What is acid rain? Acid rain is the term for pollution caused when sulfur and nitrogen dioxides combine with atmospheric moisture. The term 'acid rain' is slightly misleading, and would be more accurate if deemed 'enhanced acid rain', as rain occurs acidic naturally. Acidity is measured on what is know as the pH scale. Fourteen is the most basic, seven is the most neutral, and zero is the most acidic. Pure rain has a pH level of 7, which is exactly neutral. The acidity of rain is determined by the pH of pure water in reaction with atmospheric concentrations of carbon dioxide, resulting in carbonic acid. These particles partly dissociate to produce hydrogen ions and bicarbonate ions. A bicarbonate atom is an ion formed by one hydrogen atom, one carbon at atom, and three oxygen atoms, and is very effective in natural waters at neutralizing hydrogen ions and reducing acidity. The dissociation results in the natural acidity of pure rain, which is moderately acidic at a pH of 5.7. Rain less than 5.7 is considered 'acid rain', meaning it has reacted with acidic atmospheric gases other than carbon dioxide, such as sulfur dioxide and nitrogen dioxide. Sulfur dioxide is produced by electric utilities, industrial, commercial and residential heating, smelters, diesel engines and marine and rail transport, which creates sulfuric acid in rain. Nitrogen dioxide will also react with the rain, caused largely by transportation (cars, trucks, planes, etc.) and electric utilities, producing nitric acid. There is a certain degree of naturally occurring acidity in rain water. This acid is from reaction with alkaline chemicals, found in soils, lakes and stream, and can occasionally occur when a volcano erupts as well. Bacterial action in soils and degasing from oceanic plankton also contribute to the acidity found in rain. More than 90% of the sulfur and 95% of the nitrogen emissions which occur in North America are due to the pollution created by humans.