Acid rain is considered when pH falls below:
Unpolluted rain is mildly acidic because dissolved carbon dioxide forms carbonic acid, giving an expected pH near 5.6 rather than 7. Precipitation below approximately 5.6 is conventionally described as acid rain, usually after sulfuric and nitric acids add stronger acidity. Because pH is logarithmic, a fall of one pH unit represents a tenfold increase in hydrogen-ion activity. Sound classification links the source, atmospheric fate, exposure route, and mechanism of toxicity rather than relying only on the pollutant's name. Effective management combines cleaner fuels, emission standards, monito
Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology