The climate crisis has been named the world’s biggest health threat by the United Nations. Greenhouse gases are a cause, and, unfortunately, greenhouse gas emissions from U.S. health care have increased by more than 30% over the last decade (Nature 2020;586:248-56). U.S. health care greenhouse gas emissions account for around 8.5% of national greenhouse gas emissions and would rank 13th in the world – ahead of the entire United Kingdom – if the U.S. health care system was a country (Health Aff 2020;39:2071-9; asamonitor.pub/3KofI4c). Inhalational anesthetics account for 50% of perioperative emissions in high-income countries (Anaesthesia 2022;77:1023-9).
Various anesthetic agents have been compared to carbon dioxide’s global warming potential over a 100-year period (GWP100), where the larger the number, the higher the heat-trapping impact. The GWP100 for nitrous oxide (N2O) is 273, which means its global warming potential is 273 times that of carbon dioxide (asamonitor.pub/3GvhuPQ). For comparison, sevoflurane’s GWP100 is 144 (Environ Sci Technol 2021;55:10189-91). These numbers themselves can be an indication as to the deleterious effect; however, they become even more alarming when it is understood that this comparison is per molecule. Nitrous oxide is used in much higher concentrations due to its low potency compared with other volatile gases, thus the impact is even greater than the GWP100 alone. In clinically relevant doses, nitrous oxide has similar global warming impacts to desflurane (Br J Anaesth 2020;125:680-92).
The high GWP of N2O is partly due to its longevity in the atmosphere. Once emitted, it stays around for more than 100 years. Apart from the global warming potential, N2O has the additional effect of destroying the ozone layer, which protects us from harmful ultraviolet radiation. Since the Montreal Protocol led to a phasing out of chlorofluorocarbons, N2O is the most prevalent ozone-destroying gas (asamonitor.pub/3nPy2LV).
Nitrous oxide for labor analgesia
Nitrous oxide use for labor is common in much of Europe (Br J Anaesth 2016;117:eLetters Supplement). In the U.S., nitrous oxide has gained some popularity since approval of a self-administering system by the Food and Drug Administration that allows intermittent delivery of a fixed 50% nitrous oxide/50% oxygen mix. These systems utilize high flows, causing a large carbon footprint. For example, during four hours of labor, the system can have a carbon footprint equivalent to driving a car 1,500 km. In comparison, the carbon footprint of an epidural is equivalent to driving a car approximately 6 km (Australasian Anaesthesia 2021:193-202). Scavenging vacuum systems are set up to minimize nitrous oxide concentrations within the indoor laboring room to reduce occupational exposure. All the gas, however, is released into the outdoor atmosphere.
Nitrous oxide analgesia is often inadequate in labor, with around 40%-60% of women converting to epidural analgesia (Ochsner J 2020;20:419-21). Nitrous oxide has known nonanalgesic effects, like anxiolysis, and self-administration provides patients with a sense of autonomy that can lead to maternal satisfaction (ASA Monitor 2017;81:14-16). Perhaps selectively targeting the administration of nitrous oxide to this group of parturients could limit indiscriminate use and therefore mitigate nitrous oxide greenhouse effects, while still facilitating patient-centered care (Birth 2019;46:97-104). Environmental conservation is an important issue to many. Knowledge about the environmental impact of N2O might be a deciding factor for some parturients, and therefore potentially important to disclose when presenting analgesia options.
If an epidural is not an option, other alternatives like a remifentanil PCA might be a consideration. Some argue it is the most effective labor analgesia that is not an epidural. The short context-sensitive half-time of a few minutes means that even though placental transfer is significant, fetal drug concentration is low. Remifentanil is therefore perhaps safer for neonates than other opioids. Maternal satisfaction is high. However, potential adverse events like maternal sedation and respiratory compromise require the constant presence of a provider and a consistent infrastructure for nursing education, as the optimal timing of administration is not intuitive for patients (BJA Educ 2019;19:357-61).
During the COVID pandemic, centers found themselves not able to use N2O as there was concern for virus transmission when patients used N2O. A center in Australia compared data from their usual practice with N2O before and then during the pandemic. The results left them wondering if there is still a need to supply N2O to laboring wards: they saw no significant changes in mode of delivery or maternal or fetal outcomes, despite an increase in opioid use (Obstet Gynaecol 2022;62:910-4).
General anesthesia for C-sections
For many reasons, neuraxial anesthesia techniques are generally the preferred management for cesarean sections. General anesthesia is mostly reserved for emergencies due to the well-known concern for difficult airways and increased maternal morbidity and mortality. The minimal alveolar concentration (MAC) requirement for volatile anesthetics is reduced by 30%-40% at term. This is advantageous as volatile anesthetics have a concentration dependent impact on uterine muscular tone with risk of postpartum hemorrhage. Nitrous oxide reduces the MAC requirement for volatile anesthetics further, thereby maintaining uterine tone while preserving cardiovascular stability. As such, it has been classic practice to combine nitrous oxide with volatiles after delivery of the newborn (Br J Anaesth 2019;122:587-604). The MAC-sparing properties reduce the usage and therefore the environmental impact of the other potent volatile anesthetic used (e.g., sevoflurane). However, this is negated by the significantly higher concentration and therefore relative higher environmental impact of N2O. Using minimal fresh gas flows can help mitigate pollution, although this practice can risk accumulation of N2O in the circuit and subsequent hypoxic gas mixes (Br J Anaesth 2019;122:587-604).
Some may be concerned about achieving desired MAC with volatiles alone. In addition to possibly increasing the risk for poor uterine tone and postpartum hemorrhage, reducing volatile concentrations without the addition of nitrous oxide could risk awareness. This might leave the professional feeling stuck between a rock and a hard place, particularly knowing that awareness risk is increased during a cesarean section (asamonitor.pub/3Uta8lF). There are intravenous medications available to mitigate this risk, such as benzodiazepines. Furthermore, numerous uterotonics are available to help prevent hemorrhage.
Alternatively, total intravenous anesthesia (TIVA) can be considered either from the start of the case (which can be a logistical challenge) or as soon as circumstances allow. As propofol does not have the same relaxing effect on the myometrium in clinical practice, addition of N2O would not be required (Int J Obstet Anesth 2022;51:103548). Unlike sevoflurane, propofol does not have analgesic properties. However, administration of opioids after delivery of the infant is common and should easily counteract this. Propofol’s relative impact on greenhouse gas emissions is much smaller than any of the volatile anesthetics, by several orders of magnitude in clinically relevant doses, even when plastic waste is taken into consideration (Anesth Analg 2012;114:1086-90). Some believe the use of propofol for cesarean sections is currently understudied, particularly regarding neonatal outcomes if used pre-delivery. Studies have shown, however, that APGAR scores do not change significantly with longer duration of infusion prior to delivery (Int J Obstet Anesth 2022;51:103574; Taiwan J Obstet Gynecol 2017;56:521-6). These techniques can help minimize nitrous oxide usage if not remove it from practice all together.
There is evidence on three continents showing massive waste of nitrous oxide through hospital pipeline leakages of up to 95%, pre-use (BMJ 2022;377:o1301). Supplying nitrous oxide from portable cylinders rather than central pipelines can prevent significant environmental contamination. Throughout the U.K., the Nitrous Oxide Project is working to transition to portable nitrous oxide only (asamonitor.pub/3nW8rRz). The University of California system has made it one of their climate action goals for this year to convert from pipeline to portable cylinder nitrous oxide (asamonitor.pub/43mAslp). This will save the equivalent of 110,000 miles driven per month at UC Davis alone. In addition, there will be no pipelines installed for N2O in any new buildings at UC Davis (Personal communication, Dr. Richard Applegate III, Chair Department of Anesthesiology and Pain Medicine, email, February 22, 2023). The University of Southern California has gone even further and completely removed nitrous oxide from their anesthesia machines (Personal communication, Dr. Arash Motamed, University of Southern California, email, February 15, 2023).
Near-empty cylinders that are returned to the manufacturer can also be a problem as regulations in Europe prevent any remaining N2O from “re-use,” resulting in quantities remaining in “empty” tanks (estimated at approximately 10%) to be vented to the outdoor air prior to the cylinders being refilled (asamonitor.pub/3Kic384). Similarly, the largest manufacturer of N2O in the U.S. references FDA regulations and vents any remaining N2O (asamonitor.pub/3odJ1ip; Philippe Farner, Airgas, Personal communication, March 13, 2023). Scavenged nitrous oxide can undergo catalytic conversion into less harmful products (nitrogen and carbon dioxide), a common approach to waste disposal management in Sweden on its laboring units (Anaesth Rep 2022;10:e12182). However, the vast majority of exhaled gas is not collected for potential treatment (BMJ 2022;377:o1301).
Even though N2O has been around for decades, the discussion about its pros and cons is still ongoing (Biomed Res Int 2019; 2019:4618798). In this article, we wanted to highlight the environmental concerns regarding nitrous oxide and give practical ideas as to how its use in obstetric practice can be minimized. We are advocating for the elimination of its use, or at least for reducing its usage as much as possible. Education of staff as well as patients will raise awareness and might change preferences to more carbon-friendly alternatives. If used, we encourage low fresh gas flows. Even if you are deciding to continue its use, institutions can significantly reduce emissions by decommissioning pipeline systems and using portable cylinders only. As clinicians who have vowed to “first do no harm,” we must be the driving force for change so that we can reduce greenhouse gas emissions from our labor wards and help mitigate climate change effects for us and future generations.