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What is acid rain?

2010 April 9

Acid rain is actually a lot more complex than most people think.  I remember learning about it as a kid and thinking, “great, now I am going to melt when I go outside to play in the rain.”  As it turns out, acid rain is bad but… you don’t actually melt if you get caught in a rainstorm.

Acid rain is a broad term referring to a mixture of wet (rain, snow, fog) and dry (gases, dust) material that contains high amounts of nitric and sulfuric acids when it falls to the Earth.  These acids are created when sulfur dioxide (SO2) and nitrogen oxide (NOx) pollution mixes with oxygen and water in the air.  A large amount of this SO2 and NOx pollution comes from burning fossil fuel like coal and oil. Most of the electricity we use in the United States is created by power plants that burn fossil fuel so there were a lot of power plants contributing to this problem. This is why Congress created the Acid Rain Program in the U.S. to require power plants across the country to reduce the pollutants that form acid rain.  Here is a great graphic showing how acid rain is formed.


The weird thing is that acid rain looks, feels, and even tastes just like regular rain.  The problem is that, over time, acid rain and the pollutants that cause it result in several serious environmental and health problems.

For instance, acid rain can damage our local aquatic, or water, ecosystems and the plants and animals that live there.  Certain animals like clams, snails and some fish cannot survive when the pH level of the water is too low. The pH level is a measurement of how acidic, basic or neutral a substance is. If a lake has a low pH level, it is considered acidic.

PH scale

Aquatic ecosystems are very sensitive and if the pH level is changed due to acid rain, some of the plants and animals living in these acidic lakes and streams cannot survive, resulting in lakes with no fish.  Acid rain can also weaken trees by damaging their leaves, depriving trees of their ability to get valuable nutrients.  Not good… considering trees help keep our air clean and healthy for us humans to breathe.  This also affects how our beautiful forests look.

Does acid rain affect human health?  Well, not directly – remember, you won’t get hurt if you are in a rainstorm where the rainwater pH is low. Unfortunately, however, the same pollutants that cause acid rain also cause unhealthy levels of air pollutants like ozone and fine particulate matter. When we breathe in high amounts of ozone or fine particles of pollution, they can damage our lungs and heart.  This can lead to more asthma attacks and hospital visits, and even heart attacks for some folks.

So, what did you learn about acid rain in school? Did your teacher get it right? Did you ever think you would melt in the rain?  Share your story!

Josh Stewart is the Communications Intern with the EPA’s Clean Air Markets Division. Josh is currently working on his Master’s Degree in Political Management at The George Washington University.

Editor's Note: The opinions expressed here are those of the author. They do not reflect EPA policy, endorsement, or action, and EPA does not verify the accuracy or science of the contents of the blog.

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3 Responses leave one →
  1. April 9, 2010

    It’s still a problem in the Adirondacks, if I am not mistaken, and is often cited for the demise of native fisheries and declines in Common Loon numbers.

  2. April 12, 2010

    A map showing current & historical precipitation pH values would be a useful addition to this discussion.

  3. cwalke permalink*
    April 13, 2010

    We agree. Check back on April 21st. We will dedicate a whole post to that topic. Until then please check out>> This is a great resource with maps with ambient concentrations and wet deposition of sulfate, nitrate, and acidity (as pH).

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