In extreme cases, it has altered entire biological communities and eliminated some fish species from certain lakes and streams. In many other cases, the changes have been more subtle, leading to a reduction in the diversity of organisms in an ecosystem. This is particularly true in the northeastern United States, where the rain tends to be most acidic, and often the soil has less capacity to neutralize the acidity. Acid rain also can damage certain building materials and historical monuments. Some scientists have suggested links to human health, but none have been proven.
Acidity is measured on the per-hydrogen, or pH scale.
This is a measure of the concentration of positively charged ions in a given sample. Generally rain with a pH value of less than about 5.3 is considered acid rain. As the map below shows, most of the rainwater which falls in the eastern United States has a pH between 4.0 and 5.0.