– A woman lay on the operating table at Provident Hospital in Portland, Ore., preparing for surgeons to remove a growth in her stomach.

But first, an anesthesiologist — Dr. Brian Chesebro — put her under by placing a mask over her face.

"Now I'm breathing for her with this mask," he said. "And I'm delivering sevoflurane to her."

Sevoflurane is one of the most commonly used anesthesiology gases. The other big one is desflurane.

Whichever gas a patient gets is inhaled, but only about 5% is metabolized. The rest is exhaled. And to make sure the gas doesn't knock out anyone else in the operating room, it's sucked into a ventilation system.

And then? It's vented up and out through the roof, to mingle with other greenhouse gases.

The two gases are fairly similar medically and generally anesthesiologists tend to pick one of the two gases and stick with it. Few understand that desflurane is much worse for the environment.

And that bothered Chesebro. He grew up on a ranch in Montana that focused on sustainability.

Now he lives in the city with his three kids and has gradually started to worry about their environmental future. "When I look around and I see the stewardship on display today, it's discouraging," he said. "I hit the pause button on myself and said, 'Well, what's the very best that I can do?' "

He learned desflurane is 20 times more powerful than sevoflurane in trapping heat in the Earth's atmosphere. It also lasts for 14 years in the atmosphere, whereas sevoflurane breaks down in one year.

Opening a notebook filled with diagrams, he showed how he computed the amount of each gas the doctors in his group practice used. Then he shared their carbon footprint with them.

One of the doctors he shared his analysis with was Dr. Michael Hartmeyer, who said, "I wish I had known earlier. I would have changed my practice a long time ago." He said he was stunned when Chesebro explained that his use of desflurane was the greenhouse gas equivalent of driving a fleet of 12 Hummers for the duration of each surgical procedure. It's "only" half a Hummer if he uses sevoflurane. Hartmeyer, who noted that he drives a Prius, a hybrid electric car, said, "This was, far and away, a relatively easy thing that I could do that made a huge impact."

Other anesthesiologists made the switch, too. And it didn't hurt that sevoflurane is considerably cheaper. Hartmeyer's change saved his hospital $13,000 a year.

When Chesebro shared his findings with the anesthesia departments at all eight Providence Health hospitals in Oregon, they prioritized the use of sevoflurane. They now save about $500,000 a year.

Providence's chief executive, Lisa Vance, said the hospital system didn't change because of the money. It changed because the World Health Organization now says climate change is the No. 1 public health issue of the 21st century — and because of Chesebro.

Dr. Jodi Sherman, an associate professor of anesthesiology at Yale School of Medicine, called Chesebro's efforts remarkable and important. She said several hospitals around the country have tried to make this shift, but with mixed results. Some just gave anesthesiologists the information and not much changed. Other hospitals took desflurane away, but that left many anesthesiologists feeling disrespected.

Chesebro succeeded, she said, because he chose to persuade his colleagues — using data. And it helped that he showed them over and over, so doctors could compare their progress to their peers.

Sherman said Chesebro's efforts are needed because the health sector is responsible for about 10% of U.S. greenhouse gases. "We clinicians are very much focused on taking care of the patient in front of us," she said. "We tend to not think about what's happening to the community health, public health — because we're so focused on the patient in front of us."

Baxter International, one of the largest manufacturers of both the anesthesia gases, said it's important to provide options for patients. It said inhaled anesthetics have a climate impact of 0.01% of fossil fuels.

It's a fair point, Chesebro said, but he has a counterargument. "If I can reduce my life's footprint by a factor of six, why wouldn't you do it?' "