BACKGROUND: Inhaled anaesthetics are greenhouse gases. However, changes in the delivery of inhaled anaesthetics can mitigate environmental impact. We hypothesised that system-wide changes to the delivery of anaesthesia care would reduce environmental harm without compromising patient outcomes. METHODS: We launched the Green Anesthesia Initiative (GAIA) in March, 2022, with the aims of reducing the use of nitrous oxide, using less environmentally harmful inhaled fluorinated ethers, and increasing intravenous anaesthetic use. In this retrospective cohort study, we used electronic health record data from general anaesthetics performed on all patients older than 1 year between March 1, 2021, and Feb 28, 2023, at a single US academic medical centre across multiple sites, collecting data from before and after the introduction of GAIA. Patients with missing or invalid data recorded by the anaesthesia machine, patients given general anaesthetics for electroconvulsive therapy, and patients who met American Society of Anesthesiologists Physical Status Classification 6 were excluded. Using multivariable modelling, we compared estimated CO FINDINGS: We recorded 45 692 patients pre-intervention (23 193 [50·8%] female, 22 494 [49·2%] male, five [<
0·1%] unknown) and 47 199 post-intervention (23 981 [50·8%] female, 23 209 [49·2%] male, nine [<
0·1%] unknown). After the implementation of GAIA, CO INTERPRETATION: A health-system wide intervention reduces greenhouse gas emissions attributable to anaesthesia care without detriment to patient outcomes. FUNDING: University of Michigan Medical School and National Institutes of Health.