Acid rain is defined as any type of precipitation with a pH that is unusually low.
See also:
Dissolved carbon dioxide dissociates to form weak carbonic acid giving a pH of approximately 5.6 at typical atmospheric concentrations of CO2.
There is a strong relationship between lower pH values and the loss of populations of fish in lakes.
Below 4.5 virtually no fish survive, whereas levels of 6 or higher promote healthy populations..
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