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Clear Skies

Basic Information

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.
Unless otherwise noted, the data presented throughout this Web site reflect EPA’s 2003 modeling and analysis of the Clear Skies Act of 2003. Clear Skies legislation was intended to create a mandatory program that would dramatically reduce power plant emissions of sulfur dioxide (SO2), nitrogen oxides (NOx), and mercury by setting a national cap on each pollutant. The Clear Skies bill was proposed in response to a growing need for an emission reduction plan that will protect human health and the environment while providing regulatory certainty to the industry. The proposed legislation for air regulation never moved out of the Senate Environment and Public Works committee in 2005 and was therefore never enacted.

The emissions reductions from Clear Skies would help to alleviate our nation's major air pollution-related health and environmental problems including fine particles, ozone, mercury, acid rain, nitrogen deposition, and visibility impairment. Clear Skies would:

  • Cut sulfur dioxide (SO2) emissions by 73 percent, from year 2000 emissions of 11 million tons to a cap of 4.5 million tons in 2010 and to a cap of 3 million tons in 2018.
  • Cut emissions of nitrogen oxides (NOx) by 67 percent, from year 2000 emissions of 5 million tons to a cap of 2.1 million tons in 2008 and to a cap of 1.7 million tons in 2018.
  • Cut mercury emissions by 69 percent - the first-ever national cap on mercury emissions. Emissions would be cut from 1999 emissions of 48 tons to a cap of 26 tons in 2010 and to a cap of 15 tons in 2018.

Emissions Reductions -
Clear Skies Would Use a Proven System of Emission Caps to

  • Protect against diseases by dramatically reducing smog and fine particles which contribute to respiratory and cardiovascular problems.
    Learn More.
  • Protect our wildlife, habitats and ecosystem health by reducing acid rain, nitrogen and mercury deposition.
    Learn More.
  • Cut pollution further, faster, cheaper, and with more certainty, using a "cap-and-trade" program that would deliver rapid and certain improvements in air quality.
    Learn More.
  • Enable power generators to continue to provide affordable electricity while quickly and cost-effectively improving air quality and the environment.
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  • Encourage use of new and cleaner pollution control technologies that would further reduce compliance costs.
  • Clear Skies is modeled on the cap-and-trade provisions of the 1990 Clean Air Act's extremely successful Acid Rain program.
    • Mandatory emission reductions would be achieved through a cap-and-trade program.
    • Federally enforceable emissions limits (or "caps") for each pollutant would be established.
    • Sources would be able to transfer these authorized emission limits among themselves to achieve the required reductions at the lowest cost.
    • Clear Skies would not replace the authority of state and local government to set source-specific emissions limits to ensure that ambient air quality standards will be met.

A Better Approach to Clean Air:
Multipollutant Legislation for the Power Sector

In the United States, power generation is responsible for 63% of sulfur dioxide (SO2), 22% of nitrogen oxides (NOx), and 37% of mercury released to the environment by human activity. Once released, these pollutants, together with the gases and particles they form in the atmosphere (e.g., ozone and particulate matter), can travel long distances before being deposited. Environmental and public health problems resulting from power generation emissions include:

  • Diseases of the cardiovascular and respiratory systems associated with exposure to fine particles (particulate matter) and ozone;
  • Regional haze that impairs visibility in national parks and wilderness areas;
  • Acidification of lakes, streams, and forests;
  • Ecosystem and public health effects associated with the accumulation of mercury in fish and other wildlife;
  • Acidic damage and erosion of buildings, statuary, and other materials;
  • Ozone damage to forests; and
  • Eutrophication (a condition in an aquatic ecosystem where high nutrient concentrations stimulate blooms of algae) in coastal areas.

While the Clean Air Act has significantly improved some of these issues, additional reductions in emissions of SO2, NOx, and mercury are necessary to address persistent public health and environmental problems. Because these pollutants move beyond state and regional boundaries, individual states or localities experiencing the direct environmental effects cannot always control them. In addition, current law tends to address each environmental problem independently, even if one pollutant contributes to several problems. To more effectively address the environmental problems caused by power generation, there is a need for a national program that would take advantage of the benefits that would result from controlling multiple pollutants at the same time.

Clear Skies would do this. It is a simple, cost-effective way of improving air quality over broad, multi-state areas in a way that makes sense for everyone. The Clear Skies approach would deliver guaranteed emissions reductions of SO2, NOx, and mercury at a fraction of command and control costs, increasing certainty for industry, regulators, consumers and citizens, while maintaining energy diversity and affordable electricity.

Clear Skies Reduces and Caps Power Plant Emissions

Clear Skies is a mandatory program that will dramatically reduce power plant emissions of SO2, NOx and mercury by national cap on each pollutant at an average of 70% below today's levels. Over the next decade, Clear Skies would achieve substantially greater reductions in pollution from power plants than are attainable under current law.

NOx and SO2 requirements affect all fossil fuel-fired electric generators greater than 25 megawatts (MW). Mercury requirements affect only coal-fired electric generators greater than 25 MW.

Actual Emissions in 2000
First Phase of Reductions
Second Phase of Reductions
Reduction at Full Implementation


11.2 million tons
4.5 million tons
in 2010*
3 million tons in 2018*
5.1 million tons
2.1 million tons in 2008*
1.7 million tons in 2018*
48 tons
26 tons in 2010
15 tons in 2018*

* Because sources can reduce emissions early, earn allowances for those actions, and use those allowances later, actual emission levels will be higher than the cap in the first years of these phases. Further, the Clear Skies Act contains "safety valve" provisions for NOx, SO2, and mercury to limit the marginal costs of removal of each of the three pollutants if costs exceed a certain threshold. The 2003 modeling, based on current technological capabilities, shows that the cost of mercury removal is expected to exceed the safety valve threshold for the Phase II caps. However, technological improvements could decrease the cost of mercury control over time and cause prices to remain below safety valve levels.
** The NOx cap is divided between two zones with separate trading programs under each zone. Zone 2 includes states participating in the WRAP process as well as Nebraska and some of Western Texas. Zone 1 includes the remaining 33 states in the continental U.S. and the remaining portion of Texas.

Clear Skies Provides Significant Benefits at a Reasonable Cost

Clear Skies would result in significant benefits to public health and the environment:

  • EPA projects that, by 2020, the public health benefits alone from Clear Skies would include more than 14,000 avoided premature deaths and total $110 billion per year, substantially outweighing the annual costs of $6.3 billion.
    • An alternative methodology for calculating health-related benefits projects over 8,400 premature deaths prevented and $21 billion in health benefits - still far greater than the costs.
    • EPA projects early reductions in 2010 would result in 7,900 fewer premature deaths nationwide and $54 billion in annual health benefits at a cost of $4.3 billion. The alternative methodology projects that early reductions would prevent 4,700 premature deaths and deliver $10 billion in benefits in 2010 - again, still far greater than the cost.
  • Americans would also experience approximately 30,000 fewer visits to the hospital and emergency room, 23,000 fewer nonfatal heart attacks, and 1.6 million fewer work loss days and 200,000 fewer school absences each year under Clear Skies by 2020.
  • Benefits of improvements in visibility in our national parks and wilderness areas in 2020 would be $3 billion annually.

Clear Skies would help state and local governments attain the National Ambient Air Quality Standards (NAAQS) for fine particles (PM2.5) and ozone:

  • By 2010, an estimated additional 42 counties with 14 million people would meet the fine particle standard and an additional 3 counties with 1 million people will meet the 8-hour ozone standard.
  • By 2020, an estimated 35 additional counties with 12 million people would meet the fine particle standard.

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