Acid Rain

“Acid rain” became a household term in the 1980s when unchecked emissions from industry and motor vehicles were blamed for causing environmental deterioration. Scientific evidence has linked acid rain to decreased fish and wildlife populations, degraded lakes and streams, and human health hazards. Although the term has since faded from public consciousness, acid rain is a complex and global problem that still exists today.

What is Acid Rain?

First identified in 1872 in Sweden and studied in the U.S. beginning in the 1950s, acid rain is precipitation in the form of rain, snow, hail, dew, or fog that transports sulfur and nitrogen compounds from the high atmosphere to the ground. Sulfur dioxide (SO2) and nitrogen oxides (NO, NO2) are bi-products from burning fuels in electric utilities and from other industrial and natural sources. These chemicals react with water, oxygen, carbon dioxide, and sunlight in the atmosphere to form sulfuric and nitric acids. The acids reach the ground and change the chemistry within the environment.

The acidity of any solution is determined on the pH scale of 0 to 14. A pH level of 0 to 7 is considered acidic; 7 is neural; and a level above 7 is alkaline. As the pH number decreases, acidity increases. Unopened bottled distilled water has a pH of 7, so it is neutral. In comparison, household ammonia is an alkaline with a pH of 11.5. Milk is slightly acidic with a 6.5 pH, and soft drinks, which contain phosphoric acid, have a 3.1 pH.

Although the pH scale may seem straightforward, determining the pH of “normal” rain is much more complex. When distilled water is exposed to air, an interaction with carbon dioxide increases acidity through the formation of carbonic acid, H2CO3, and the pH level falls. Many scientists agree that the normal pH of rain is a slightly acidic 5.6 because of perpetual chemical interactions in the air.

What’s more, rain pH levels can vary significantly over short distances and in a short amount of time, even during the same rainfall. Seasons, climate, and a host of other factors can also influence the acidity of rain.

Rain and snow are not the only processes that deposit sulfur and nitrogen acids from the atmosphere to the ground. These compounds are also present in gases and dry particles, which are more difficult to measure. Like wet deposition, the occurrence of “dry deposition” of acids varies in different areas, depending on distance from the emission source and climatic conditions.

What Causes Acid Rain?

Acid rain is linked to both natural and man-made sources. Nitrogen oxides are formed through the extreme heating of air when a thunderstorm produces lightning. Also, sulfurous gases are discharged from erupted volcanoes and rotting vegetation.

However, most public attention has been focused on man-made sources of acid rain, which include the burning of any fuel that contains sulfur and nitrogen compounds, including public utilities, industrial broilers, motor vehicles, and chemical plants. Electric power generation accounted for 69 percent of total sulfur dioxide emissions in the U.S. in 2007 and 20 percent of nitrogen oxides, according to the U.S. Environmental Protection Agency (USEPA).

Many industrial sources of sulfur dioxide are located in the eastern U.S., particularly in the Midwest and the Ohio Valley where coal combustion and power generation frequently occur. Typically, the highest nitrogen oxide emissions are found in states with large urban areas, a heavy population density, and significant automobile traffic.

Acid rain is not limited to the region where sources are located. Prevailing winds can blow chemicals in the atmosphere for hundreds or even thousands of miles before being deposited, regardless of state and country boundaries. For instance, compounds from industry in China can potentially be deposited in the U.S. Midwest. For this reason, acid rain is considered a global problem.

What are its Effects?

Acid rain has been linked to detrimental effects in the environment and in human health.

Forests, lakes, and streams: Acid rain can cause widespread damage to trees. This is especially true of trees at high elevations in various regions of the U.S. Acidic deposition can damage leaves and also deplete nutrients in forest soils and in trees so that trees become more vulnerable to disease and environmental stress.

When lakes and streams become more acidic than normal, they cannot continue to support the same types of fish and aquatic life as in the past. Fish communities dwindle due to high mortality, a reduced growth rate, skeletal deformities, and failed reproduction. Lakes ultimately become home only to species that can tolerate high-acid conditions. Game fish, such as trout, are particularly sensitive to acidic water conditions.

A healthy lake has a pH of 6.5 or higher. Only a few fish species can survive at a pH of below 5; at a pH of 4, the lake is considered dead. A decrease in fish populations is often the first sign of an acidification problem.

Not all lakes are equally vulnerable to acid rain, however. In some areas, such as in Illinois, the average pH of a freshwater lake is an alkaline 8 to 9 because soils and rocks in the bottom and sides of the lake contain high levels of calcium and magnesium, which neutralize the acidity of rain. Lakes surrounded by granite, such as in New England and northern New York, don’t fare as well.

Plants and crops: Acid rain can potentially reduce agricultural production by changing the chemical properties of soil, slowing the rate of microbiological processes, and reducing soil nutrients. Roots of natural vegetation and crops can become damaged due to stunted growth. Human effects: Acidic water moving through pipes causes lead and copper to leach into the water. Most public water suppliers remove such dangerous chemicals at the plant, but tainted water could be a problem for residents who don’t rely on public water supplies for their drinking water.

Acidic fog can be more hazardous to health than acid rain as small droplets can be inhaled. These atmospheric acids can cause respiratory problems in humans such as throat, nose, and eye irritation, headache, and asthma. Acid fog is particularly dangerous for the elderly, those who are ill, and people who have chronic respiratory conditions.

Man-Made Materials: Although sunlight, heat, cold, and wind contribute to the deterioration of man-made structures and objects, acid deposition speeds up this process. Metal structures and vehicles become corroded, and limestone buildings, tombstones, statues, and monuments deteriorate faster when rain is acidic.

How is Acid Precipitation Collected?

Wet deposition samples can be measured to determine chemical concentrations in almost any area. The National Atmospheric Deposition Program (NADP) at the Wisconsin State Laboratory of Hygiene, University of Wisconsin-Madison, maintains five networks with more than 350 deposition monitoring sites. The NADP National Trends Network has 250 sites in the U.S. located far from the point sources of pollution. Each site has an automated precipitation collector and gage to gather samples only during rain or snowfall.

Weekly samples are collected and sent to the NADP for analysis. The network measures acidity and calcium, magnesium, sodium, potassium, sulfate, nitrate, chloride, and ammonium ions. Data are available online at These monitoring efforts support research and policy on air quality issues.

NADP monitoring data show that wet sulfate deposition has decreased an average of 30 percent since the early 1990s in the eastern United States. The largest decreases occurred in Maryland, New York, Virginia, West Virginia, and Pennsylvania. Nitrogen deposition has decreased as well, but to a lesser extent.

Legislative mandates, federal government programs, and environmental-friendly changes in fossil fuel use in electrical power plants have successfully lowered the emission of SO2 and NOx and the resulting acid deposition in the U.S. since the 1980s. However, the problem still exists. Scientists continue to try to fully understand acid rain and its long-term effects on the environment and on human health.


American Chemical Society. 1982. Acid Rain. American Chemical Society. Washington, D.C.

H. John Heinz III Center for Science, Economics and the Environment. 2008. The state of the nation’s ecosystems: 2008. The Heinz Center, Washington, D.C.

Likens, G. (Lead Author); Environmental Protection Agency (Content source); W. Davis, L. Zaikowski, and S. C. Nodvin (Topic Editors). 2007. Acid rain. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment).

Stanitski, C. L., L. P. Eubanks, C. H. Middlecamp, and W.J. Stratton. 2000. Neutralizing the threat of acid rain. in Chemistry in Context (third ed.). American Chemical Society. U.S. Environmental Protection Agency. 2009, January. Acid rain and related programs: 2007 Progress Report. USEPA. Washington, D.C. (Additional information available at