On a quiet country road, four thin metal cylinders containing the world’s most potent greenhouse gas are lashed to the base of a Duke Energy substation. Sulfur hexafluoride (SF6), an electrical insulator that was once used to fill race car tires, tennis balls, and even Nike Air Max shoes, is stored in the tanks.

According to the United Nations Intergovernmental Panel on Climate Change, a single pound of SF6 heats the planet as much as 25,200 pounds of carbon dioxide and remains in the atmosphere for 3,200 years. According to the US Environmental Protection Agency, SF6 is one of several long-lived, synthetic, fluorine-containing chemicals released by heavy industry, chemical manufacturers, semiconductor manufacturers, and electric utilities.

The EPA, which does not regulate any of these potent greenhouse gases, observes that once released, they are “essentially permanent additions to the atmosphere.”

According to mandatory reports filed with the EPA each year and reviewed by Inside Climate News, Duke Energy, which provides electricity in six states, leaked nearly 11 metric tons of SF6 into the atmosphere from its electric substations in North and South Carolina alone in 2020, more than any other utility in the country. According to the EPA’s greenhouse gas equivalency calculator, the emissions were equivalent to the annual greenhouse gas emissions of more than 59,000 automobiles.

According to Jeff Brooks, a spokesperson for Duke Energy, which is based a half-hour south of here in Charlotte, one reason the company’s emissions were higher than all other utilities in 2020 could be due to how frequently Duke Energy inspects equipment and refills leaked gas in comparison to other companies. If, for example, Duke Energy only refilled leaked gas on an individual piece of equipment every five years while other companies refilled leaked gas every two years, Duke would report five years of emissions for that piece of equipment for the year it was refilled while other companies would only report two years, according to Brooks.

Under this scenario, Duke Energy would most likely have years of low emissions followed by a single year of high emissions. This, however, is not the case. According to company data reported to the EPA, SF6 emissions from Duke Energy’s North and South Carolina electric utilities have been the highest of any electric power company in the country for six of the last seven years.

While other utilities have been participating in a voluntary program with the EPA for decades to reduce SF6 emissions to near zero, Duke Energy has not. Duke Energy’s leak rate in North and South Carolina, where it reports combined emissions from subsidiaries Duke Energy Carolinas and Duke Energy Progress as a single entity as a single entity, was 5.2% in 2020, or more than five times higher than the average leak rate of utilities participating in the EPA’s voluntary emissions reduction program.

As part of a grid strengthening and resiliency initiative, Brooks stated that the company is currently replacing older equipment across all six states that it serves. The initiative will replace older, leakier circuit breakers from the 1970s with newer, more leak-resistant equipment that still uses SF6. According to Brooks, Duke Energy is also prioritizing the replacement of its most leaky equipment.

According to the EPA, portable SF6 cylinders are frequently used by electric utilities to refill, or “top-up,” leaking electrical equipment. Duke Energy confirmed that the cylinders at its Davidson substation held SF6 but refused to say why they were there or whether they were full or empty. Brooks explained, “We sometimes stage cylinders at our sites for upcoming equipment commissioning or maintenance.”

Now, the 88 utilities that are currently members of the EPA emissions reduction program, which represent nearly half of the US electric grid, are looking for ways to completely eliminate SF6 emissions.

Unless the rate at which utilities leak SF6 slows, emissions in the United States will likely increase in lockstep with the electric grid. According to recent studies, the electric grid’s transmission capacity will need to be increased to two to five times its current level by midcentury if the United States is to meet its net-zero emissions targets.