Many types of human industry and activity produce waste products that pollute the air. The major source of air pollution in many parts of the United States and other countries is the exhaust from automobiles and other motor vehicles. In some cities, the exhaust develops into a highly noticeable form of air pollution called smog. Air pollution is believed to contribute significantly to bronchitis, asthma, emphysema, lung cancer, cardiovascular diseases, and allergies. Many kinds of plant damage have also been attributed to air pollution. In addition, there is concern that air pollution—primarily large amounts of carbon dioxide and smoke—may have long-range effects on climate.