Acid Rain: Effects & Causes

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  • 0:01 What Is Acid Rain?
  • 1:11 Causes of Acid Rain
  • 1:45 Measurement of Acid Rain
  • 2:37 Effects of Acid Rain
  • 4:14 Reducing Acid Rain
  • 4:58 Lesson Summary
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Lesson Transcript
Instructor: Margaret Cunningham


This lesson will focus on the environmental problem known as acid rain. It will also explore the causes and effects of acid rain, how it is measured, and what is being done to reduce it.

What Is Acid Rain?

Most people think of rain as refreshing and beneficial to the environment, but not all types of rain are a good thing. Acid rain is one type of rain that is harmful to the environment. Although the name might give you the impression that it's pure acid falling from the sky, acid rain is actually created when certain gases are mixed with atmospheric moisture to create precipitation that is more acidic than normal.

Sulfur dioxide and nitrogen oxides are two of the gases involved in the creation of acid rain. When these gases are released into the environment and mixed with water, oxygen, and other chemicals, they create acidic compounds, such as sulfuric acid and nitric acid, which results in the formation of acid rain.

Although rain is most commonly thought of as wet, acid rain can come in both a wet or dry form based on how the acidic materials fall from the atmosphere. When the materials are in a wet form, they fall as rain, sleet, snow, or fog. In the dry form, they can fall as gases or particles. Both the wet and dry forms of acid rain can be carried by the wind and travel a long distance before being deposited.

Causes of Acid Rain

The causes of acid rain can be both natural and man-made. Both volcanoes and decaying vegetation release gases that result in the formation of acid rain. However, the majority of gases come from man-made sources, such as fossil fuel combustion.

In the United States, around two-thirds of sulfur dioxide and one-quarter of nitrogen oxides in the atmosphere are released from electric power generation due to burning fossil fuels. The exhaust from vehicles also releases both gases into the air, so the more vehicles, the more risk of acid rain.

Measurement of Acid Rain

We use the pH scale to measure acid rain. The pH scale measures how acidic a substance is. It runs from 0 to 14; 0 is the most acidic, 7 is neutral, and 14 is the most basic. Although pure water is known to have a pH of 7, normal rainwater has a slightly more acidic pH of around 5.6. This pH level is due to the carbon dioxide in the atmosphere that dissolves into a weak carbonic acid. Acid rain has an average pH of 4.2 to 4.4, which is almost ten times more acidic than normal rain.

The level of acidity can be determined using high-tech devices or using litmus paper. When litmus paper is exposed to a substance, it changes color depending on the acidity: red for acidic, blue for basic, and a variety of colors for anything in between.

Effects of Acid Rain

Overall, the environment and its inhabitants are adapted to survive within a certain acidity level. When acid rain falls, it can dramatically alter the acidity level of the habitat and cause a great deal of damage to the living and non-living things within.

Acid rain can negatively affect human health by creating particles in the air that can cause respiratory problems or make breathing more difficult. Acid rain can also cause building materials to decay more rapidly and paint more likely to peel. The acidity also wears down stone statues, making them appear older and reducing their value.

The most drastic effects from acid rain can be seen in forests and waterways. Acid rain does not kill trees directly but seeps into the soil and dissolves many of the essential nutrients the trees need. Acid rain also releases excess aluminum into the soil around trees, which makes it difficult for them to take up the water. Trees become more susceptible to infection, insects, and are less resistant to cold weather. In Germany, there is a place called the Black Forest, which received its name because acid rain caused the trees to drop their needles and are now simply black trunks and branches.

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