Geothermal fluids contain variable concentrations of gases, largely nitrogen and carbon dioxide with some hydrogen sulphide and smaller proportions of ammonia, mercury, radon and boron. Most of these chemicals are concentrated in the disposal water which is usually reinjected back into the drill holes so that there is minimal release into the environment. The concentrations of the gases are usually low enough not to be harmful or else the abatement of toxic gases can be managed with current technology.
Carbon dioxide is the major component of the noncondensible gases in the steam, but its emission into the atmosphere per kWh is well below the figures for natural gas, oil, or coal-fired power plants. Hydrogen sulphide is the pollutant of most major concern in geothermal plants yet even the sulfur emitted with no controls is only half what is emitted from a coal-fired plant. Overall, with present technology able to control the environmental impact of geothermal energy development, it is considered to be a relatively benign source of energy.