What is E15 fuel? Why higher-ethanol gasoline could raise summer smog levels
As the conflict in Iran chokes off oil and gas supplies, prices at the pump have risen. The economic pain could be acute as many hit the road over Memorial Day weekend.
In an effort to reduce fuel costs for drivers, Environmental Protection Agency Director Lee Zeldin announced in March that he would waive the ban on the sale of so-called E15 gasoline, which contains 15 percent ethanol, during the summer. The waiver took effect May 1 and was initially scheduled to last 20 days, but Zeldin said he would likely extend it into the summer to increase “fuel supply and consumer choice.” The EPA has previously issued short-term waivers in response to summer gas prices, but, unlike in previous years, the House of Representatives passed a bill on May 13 to make year-round sales of E15 permanent.
If this bill passes the Senate, it could also increase air pollution in the summer. Here’s what you need to know about the change.
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Why is ethanol mixed with gasoline?
Most gasoline sold year-round in the United States contains 10 percent ethanol, a fuel made from fermented biomass — usually corn — instead of petroleum. The use of ethanol in fuels dates back to the Clean Air Act Amendments of 1990 and the U.S. Renewable Fuel Standard that was established by the Energy Policy Act of 2005. Ethanol contains more oxygen than gasoline, which means ethanol blends release fewer toxic carbon monoxide emissions through the tailpipes. Ethanol also releases fewer nitrogen oxides – another set of exhaust pollutants – compared to gasoline. At the time, lawmakers hoped ethanol would have a smaller climate effect than gasoline, says Holly Gibbs, a geographer at the University of Wisconsin-Madison who studies land use.
“The basic idea behind ethanol blending was that replacing some of the oil with plant-based fuel would reduce transportation emissions, because it was assumed that the carbon released by burning the ethanol was partly offset by the corn absorbing CO₂ as it grew,” she says. This hope, however, has not been realized due to the climatic effect of the increasing footprint of corn fields, she adds.
Ethanol is popular with American farmers, however, because it provides a market for their crops. About 40 percent of the U.S. corn crop is used for ethanol production.
What is E15 fuel and why is it not sold in summer?
E15 fuel contains between 10.5 and 15 percent ethanol. In the United States, it is approved for use in all light-duty vehicles manufactured in 2001 or later and for so-called flexible fuel vehicles, designed to run on blends containing up to 83% ethanol. However, it is generally not sold in summer because burning ethanol releases volatile organic gases that react with sunlight and nitrogen oxides in the atmosphere to produce acetaldehyde, a major component of ground-level ozone. On the Earth’s surface, ozone is a toxic gas that causes wheezing, coughing and asthma attacks; it also contributes to smog. (Higher in the stratosphere, ozone protects against ultraviolet rays.)
This reaction is more pronounced in summer because the heat and the sun accelerate these reactions. Its impact is also greater in places where there are not already many reactive organic gases in the air. That’s the case in most of the United States, except the Southeast, where abundant vegetation naturally releases reactive gases, says Mark Jacobson, a professor of civil and environmental engineering at Stanford University who has studied the polluting impacts of ethanol-based fuels. So while the increase in ozone from E15 isn’t necessarily significant in a place like Atlanta in the summer, it is in a place like Los Angeles, where it’s important to keep reactive organic gases low to reduce smog, Jacobson says. For this reason, sales of E15 are generally limited in most of the United States during the summer months.
What effect will the fuel exemptions have?
Farmers will likely welcome the waivers because the move could push up corn prices, which could help them offset the financial hit caused by rising diesel and fertilizer prices resulting from the war in Iran. And the Trump administration is under pressure to maintain farmers’ support, says Carlisle Runge, a professor of applied economics and law at the University of Minnesota.
About 3,000 gas stations, or 2 percent nationwide, are equipped to dispense E15 gas, according to the EPA. Although the cost effect of blending more ethanol into gasoline varies depending on the price of corn and the price of oil, E15 will likely sell between five and 40 cents per gallon less than E10, or regular gasoline containing up to 10 percent ethanol, this summer. Ethanol, however, has a lower fuel efficiency than gasoline, meaning drivers may need to refuel more often, according to the American Consumer Institute.
Given the limited distribution of E15 stations and the relatively low concentration of ethanol in E15, the health effects of this summer’s waivers will likely be small, Jacobson says.
What is the environmental effect of ethanol fuel in general?
This is not the first time E15 waivers have been granted: E15 sales authorizations have occurred every summer since 2022. The US Renewable Fuel Standard for 2023 to 2025 also calls for an increasing proportion of biofuels, including ethanol, in gasoline, with its supporters – often in the agricultural sector – calling for up to 85% ethanol blends.
An influential 2019 study from Argonne National Laboratory found that between 2005 and 2019, the use of corn ethanol reduced emissions by 544 million tons of CO.2a 23 percent reduction in the carbon intensity of fuel. These results are, however, highly contested. A 2022 study co-authored by Gibbs found that when taking into account the effects of converting land to agricultural land for growing corn, corn-based ethanol actually increases greenhouse gas emissions by 24%.
The intensive land use required to grow corn for ethanol also has its own effects on water quality and fertilizer use, Runge says. For these reasons, as well as the effects of air pollution, many researchers argue that ethanol is not as environmentally friendly a fuel as is sometimes claimed. And Jacobson’s work found that a shift toward gasoline containing 85 percent ethanol would result in between 71 and 213 additional ozone-related deaths and 1,200 additional emergency room visits for asthma in the United States each year.
“Gasoline and ethanol are bad,” Jacobson says. “And the solution is really to move to electric vehicles.”
