical reactions of pollutants like sulfur dioxide and nitrogen oxides with oxygen and water in the air. The acidic pollutants formed in these reactions are the cause of acid rain. In addition to environmental pollution due to human activities, volcanic activity can also cause acid rain. Acid rain can be harmful to natural plant life, farm crops, animals, and marine life. It can also be damaging to buildings, depending on what materials were used in their construction.