Understanding how acid rain forms, its effects on plants, animals and humans, and how to promote pollution prevention are steps being taken to combat its destruction.

What is Acid Rain?

Acid rain is precipitation containing higher than normal amounts of nitric and sulfuric acid. According to the EPA, acid rain can either occur by wet deposition or dry deposition, occurring when sulfur dioxide and nitrogen oxide react with the atmosphere. During wet deposition the gases react with water, oxygen and other chemicals. When this happens, various acidic compounds form, most commonly sulfuric and nitric acid. These acids are blown by the wind into areas with wet weather, where they fall in the form of rain or snow. If the acids are blown to areas with dry weather, they fall as dry deposition, clinging to dust or smoke, sticking to buildings and trees. The acid is then washed into the water system by rain.

Acid rain forms from both natural and man-made sources. Volcanoes and decaying vegetation can form sulfur dioxide and nitrogen oxides. Emissions from burning fossil fuels also produce acid rain. According to the EPA, in the United States most of the sulfur dioxide and a quarter of nitrogen oxides “come from electric power generation that relies on burning fossil fuels, like coal.” Winds have been known to carry the acid rain into parts of Canada.

Measuring Acid Rain

Acid rain is measured using a pH scale.

• Pure water – pH 7.0
• Normal rain, containing some carbon dioxide mix – pH 5.6
• Acid rain – pH 4.3

Acid Rain Effect on Plants and Animals

Acid rain has a negative effect on plants and animals. Once the acid gets into the water cycle it can cause the acidification of lakes and streams. The National Surface Water Survey (NSWS) identified over a thousand lakes and thousands of streams in the United States, where some form of acidification has taken place. Of these lakes and streams, 75 percent of the acidity in the lakes was directly due to acid rain. 50 percent of the streams had been affected by acid rain. Many areas of the Northeastern United States and Eastern Canada contain lakes and streams affected by acidification from acid rain.

Changes in the pH of lakes and streams affected by acid rain can result in a decrease in the variety of fish, plants and animals living in or near the water. Some animals and plants cannot tolerate the higher levels of acid. Snails, clams and bass are examples of animals that can only tolerate a small increase in acidity.

Acid rain destroys trees and plants by causing damage to leaves and dissolving nutrients in the surrounding soil. Trees in higher elevations, have a greater risk for damage due to acid rain because they have greater exposure to clouds carrying sulfur dioxide and nitrogen oxide.

Acid Rain Effect on Humans

Acid rain can also have a devastating effect on man-made structures, such as those made of stone and metal. Bronze statutes and marble monuments are deteriorated by acid rain. Costly repairs and maintainence is required to clean acidic compounds resulting from dry deposition collecting on buildings.

One important misconception to clear up about acid rain is its direct harm to humans. Acid rain falls from the sky just like regular rain, without odor or taste. Acid rain is not directly harmful to humans. Yet, sulfur dioxide and nitrogen oxide are harmful pollutants before they combine with water and oxygen to form acid rain. These gases cause harmful particles that can be inhaled by humans, causing lung and heart disorders.

Reducing Acid Rain Through Clean Air Act

Education on how acid rain forms and how it damages both the natural and material environment must be done to tackle the problem of acid rain. The Clean Air Act, which was last amended in 1990, aims to reduce emissions of sulfur dioxide and nitrogen oxides by capping emissions from power plants, promoting the use of alternative energy and pollution prevention programs.