Acid rain may not have quite as much prominence in public discourse as it did years ago, but that doesn't mean that the problem has gone away. Acid rain effects can be devastating, particularly to forests and aquatic ecosystems, making waters toxic and depriving the soil of essential nutrients.
When fossil fuels like coal and oil are burned by power companies and other industries, sulfur is released into the air, which combines with oxygen to form sulfur dioxide. This compound, along with the nitric acid that forms due to car exhaust, dissolves into the water vapor in the air, which then pours down in the form of acid rain. While acid rain gases originate in urban areas, they can drift hundreds of miles into rural areas to wreak havoc on forests and lakes.
According to the United States Environmental Protection Agency (EPA), these effects are most dramatic in water environments like streams, lakes and marshes. Most bodies of fresh water have a pH between 6 and 8, meaning they are on the alkaline, or 'base' side of the pH scale. As acid rain falls into the water, it lowers this pH, and the surrounding soil is often unable to buffer it. The acidic water leaches aluminum from the soil, which is highly toxic to many species of aquatic organisms.
A 2000 study by the University of Wisconsin-Madison focusing on acid rain effects on Wisconsin's Little Rock Lake found that while bodies of water can correct themselves naturally from this change in pH, the nature of the food chain changed dramatically, with many species dying off. These impacts, observed in many other bodies of water around the world, spread to non-aquatic species such as birds.
Scientists have noted that acid rain has slowed the growth of some forests, and in extreme cases, caused them to die off altogether. Differences in the ability of soils to buffer acidic rain are a large part of the reason why some geographical areas, like high-elevation forests in the Appalachian Mountains from Georgia to Maine, seem to be affected more than others. High mountain regions are also more affected because they are surrounded by clouds and fog that contain more acid than rainfall.
Acid rain leaches nutrients both from the soil and from the leaves of trees, dissolving them and washing them away. As in bodies of water, acid rain falling in a forest causes the release of toxic substances like aluminum.
Just how harsh are the acids in acid rain? The effects on stone such as marble and limestone buildings give us an idea, as sharp edges and carving details gradually erode away. Even sheltered areas show damage as blackened crusts of gypsum – a mineral that forms from the reaction between calcite, water and sulfuric acid – blister and crumble. Acid rain has also been known to wear away automotive coatings and contribute to the corrosion of metals.
Acid rain affects our health, too. While standing outside in falling acid rain won't necessarily cause any harm, sulfur dioxide and nitrogen oxides, the pollutants that cause acid rain, are toxic. Fine particles of these gases can be inhaled deep into our lungs, potentially causing heart and lung disorders including asthma and bronchitis. The Acid Rain Program, fully implemented under the Clean Air Act in 2010, aims to reduce these effects by regulating the output of sulfur dioxide and nitrogen oxide emissions from power plants.
Inhalation of the gases isn't the only way humans are affected by acid rain, however. A 1985 study found that the increase in water and soil content of lead and cadmium caused by acid rain poses a risk, and that acidification increases bioconversion of mercury into methylmercury in fish, increasing its toxicity for those who eat it.
The only way to combat acid rain is to reduce the release of the pollutants that cause it. If you'd like to help, National Geographic recommends conserving energy in the home, because the less electricity we use, the fewer chemicals power plants will emit.