The benchmark of "natural rain" is 5.6. Acid precipitation
in the range of 4.2-5.0 has been recorded in most of the Eastern
United States and Canada. EPRI (Electric Power Research Institute)
likes to compare these values to familiar objects to give the
impression that these pH values are not harmful.
Examples: Carrots = 5.0, Bananas = 4.6, Tomatoes = 4.2, Apples
and soft drinks = 3.0, Lemon juice = 2.0.
EPRI also contends that pH 5.6 may or may not be a valid reference
point. It should not be considered the "background"
or "natural" acidity of precipitation.
Even without man-made influences, there are natural sources
of sulfur oxides, nitrogen oxides, and other species important
to determining the precipitation acidity at any given time. Hence,
trying to quantify man's contribution to the natural condition
will never be possible, since the "natural background"
condition cannot be known.
In the forest areas of Brazil at the headlands of the Amazon
River, an area remote from civilization, the monthly average
of 100 rain events in the 1960s ranged from pH 4.3 to pH 5.2,
with the median value of pH 4.6 and one reading as low as pH
On the island of Hawaii, remote from all industrial activity,
the weighted average of precipitation over a 4 year period was
pH 5.3, with a minimum value of pH 3.8.