- ★ Volatile anesthetics and nitrous oxide (N2O) are potent greenhouse gases. N2O also contributes to depletion of the ozone layer.
- ★ Desflurane is the inhaled anesthetic that is the most potent greenhouse gas, with a global warming potential (GWP100) of 2540, followed by isoflurane (539), nitrous oxide (273), and sevoflurane (144). However, clinical doses must also be considered.
- ★ Because N2O has relatively low potency, higher concentrations (and therefore mass quantities) are required to achieve clinical effects compared with volatile anesthetics. In clinically equivalent doses, ranked climate impacts: desflurane > nitrous oxide > isoflurane > sevoflurane.
- ★ Life cycle greenhouse emissions from inhaled anesthetics are several orders of magnitude greater than for propofol.
- ★ Mitigation strategies include:
- Avoiding inhaled anesthetics and using intravenous and regional anesthesia when clinically appropriate.
- If inhaled anesthetics are used, desflurane and nitrous oxide should be avoided unless there are clear clinical indications.
- Minimizing fresh gas flows.
- Abandoning central nitrous oxide pipelines and substituting portable tanks that remain closed between uses.
- ★ Waste anesthesia gas capture and destruction technologies appear promising but require additional research to verify extraction efficiencies and overall emissions reduction benefits.
Inhaled Anesthetics are Potent Greenhouse Gases
Inhaled anesthetics are potent greenhouse gases with heat trapping properties (global warming potential) hundreds to thousands of times greater than an equivalent mass of carbon dioxide.64 In addition, some inhaled anesthetics, notably nitrous oxide (N2O), also contribute to the depletion of the ozone layer.65 During clinical use, inhaled anesthetics are mostly vented through gas scavenging (vacuum) systems to protect against indoor occupational exposure but are ultimately released (scavenged and unscavenged) to the outdoor atmosphere in an uncontrolled manner.66,67
Volatile hydrofluorocarbon anesthetics (desflurane, sevoflurane, isoflurane, halothane), and N2O are used routinely during intraoperative care. Although insufficient data are available on the global production and consumption of these medical gases, estimates of their contribution to total global greenhouse gas emissions range from 0.01% to 0.1%.10,68 In clinical contexts, inhaled anesthetics can account for 50% of perioperative emissions,88 5% of emissions from hospitals,69 and 3% of total national health care emissions.10,41,69
Atmospheric concentrations of desflurane (which has the highest global warming potential of all inhaled anesthetics) are increasing, while sevoflurane and isoflurane are constant or decreasing, respectively.70 It is now estimated that desflurane is responsible for 80% of the greenhouse effect from all volatile anesthetic pollution.10,70
Atmospheric Lifetime of Inhaled Anesthetics and Their Global Warming Potential
The degree to which each anesthetic agent will act as a greenhouse gas depends on both its unique infrared absorption spectrum and its atmospheric lifetime. Greenhouse gases absorb infrared radiation, preventing radiation of this energy back into space. The absorbed radiation is subsequently reemitted and retained as heat in the atmosphere. This process occurs until the gases undergo degradation in the atmosphere.10,64,71 Because inhaled anesthetic agents undergo minimal metabolism during clinical use, the vast majority of exhaled gases remain chemically intact upon release from health care facilities.11,72 The atmospheric lifetimes of the most commonly used anesthetic gases vary considerably between desflurane (14 years), isoflurane (3 years) and sevoflurane (2 years). Nitrous oxide has a particularly long atmospheric lifetime of about 114 years.10,64,71
Global warming potential (GWP) is a mass-based measure of how much a greenhouse gas contributes to global warming over a specified period of time, and the international standard is 100 years. GWP is a relative scale that compares the contribution of the gas in question to an equivalent mass of carbon dioxide. By definition, the GWP of carbon dioxide is one and is the unit of comparison. Among inhaled anesthetics, desflurane has the highest GWP100 (2,540), followed by isoflurane (539), nitrous oxide (273), and sevoflurane (144).71 These factors are routinely updated as atmospheric chemistry is continuously changing.71
Anesthetic Choice and Fresh Gas Flow Management Affect Environmental Impact
Several studies compare the relative environmental impact of anesthetic agents in clinical practice and consider ways to minimize the associated greenhouse gas emissions.72-75 Inhaled anesthetic emissions are driven by two clinical management decisions: 1) the choice of inhaled anesthetic (with its associated heat trapping properties or GWP; see section above); and 2) the total amounts used and released to the atmosphere. The amount of inhaled anesthetic used heavily depends on the fresh gas flow rates as well as the clinical potency (mean alveolar concentration [MAC]) of the anesthetic selected.
Greenhouse Gas Emissions of Common Inhaled Anesthetic Agents. (Adapted from Ryan, et al.72)
MAC inhaled agent
Atmospheric lifetime (years)
100-year Global Warming Potential (GWP)71 (per kg, in comparison with 1 kg CO2, where GWP CO2 = 1)
Equivalent auto miles* driven MAC-hour of anesthetic use at 1 L/min
60% Nitrous Oxide (0.6 MAC)
MAC = mean alveolar concentration
* Based on EPA 2022 emission factor of 4.03 x 10-4 metric tons of CO2- equivalent/mile
Beyond GWP, the role of clinical potency in anesthetic consumption and subsequent greenhouse gas emissions is an important concept that is often overlooked. (See table 1 above.) Desflurane not only has the highest GWP100 but also has the lowest clinical potency (MAC 6.7%) of the volatile drugs, requiring three-to-five times the concentration of sevoflurane (MAC 2.2%) or isoflurane (MAC 1.2%), respectively, to achieve an equivalent clinical effect at similar fresh gas flow rates. While N2O has a lower GWP100 (273) than isoflurane (539), it is usually delivered at a concentration of 50-70%, resulting in a higher overall environmental impact.35,71,72
Life Cycle Assessment of Inhaled Anesthetics
Global warming potential of the anesthetic gases does not tell the whole story, however. Cradle-to-grave life cycle emissions must also be considered, including those stemming from manufacturing, packaging, and transportation. A life cycle assessment of inhaled anesthetic agents and intravenous propofol was performed comparing 1-MAC-hour equivalent quantities, including plastic syringes and tubing as well as energy to run infusion pumps.75 Compared with intravenous propofol, inhaled anesthetic life cycle emissions were several orders of magnitude greater. Between inhaled anesthetics, desflurane emissions were 15 times greater than isoflurane and 20 times greater than sevoflurane. However, this analysis used 2L/min fresh gas flow for sevoflurane and 1L/min fresh gas flow for desflurane and isoflurane. Lowering the fresh gas flows of sevoflurane to 1L/min would result in a desflurane footprint that is 40 times greater than sevoflurane. Combining N2O with volatiles significantly increased emissions compared with isoflurane or sevoflurane alone. The direct greenhouse gas emissions from the waste disposal life cycle phase constituted the overwhelming portion of the environmental impacts of inhaled anesthetics, whereas the manufacturing and energy to run the infusion pump constituted the majority of emissions from propofol, suggesting additional areas for pollution mitigation.
Strategies to minimize environmental impact include avoiding desflurane and N2O unless there are clear clinical indications. Total intravenous anesthesia and regional anesthesia should also be considered, when clinically appropriate, to avoid inhaled anesthetic pollution altogether.66,67,75 When inhaled anesthetics are used, fresh gas flows should be minimized, including on induction. (See ‘Fresh Gas Flow Management’ section, below.) The FDA package insert recommends sevoflurane be used at a minimum of 1L/min FGF for up to 2-MAC-hours, and 2L/min thereafter, when using CO2 absorbents with sodium hydroxide content above 2%. However, this is not observed globally, and renal injury secondary to Compound A has not been demonstrated. Sevoflurane is generally considered safe even at low fresh gas flows (<= 1L/min), particularly with low or no sodium hydroxide in CO2 absorbents.10,76-79 The ASA Statement on the Use of Low Gas Flows for Sevoflurane states, “The ASA has evaluated current scientific studies and concludes there is no reasonable evidence to support a lower limit of fresh gas flow when using sevoflurane. Therefore, the ASA supports the use of low fresh gas flows when sevoflurane is administered."< />
Waste Anesthetic Gas Treatment
Volatile gas capture and nitrous oxide destruction are two methods of potentially treating scavenged waste to mitigate their emissions.10,80 Several waste anesthetic gas capture technologies are emerging in the market that either directly adsorb volatiles onto a material matrix or cryogenically condense volatiles into a liquid.80 Captured and purified volatile anesthetics could potentially be sold for reuse; however, absence of broad regulatory approval means that captured drug must be transported and stored. Due to nitrous oxide’s low boiling point and low cost, capture is not commercially viable. However, photochemical destruction technologies are available and are in routine use in Scandinavia.81 Photochemical destruction of volatiles is also feasible, though not yet commercially available,82 and would ameliorate transportation, purification, and resale or storage issues that capture technologies create. Additional challenges include the efficiency of waste anesthetic gas treatment. One recent estimate of extraction efficiency for desflurane was only 25%,83 while another researcher found treatment efficiency to be variable and reduced as fresh gas flows are increased.82
Notably, these capture and destruction technologies apply only to anesthetic gases entering the waste anesthesia gas scavenging system. Yet many clinical practices, including inhaled inductions and deep extubations, as well as improper self-administration of N2O analgesia (e.g., by parturients), lead to significant quantities of un-scavenged anesthetic agents.
These studies highlight the need for further peer-reviewed efficiency assessments of these waste treatment systems. In addition, these technologies should undergo full life-cycle assessments to quantify the environmental impacts of the devices themselves in addition to transportation, processing, and storage of reclaimed waste.66,67 Waste treatment technologies should be considered only after mitigation efforts are maximized. Avoiding anesthetics with large climate impacts, decommissioning central pipelines, and minimizing fresh gas flow rates remain higher-priority interventions to reduce greenhouse gas emissions.66,67
Nitrous Oxide Pre-Utilization Losses
Recent reports from health systems in the U.K., New Zealand, and the U.S. found significant losses of N2O, ranging from 77-95%, occurring prior to clinical use via central pipeline system leaks around manifolds.84,85 This is leading to a growing movement of facilities abandoning central N2O piping, and substituting portable tanks. This was first promulgated through the Nitrous Oxide Project out of the University of Edinburgh86,87 and is advancing across the U.K. through the Association of Anaesthetists.88 Additional mitigation solutions include ensuring that portable tanks are systematically closed between uses or at the end of the day, or even electing to eliminate N2O from the formulary.66,67