EEI > Issues & Policy > Environment > Clean Air > Ozone


Ground-level ozone is created by chemical reactions between emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Health effects are far from certain due to questions involving uncertainties of studies trying to link ozone exposure to health effects via modeling and importance of exposures at low levels.
Motor vehicles, industrial facilities, electric companies, construction equipment, locomotives and ships are some of the major sources of NOx and VOCs. Currently, electric companies emit only about 6 percent of the nation’s combined emissions of NOx and VOCs.  The power sector has reduced emissions of the NOx “precursor” to ozone by 82 percent since 1990. 

Air Quality Standards for Ozone

The Clean Air Act (CAA) requires the U.S. Environmental Protection Agency (EPA) to establish national ambient air quality standards (NAAQS)—at levels that are protective of public health and welfare (environment)—for six pollutants, including ozone.
EPA reviewed its ozone standards in 1997 and set a tighter limit of 0.08 parts per million (ppm) for a new measuring time period, the 8-hour average (with rounding, the standard actually was 0.084). The CAA requires EPA to complete a review of each NAAQS, including ozone, at five-year intervals and make any necessary revisions. In 2008, EPA tightened the standard again to 0.075 ppm and in 2015 the Agency further lowered the ozone NAAQS to 0.070 ppm.
Each tightening of the standard has consequences for state governments, businesses like electric companies, and the cost of many products and processes.

The Energy Industry is Reducing NOx Emissions that Help to Reduce Ozone

The electric power industry nationally has reduced its national total NOx emissions by 82 percent since 1990. NOx emissions are one of two man-made chemicals that contribute to ozone formation; electric power sources emit very little of the other, called volatile organic compounds (VOCs) or hydrocarbons. NOx reductions from electric companies have been achieved by installation of control technologies as a result of environmental regulations – such as the 1997 and 2008 ozone NAAQS; the 1997, 2006 and 2012 fine particulate matter NAAQS; the Acid Rain Program; the “NOx SIP Call;" the Clean Air Interstate Rule (CAIR); the Cross-State Air Pollution Rule (CSAPR); the Mercury and Air Toxics Standards (MATS); and regional haze requirements – and advancements in clean fuels and renewables.
The industry will achieve greater NOx reductions in the future due to regulations like EPA's 2015 ozone NAAQS, the CSAPR Update rule and visibility rules.  In addition, advancements in clean fuels and renewables will continue to help the industry reduce NOx emissions.


Learn more at EPA’s Ozone site.