Jump to main content.


Acid Rain


Note: EPA no longer updates this information, but it may be useful as a reference or resource.

Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.


Sulfur Dioxide and Nitrogen Oxides emissions are the main ingredients in the formation of acid rainNature and Source of the Problem: Acidic deposition or "acid rain" occurs when emissions of sulfur dioxide (SO2) and oxides of nitrogen (NOx) in the atmosphere react with water, oxygen, and oxidants to form acidic compounds. These compounds fall to the Earth in either dry form (gas and particles) or wet form (rain, snow, and fog). Some are carried by the wind, sometimes hundreds of miles, across State and national borders. In the U.S., about 70 percent of annual SO2 emissions and 30 percent of NOx emissions are produced by electric utility plants that burn fossil fuels.

Health and Environmental Effects: Before falling to Earth, SO2 and NOx gases and related particulate matter (sulfates and nitrates) contribute to poor visibility and impact public health. Major human health concerns associated with their exposure include effects on breathing and the respiratory system, damage to lung tissue, cancer, and premature death. In the environment, acid rain raises the acid levels of lakes and streams (making the water unsuitable for some fish and other wildlife) and damages trees at high elevations. It also speeds up the decay of buildings, statues, and sculptures, including those that are part of our national heritage.

Program Structure and Benefits: The overall goal of EPA's Acid Rain Program is to improve public health and the environment by reducing emissions of SO2 and NOx . In order to accomplish its goals cost-effectively, the program employs both innovative and traditional approaches for controlling air pollution and encourages energy efficiency and pollution prevention.

Specifically, to achieve its goal of reducing annual SO2 emissions by 10 million tons between 1980 and 2010, the 1990 Clean Air Act Amendments require a two-phase tightening of the restrictions on fossil fuel-fired power plants. Phase I began in 1995 and affected 445 electric utility units, including the biggest and dirtiest in the country. Phase II, which begins in the year 2000, will tighten the annual SO2emission limits for these plants and also set restrictions on smaller, cleaner plants fired by coal, oil, and gas. The Clean Air Act also requires significant reductions in NOx emissions beginning in 1996, most of which will be achieved by requiring coal-fired utility boilers to install low-NOx burner technologies and meet new emissions standards.

Reductions in SO2 and NOx will decrease levels of sulfates, nitrates, and ground-level ozone (smog), leading to improvements in public health and other benefits such as better water quality in lakes and streams. Visibility will also improve, enhancing the beauty of our country's scenic vistas, including those in national parks. Likewise, damage to the trees that populate mountain ridges from Maine to Georgia will be reduced, and deterioration of our historic buildings and monuments will be slowed.

Sulfur Dioxide Emissions Chart

Emissions and Atmospheric Trends: Emissions reductions have been significantly greater in 1995, the first year of compliance with EPA's Acid Rain Program. As shown below, for the 445 electric utility units participating in Phase I of the Acid Rain Program, actual emissions measured by continuous emission monitoring systems were reduced by more than half relative to 1980 levels, plummeting from 10.9 to 5.3 million tons. These emissions were 3.4 million tons (or 39 percent) below the 1995 allowable emissions level of 8.7 million tons required by the Clean Air Act.

According to a recent study released by the U.S. Geological Survey, reductions in SO2 emissions resulted in less acidic rain in 1995. In the study, 1995 precipitation data were compared with reference data from 1983 through 1994. As shown in the map below, the study reports a 10 to 25 percent drop in wet deposition sulfate concentration and rainfall acidity, particularly at some sites located in the Midwest, Northeast, and Mid-Atlantic Regions. These areas are some of the most acid­ sensitive regions of the country. Some areas on the map show increases in sulfate levels. These are attributed to drought conditions that occurred during 1995. Had rainfall conditions been more normal in 1995, even greater reductions in sulfate levels might have occurred.

The Acid Rain Program has also led to improvements in SO2 concentrations and emissions. Between 1994 and 1995, national SO2 concentrations decreased 17 percent and SO2 emissions (all sources combined) decreased 13 percent.

Map showing percent change in sulfate levels in 1995 rainfall
The level of sulfates in rain is an indicator of acidity. A 10 to 25 percent decrease in sulfate levels in rainfall was observed in 1995, particularily in some of the most acid-sensitive regions in the U.S.

This document is provided for historical purposes only. The most recent version can be found at AIRTrends


Local Navigation


Jump to main content.