Rain water is naturally slightly acidic, witha pH of about 5.0. Natural variations and human pollutants may cause rain to be more acidic. Depending on region, season and presence of pollutants, the pH of rain may drop to as low as 2.0 (the acidity of vinegar).

The acidity of "normal" rain is attributed to carbonic acid, a natural compound that forms during the water cycle.

Even in areas that are minimally affected by human pollution, the pH of rain can range from 4.5-5.0. Volcanic areas, including Hawaii, may experience more acidic rain because of the sulfur-based compounds that are released into the atmosphere by volcanic activity.

In non-volcanic areas, acid precipitation is generally caused by human pollution. Coal power plants release compounds that form sulfuric acid, causing rain to become as acidic as lemon juice in some areas.

Acid rain is associated with river die-off, erosion, loss of vegetation and human health problems.

While there is no way to establish the exact natural pH of water in any given area, ecologists agree that the threat of acid precipitation can be mitigated through decreased industrial dependence on fossil fuels.