Since Fox et al. first published their warning in 1975,1 concern has been repeatedly expressed about the potential harm that the release of halogenated general anesthetic gases poses to the global environment.2–8 All the volatile anesthetics that are currently used (halothane, isoflurane, enflurane, sevoflurane, and desflurane) are halogenated compounds potentially destructive to the ozone layer. The widely used anesthetic gas nitrous oxide (N2O) is an established greenhouse gas.5,9 A recent report suggests that N2O is also an important ozone-depleting gas.10
As the world population continues to grow and as modern anesthesia becomes available to more regions of the world, the global use of volatile anesthetics and N2O will rapidly grow. General anesthetics were administered to approximately 50 million patients in the United States in 2006, according to data released by the National Center for Health Statistics.a Anesthetic vapors and gases (for the purposes of this article we combine both the vapors and gases, calling all of these anesthetic gases) are also widely used in dentists' offices, veterinary clinics, and laboratories for animal research. A key attribute that differentiates all of these anesthetic gases from other medical drugs is that they are substantially eliminated through exhalation, without being metabolized in the body. At present, most anesthesia systems transfer these waste gases directly and unchanged into the atmosphere. Although the introduction of scavenging systems has significantly reduced spillage of general anesthetics into the operating room,11 they are still exhausted into the environment. Little consideration has been given to the ecotoxicological properties of gaseous general anesthetics.
VOLATILE ANESTHETICS AND THE GLOBAL ENVIRONMENT
Chemically, halogenated volatile anesthetics are closely related to the chlorofluorocarbons (CFCs), which play major roles in ozone depletion. The effect of a volatile anesthetic on ozone depletion will depend on its molecular weight, the number and type of halogen atoms, and its atmospheric lifetime12 (defined as the time taken to remove or transform 1/e, or 63%, of an emitted gas13). The atmospheric lifetime of these trace gases depends on their removal by chemical reaction with radicals, photolysis, and dry or wet deposition, such as “rainout.” Those species with a tropospheric (Fig. 1) lifetime of more than 2 years are then believed to reach the stratosphere in significant quantities. The tropospheric lifetime of halogenated anesthetics is much shorter than that of CFCs (Table 1), because hydrogen atoms of the anesthetic molecules are susceptible to attack by hydroxyl radicals in the troposphere,2 making them less likely to reach the stratosphere. However, a concern has been raised about very short-lived compounds (with a lifetime of a few months or less) and their potentially significant contribution to ozone destruction.14 Once anesthetics reach the stratosphere, chlorine-containing anesthetics such as halothane, isoflurane, and enflurane may be more destructive to the ozone layer than are newer drugs, such as sevoflurane and desflurane, which are halogenated entirely with fluorine.
By measuring the rate of reaction with hydroxyl radicals, Brown et al. have calculated that the tropospheric lifetimes of halothane, enflurane, and isoflurane are 2, 6, and 5 years, respectively.2 A more recent evaluation of the lifetimes of halogenated volatile anesthetics and their potential contribution to ozone depletion has been reported by Langbein et al.4 Using measurements of hydroxyl radical reaction kinetics and ultraviolet absorption spectra of anesthetics, we estimated the total atmospheric lifetimes of these anesthetics at 4.0 to 21.4 years. Contributions to total stratospheric ozone depletion were reported as approximately 1% for halothane and 0.02% for enflurane and isoflurane, suggesting that these anesthetics can play important roles in ozone depletion.
The global warming potential (GWP) of halogenated anesthetics is reported to range from 1230 (isoflurane) to 3714 (desflurane) times the GWP of carbon dioxide (CO2) (Table 1). Recently, Ryan and Nielsen reported on the impact of halogenated volatile anesthetics on global warming within the framework of common clinical practice, an approach that has not been taken before.15 Their study suggests that all the anesthetics (isoflurane, sevoflurane, and desflurane) can have a significant influence on global warming with the greatest impact produced by atmospheric desflurane.15
NITROUS OXIDE AND THE GLOBAL ENVIRONMENT
With an atmospheric lifetime of approximately 120 years, N2O is a remarkably stable gas.16 N2O traps thermal radiation escaping from the Earth's surface, contributing to what is known as the “greenhouse effect” (Fig. 1). The GWP of N2O is approximately 300 times more than that of CO2 (Table 1).17,18 N2O, along with CO2 and methane, are the most influential long-lived greenhouse gases among all gases encompassed by the Kyoto Protocol.19 N2O is produced by human sources including agriculture (nitrogen-based fertilizers) and the use of fossil fuels, as well as natural sources in soil and water, such as microbial action in moist tropical forests. The N2O concentration is reported to be steadily increasing at a rate of 0.7 to 0.8 parts per billion (ppb) per year in past decades, and N2O currently contributes about 6% of the total radiative forcing (difference between incoming and outgoing radiation energy within the Earth's atmosphere).20 In addition, N2O is a primary source of stratospheric nitrogen oxides, referring specifically to NO and NO2. Both destroy ozone. Although the ozone depleting potential (ODP) of N2O (0.017) is lower than that of CFCs (only 10% of N2O is converted to nitrogen oxides), N2O emission is reported to be the single largest ODP-weighted emission and is expected to remain the largest for the rest of this century.10
Sherman and Cullen first reported in 1988 that N2O, the most popular anesthetic gas, could contribute to global warming, and estimated that approximately 1% of total N2O production was used for clinical anesthesia on the basis of the number of surgical procedures in the United States, approximately 21 million cases at that time.9 They estimated the worldwide annual use of N2O for anesthesia to be 0.5 to 1.0 × 109 moles (2.2 to 4.4 × 104 tons).
Although the precise quantities manufactured for medical use are unavailable to the public, we can estimate the most recent consumption of N2O for anesthetic purposes. Our institution consumed 20.2 tons of N2O for anesthetic use in 2006 for approximately 40,000 procedures that were performed with an anesthesiologist present. In the United States, approximately 70 million procedures were performed in 2006 with an anesthesia provider (all types of anesthesia included), according to data from the National Center for Health Statistics.b Extrapolating from these figures, we estimate that approximately 3.5 × 104 tons of N2O were used for anesthetic purposes for 70 million patients in 2006 in the United States. The latest inventory of greenhouse gas emissions by the United States Environmental Protection Agency reports that total United States emissions of N2O were 1.187 × 106 tons in 2006, a reduction of 8% from 1996 levels.21 The anesthetic use of N2O is therefore estimated to be 3.0% of total 2006 N2O emissions in the United States. These numbers are provided only as an example of the volume of N2O liberated by 1 country, because there seems to be a declining trend in the use of N2O in European countries. However, the data on the worldwide anesthetic use of N2O, including all developed and developing countries, are not available. Until those data are obtained, a warning that the medical use of N2O can be a significant contributor to overall greenhouse gas emissions should be maintained.
CURRENT AND FUTURE ALTERNATIVES
Closed or Low Flow Anesthesia and Total IV Anesthesia
The use of volatile anesthetics could be reduced by up to 80% to 90% if closed circuit anesthesia were widely used for all patients, and to a lesser degree if “low-flow” anesthesia were routinely used. Although closed-circuit anesthesia is not a difficult technique with modern anesthesia systems for well-trained anesthesiologists, continuous accurate gas monitoring is required to prevent inadequate oxygenation or volatile anesthetic concentration.22 Shifting to total IV anesthesia would eliminate the use of anesthetic gases. Nevertheless, many anesthesiologists may still prefer volatile anesthetics and N2O, and their use is almost always required for anesthesia in infants and children. Modifying our practice towards more conservation of anesthetic gases can usually be done without compromising patient care if appropriate monitoring is used, and these techniques should be available to most anesthesiologists in developed countries.
New Technologies to Prevent Wasting Anesthetics into the Atmosphere
Doyle et al. have shown that silica zeolite (Deltazite™) was effective at completely removing isoflurane (1% in exhaled gases) in the scavenging line for a period of 8 hours.23 The trapped halogenated agents could then be reprocessed by steam extraction or fractional distillation for reuse. Reprocessing techniques are essential to reducing the amount of the anesthetics released into the atmosphere because disposal does not change the eventual fate of the anesthetics. A technique for conserving halogenated anesthetic vapors using a zeolite filter at the Y-piece connector has been proposed by Thomasson et al.,24 and the principles of this technique have been used to develop an anesthetic conserving device (ACD).25,26 The system is closed to volatile anesthetics, but it is open to oxygen; volatile anesthetics are supplied to the ACD through a syringe pump. This system has been shown to successfully reduce the total amount of volatile anesthetics released by 40%–75%, suggesting that the ACD may provide an alternative to low-flow systems.
Xenon and Future Anesthetic Gases
First reported >50 years ago,27 the anesthetic property of xenon has been revisited.28,29 Xenon is a naturally occurring atmospheric trace gas, existing at 0.08 parts per million (ppm), with no known detrimental ecotoxicological effect. The pharmacokinetic benefits of xenon include profound analgesia, neuroprotection, and hemodynamic stability.28,30 Xenon also has an extremely low blood–gas partition coefficient, which lends itself to rapid induction and emergence. However, clinical use of xenon has been limited mostly by its high cost of manufacture, which involves fractional distillation of liquid air. Furthermore, the production of xenon consumes enormous amounts of energy (220 W/h per 1 L of xenon gas),c significantly more energy than that required for N2O production.31 Routine use of xenon for clinical anesthesia would only be economically possible with a closed-circuit system that recycles the rare gas.
An ideal inhaled anesthetic should be safe, effective, and environmentally benign. This third characteristic has received insufficient consideration in part because of uncertainties on the environmental effects of gaseous anesthetics. Key criteria that will determine the global environmental impact of alternatives to halogenated anesthetics and N2O are their atmospheric lifetime, GWP, and ODP. These characteristics should be determined for existing anesthetics, and for any new anesthetic gases before widespread clinical use. Novel anesthetic gases should be adopted only if the clinical benefits outweigh any adverse environmental consequences.
Although anesthetic gases are considered medically essential, an appreciable change is occurring in medical society. CFC propellants were previously considered medically essential for metered dose inhalers, but these have now been replaced with hydrofluoroalkane propellants.32 Current evidence may be insufficient for determining whether the contribution of waste anesthetics to the global climate change is significant. However, it is likely that anesthetic gas contributions, calculated in full carbon equivalents,33 will become an important part of efforts to limit the production of greenhouse and ozone-depleting gases.
In summary, the use of N2O in medicine contributes to both global warming and ozone depletion.10 The use of halogenated anesthetics is a concern for producing global warming.15 In addition, the influence of halogenated anesthetics on ozone depletion will be of increasing relative importance, given the decreasing level of CFC usage globally.4 Furthermore, it should be recognized that other uses of anesthetic gases, including the use of N2O in dental offices and anesthetic gases in veterinary clinics and animal laboratories, may make significant additional contributions to adverse environmental change. It is essential to collect primary information on the quantities of N2O and halogenated volatile anesthetics manufactured or used, especially in consideration of serious international efforts to successfully reduce the emissions of ozone-depleting substances and greenhouse gases.21,34,35 We should develop tools for monitoring the use of ecotoxic gases, and initiate an international dialogue on these medically useful pollutants.
The author thanks Drs. Warren M. Zapol (Reginald Jenney Professor of Anesthesia at Harvard Medical School, Massachusetts General Hospital), Peter Huybers (Department of Earth and Planetary Sciences, Harvard University), and Federico M. San Martini (Department of Chemical Engineering, Massachusetts Institute of Technology) for expert advice and Christine Eckhardt for skillful graphic design.
a The data show that in 2006 total 103.8 million surgical and nonsurgical procedures were performed in the United States, including 46 million inpatient (National Hospital Discharge Survey) and 57.8 million ambulatory (preliminary data, National Survey of Ambulatory Surgery) procedures. On the basis of the preliminary results for the ambulatory cases, the author estimates approximately 50% of the total procedures were performed under general anesthesia (general anesthesia and multiple types of anesthesia).
b According to the data released by the National Center for Health Statistics, approximately 70% of the total procedures (103.8 million) were performed with an anesthesia provider (anesthesiologist, certified registered nurse anesthetist, or multiple providers). This includes all types of anesthesia.
c This amount of energy is equivalent to approximately 0.135 kg CO2. The author calculated the emissions from electricity generation on the basis of the data by the United States Energy Information Administration (http://www.eia.doe.gov/). On average, electricity sources emit 1.35 pounds (0.6 kg) CO2 per kilowatt-hour. Approximately 16 L of xenon gas is required to anesthetize a 70-kg adult for 4 hours using a closed circuit.31 The amount of xenon manufacturing energy for the anesthetic for 6 patients is comparable to the CO2 emissions of 1 passenger car per day.
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