Acid rain is measured through pH tests which determine the concentration of hydrogen ions. Pure water has a neutral pH of 7. When the pH is greater than 7, the material is said to be alkaline, or basic; acids have a pH less than 7. At a natural pH of 5.7, pure rain has a little bit of acid due to its mixture with naturally-evolving carbon dioxide in the atmosphere. However, when its pH is further reduced, the rain becomes pollution. In the worst cases, acid rain has shown a pH of 2.4, which is as acidic as vinegar. When pH levels are drastically skewed in soil and water, entire lakes and forests are jeopardized. Evergreen trees in high elevations are especially vulnerable. Although the acid rain itself does not kill the trees, it makes them more susceptible to other dangers and stresses. Plants and trees that are exposed to acid rain are less effective at photosynthesis and respiration. Also, high acid levels in soil cause leaching, or removal, of other valuable minerals such as calcium, magnesium, and potassium. According to the World Watch Institute in 1988, forest damage in Europe ranged from a low of 4% in Portugal, to a high of 71% in Czechoslovakia, averaging 35% overall.
