Geothermal power plants can meet the most stringent clean air standards. They emit little carbon dioxide, very low amounts of sulfur dioxide, and no nitrogen oxides. See Charts 1, 2, and 3 below.

To put this in perspective, electricity produced from U.S. geothermal resources, compared to conventional coal-fired plants, annually offsets the emission of:

  • 4.1 million tons of carbon dioxide
  • 80,000 tons of nitrogen oxides
  • 110,000 tons of particulate matter

The small quantities of gases emitted from geothermal power plants aren't created during power production because there's no combustion. These gases are natural, minor constituents of all geothermal reservoirs. They eventually would vent to the atmosphere without geothermal power development, although at much slower rates. Dry steam and flash steam power plants emit mostly water vapor. Binary-cycle power plants emit virtually no gases because they operate using a closed-loop system.

Carbon Dioxide

When geothermal power plants do emit gases, it's mostly carbon dioxide, which isn't a pollutant but a greenhouse gas. Still, geothermal power plants emit much less carbon dioxide than fossil fuel power plants.

Also, the common practice by geothermal power plants to inject geothermal fluids back into reservoirs to sustain resources has diminished their carbon dioxide emissions. Carbon dioxide emissions from the Dixie Valley geothermal "flash" power plant in Nevada decreased 39 percent when it started using this practice in 1992.

Hydrogen Sulfide

Gases released from geothermal fluids may also include hydrogen sulfide, which causes the characteristic sulfurous odor often evident near natural hot springs. But typical emissions of hydrogen sulfide from geothermal power plants are less than 1 part per billion—well below what people can smell. In fact, most geothermal power plants produce such low concentrations of hydrogen sulfide that they require no special controls to comply with most state and federal emission standards. However, at The Geysers in California, the steam contains up to 0.15 percent hydrogen sulfide by weight, but treatment processes remove more than 99.9 percent of emissions.

As a result of the hydrogen sulfide treatment processes at The Geysers, Lake County became the first and only county in compliance with California's stringent air quality regulations in 1990. The State of California has also honored Pacific Gas & Electric Company (initial owner) and Calpine Corporation (present owner) for air pollution prevention at The Geysers. Calpine also received the 2004 Clean Air Award for Technology Development from the American Lung Associations of the Bay Area. See the Award-Winning Environmental Performance of Geothermal Power in California for more information.


U.S. Department of Energy laboratories continue to investigate even more efficient processes for detecting and treating gases and particulate matter produced by geothermal power plants. See Geothermal Process Gas Monitorsfor more information.

More Information

Visit the U.S. Environmental Protection Agency Web site for more information on:

The Agency for Toxic Substances and Disease Registry Web site has more information on hydrogen sulfide.