Climate change is making asthma worse, as hotter temperatures increase ozone pollution and wildfires and make pollen season longer. But asthma, ironically, is also making climate change worse: Inhalers use chemicals that are as much as 3,600 times more powerful than CO2 at trapping heat in the atmosphere.

GSK, the U.K.-based pharma giant, says that 45% of its total global emissions come from the use of its Ventolin brand inhalers. Using one of the inhalers has roughly the same climate impact as driving a gas car for 175 miles. Globally, 35 million patients use the inhalers now.

Two years ago, the company started working on a low-carbon version of the inhaler. “We are committed to developing a safe and effective medicine for patients, which is much lower in global warming potential and without additional environmental concerns, such as PFAS,” says Margaret Rumpf, global commercial head of established pharmaceuticals at GSK.

The high carbon footprint comes from the propellant, a chemical that helps push the medicine out of a metered dose inhaler and into the lungs. Right now, the devices use chemicals called hydrofluoroalkanes. GSK already makes a “dry powder” version of the inhaler that has a low carbon footprint because it doesn’t include the propellant. But it wanted to also update its metered dose inhalers because they can be easier to use when someone is experiencing an asthma attack.

The company developed a new version that uses a different propellant, and plans to begin Phase III clinical trials next year. “The development of this low-carbon inhaler is complex and involves clinical and nonclinical programs, as well as establishing new manufacturing facilities,” Rumpf says. If all goes as planned, the device could be submitted to regulatory agencies for approval in 2025.

Affordability might be one challenge: Rumpf said that it was too early to say how the change might affect pricing. But there is a clear environmental benefit. The new inhaler is expected to have a carbon footprint that’s 90% smaller than the current version. It’s a major piece of GSK’s plans to reach net zero, which include an 80% drop in emissions across its value chain by 2030, and a 90% reduction by 2045.

It’s not the first time that inhalers have switched propellants—companies previously used CFCs, but stopped after the Montreal Agreement phased that chemical out because it was harming the ozone layer.