When polluted air mixes with rain, snow, and fog, acid precipitation forms. This acidity has caused people to worry about the environment; some reports show that acid rain has affected lakes, trees, and fish populations in the Northeastern United States and Canada. Another concern is its effect on historic buildings and monuments.

This booklet focuses on acid rain and its impact on our Nation's capital. Rain in Washington, D.C., has an average acidity of 4.2, about as acid as a carbonated drink and more than ten times as acid as clean, unpolluted rain. This booklet will define acid rain, explain what effects it has on marble and limestone buildings, and show, on a walking tour, some of the places in our Nation's capital where you can see the impact of acid precipitation.

The pH scale:
pH = 7 is neutral, neither acid or alkaline;
smaller pH values are acid, larger pH values are alkaline.
A liquid with a pH of 3 is ten times as acid as one with a pH of 4.