Which pH value is indicative of highly acidic rain?
Rain at pH 4.3 is strongly acidic relative to normal rain, whose pH is near 5.6 because of dissolved carbon dioxide. Since pH is logarithmic, pH 4.3 has about twenty times the hydrogen-ion activity of pH 5.6. Values of 6 or 5.5 are near or only slightly below natural rain acidity, while 7.2 is mildly alkaline. 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, monitoring, urban planning, and protection of populations with high exposure or physiological vulnerability. The distinction is ecologically important because emission control must target either the directly released precursor or the product formed after atmospheric transport and reaction. Exposure depends on concentration, duration, ventilation, weather, and individual susceptibility, so a pollutant's presence alone does not fully predict biological harm. Atmospheric inversions can trap contaminants near the surface, while wind, turbulence, precipitation, and chemical transformation determine their eventual distribution and removal.
Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology