Air and water pollution are caused by a variety of human activities and can harm human health and the environment. Air pollution includes smog from vehicle emissions and industrial activities, acid rain from sulfur dioxide and nitrogen oxide, the greenhouse effect from carbon dioxide trapping heat, and destruction of the ozone layer from CFC emissions. Water pollution occurs when waste is dumped into water sources, contaminating the water and affecting both wildlife and humans who use the water. Reducing pollution requires changes in individual lifestyles as well as industrial practices.