Carbon Footprint of Anesthesia: Reply

We thank all authors for the correspondence^1-6  relating to our life cycle assessment of anesthesia for knee replacements.⁷ 

In response to Kalmar et al.¹ : We used the Intergovernmental Panel on Climate Change preferred⁸  global warming potential of 100 yr, given that it is the recognized compromise between short- and long-lived greenhouse gases. The third reference in the article by Kalmar et al.⁸  gives sevoflurane’s global warming potential as 195. The fourth reference in the article by Kalmar et al.⁹  is cited as being more recent and accurate for sevoflurane’s global warming potential (130), yet we referenced that article.^7,9  We note sevoflurane’s recently updated global warming potential of 144 by Andersen et al.,¹⁰  with concerns that the Intergovernmental Panel on Climate Change’s global warming potential for sevoflurane⁸  of 195 is inaccurate. The global warming potential for carbon dioxide itself requires intermittent updating as new data arrive, leading to adjustment of the global warming potentials for anesthetic gases.¹¹  There are also uncertainties with all global warming potentials, particularly for trace anesthetic gases.¹²  Nevertheless, we had used the most accurate global warming potential for sevoflurane (130), but acknowledge that a 10% [(144 – 130)/130] adjustment upward to a global warming potential of 144 is required now.

We did not include the carbon dioxide equivalent emissions from the production of volatile anesthetics as the article by Hu et al.¹³  is very recent. Hu et al. indicated two methods of producing sevoflurane, with manufacturing method A leading to approximately fivefold greater production of greenhouse gases than the clinical use of sevoflurane itself. The lower carbon manufacturing method B produced a similar magnitude of greenhouse gases as clinical use of sevoflurane. It is unclear why Hu et al. found much greater carbon dioxide equivalent emissions from the manufacture of sevoflurane than estimated by Sherman et al.,¹⁴  particularly as Hu et al. noted, “The processes described in Method-B are similar to the ones modeled by SciFinder in Sherman.”¹³  Neither paper had access to commercial pharmaceutical manufacturing data.

We sought production information from Baxter Healthcare (Deerfield, Illinois), a multinational supplier of sevoflurane. Baxter’s February 2022 letter of response (from Jason Vollen, M.B.A., Baxter Healthcare) was as follows: “On the basis of the evidence…the majority of our sevoflurane comes from a process that most aligns with ‘Sevo B’ (i.e., the lower [greenhouse gas] emissions’ method).” We thus note the much higher greenhouse gas numbers for sevoflurane calculated by Kalmar et al., but indicate that the majority of these concerns are moot. Collaborative industry research to clarify the true environmental cost of sevoflurane manufacture is urgently required.

In response to Norman et al.² : Norman et al. raise important concerns about single-use plastics. Our study focused upon the carbon footprint of anesthesia, although, as in all robust life cycle assessments, we obtained data (unpublished data about other environmental effects such as physical waste and aquatic toxicity, among others) about the end of life of all waste. Using more single-use plastics will evidently create more trash with attendant concerns about the ultimate resting place of such rubbish.¹⁴ 

With the rapid move toward electricity decarbonization in Australia¹⁶  (and elsewhere), the aphorism “renewable (electricity) makes reusable (equipment) better” becomes more relevant. The combination of reduced carbon emissions, reduced plastic waste, and lower financial costs when anesthesiologists use reusable equipment¹⁷  becomes a powerful argument to abandon single-use plastics.

In response to Gobert and Dernis³ : Thank you for emphasizing the variability in how often anesthetic breathing circuits are changed despite studies indicating the safety of less frequent changes.^18,19  Weekly circuit changes, reusable or disposable, are certainly financially and environmentally more sound, and clinically no less safe than changing with each patient. We (and others)²⁰  suggest engagement with infection prevention to challenge the dominant infection prevention paradigms that (1) single use is safer, and (2) the financial and environmental costs of clinical care are simply externalities. Anesthesiologists can lead the way collaboratively just as they have for safety and quality assurance.

In response to Tsai et al.⁴ : We have previously corresponded with Tsai et al. about water use required for cleaning reusable anesthetic equipment,²¹  and remain in agreement that water use is an important local environmental consideration. The concerns of Tsai et al. about the limitations of life cycle assessment are also correct, but we note that methodologic techniques are evolving to incorporate life cycle assessment into Herman Daly’s ideas about ecological economics, e.g., by Pelletier et al.²²  Nevertheless, we remain focused upon carbon dioxide equivalent emissions as global climate change is an existential threat.

As to water, we remain committed to running hospital steam sterilizers more efficiently.²³  Kaiser Permanente (USA) has emulated our efforts and saved approximately US$300,000 per annum by more efficiently using their steam sterilizers.²⁴  We encourage anesthesiologists to collaborate toward a more environmentally sustainable healthcare system.

In response to Carter and Davies⁵ : Carter and Davies indicate the importance of interpreting our study within the context of one’s institution and practices (e.g., energy use, efficiency of resource use, and behaviors). Importantly, as the life cycle carbon footprint of single-use plastic (e.g., polypropylene) is less than 10% attributable to electricity, a switch to 100% renewable energy for plastic manufacture will have a much lower effect on single-use plastic’s overall carbon dioxide equivalent emissions than moving to 100% renewable electricity for cleaning reusables. With Australia’s movement toward 100% renewable energy,²⁵  the carbon footprint of reusable anesthetic equipment will decrease to levels similar to those in Europe. We encourage anesthesiologists to return to reusables where possible.

In response to Schroeder et al.⁶ : Schroeder et al. emphasize the significant environmental impact of reusable sterile gowns in our study.⁷  Schroederet et al. indicate the American Society of Anesthesiologists (Schaumburg, Illinois) does not recommend sterile gowns for neuraxial procedures in recent practice guidelines.²⁶  Nevertheless, the Australian and New Zealand College of Anaesthetists (Melbourne, Australia)²⁷  and the New York School of Regional Anesthesia (New York, New York)²⁸  recommend gown use for spinal anesthesia.

A welcome outcome of our research could be to promulgate greater understanding of regional and international variation in anesthetic practice, and the corresponding rationale. Since it is unlikely that there is a difference in infection rates from spinal anesthesia with or without a sterile gown, the focus of guidelines could shift to include protecting the patient and the environment.²⁹ 

We appreciate concerns about high oxygen flow rates in our study.⁷  Oxygen can be titrated to low flows via a facemask (4 l/min) while avoiding rebreathing,³⁰  or via nasal prongs with close monitoring. Our observational study⁷  revealed surprising practice variations that could lead to large environmental footprints nationally from anesthesia. We encourage others to clarify such practice variations and begin the journey to safely reducing anesthesia’s environmental footprint.

Dr. McGain has received grant funding from the Australian National Health and Medical Research Council (Canberra, Australia), Australian Medical Research Future Fund (Canberra, Australia), and the Australian and New Zealand College of Anaesthetists (Melbourne, Australia). S. McAlister has received grant funding from Wiser Care and the Australian National Health and Medical Research Council. The other authors declare no competing interests.