United Airlines made history last week on a flight from Chicago to Washington, D.C. when it became the first commercial carrier with a passenger flight to use 100 percent undiluted sustainable aviation fuel.

The flight took off Wednesday with more than 100 passengers from Chicago O’Hare to Reagan National Airport on a 737 MAX 8. The flight used 500 gallons of sustainable aviation fuel (SAF) in one engine and the same amount of conventional jet fuel in the other engine.

Currently, airlines are only permitted to use a maximum of 50 percent SAF on a flight, and that is blended with conventional fuel in all tanks.

“(The) SAF flight is not only a significant milestone for efforts to decarbonize our industry, but when combined with the surge in commitments to produce and purchase alternative fuels, we’re demonstrating the scalable and impactful way companies can join together and play a role in addressing the biggest challenge of our lifetimes,” said United CEO Scott Kirby, who flew on the historic flight.

When the first sustainable aviation fuels were produced, they needed to be blended with petroleum-based fuels to have the right chemical composition to meet the industry standard. But that’s no longer the case, according to Fast Company.

The SAF used on the flight was made from fats, cooking oils and grease. It’s blended with synthesized compounds called aromatics—normally made from fossil fuels, but in this case derived from plant sugars.

The fuel is drop-in ready and compatible with existing aircraft fleets, according to United.

Overall this year, United purchased more than 7 million gallons of SAF, which it said produces nearly 80 percent lower greenhouse gas emissions over its lifecycle compared to conventional jet fuel, according to Travel and Leisure.

Airlines have been moving to become more environmentally friendly in recent years. British Airways flew a carbon-neutral flight from London to Glasgow in September using a combination of sustainable aviation fuel and carbon offsets.