If you don't remember your password, you can reset it by entering your email address and clicking the Reset Password button. You will then receive an email that contains a secure link for resetting your password
If the address matches a valid account an email will be sent to __email__ with instructions for resetting your password
Anesthesia providers have the ability to choose an anesthetic plan that minimizes environmental impact without affecting patient care.
•
Volatile anesthetic agents are greenhouse gases with significant environmental impacts that can be reduced by choosing to avoid desflurane and nitrous oxide and to use low fresh gas flows.
•
Wasted medications and single-use devices, particularly disposable laryngoscopes, contaminate the environment and are a source of considerable cost.
•
Anesthesia sustainability initiatives save money and decrease the carbon footprint of the operating room, which is appealing not only to clinicians’ individual sense of responsibility and stewardship but also to the collective duty of physicians to improve global health.
Introduction
Climate change will be the defining health crisis of the twenty-first century and represents the greatest threat to global health.
Although severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has captured attention and will be at the forefront of public health measures for the foreseeable future, the effects of climate change on public health are further reaching, longer lasting, and more difficult to mitigate than even this very contagious virus. The scope of the problems arising from climate change is immense, including a rise in sea levels, increases in extreme weather events, increases in atmospheric carbon dioxide (CO2) to unprecedented levels, the spread of infectious diseases, the loss of biodiversity, and the declining health status of the population as a whole. Efforts are underway in many countries to curb CO2 emissions and thus slow and, it is hoped, eventually reverse the current trends. Ironically, in striving to improve population and individual health, the health care system contributes significantly to climate change, which ultimately negatively affects human well-being. Many analyses have verified the contribution of developed countries’ national health care systems to their countries’ greenhouse gas (GHG) emissions to be between 3% and 10%. The UK National Health Service was estimated to contribute 4.6% of national GHGs in 2015,
whereas an analysis of 36 generally first-world countries as well as India and China found that the health care sector was responsible for an average of 5.5% of each nation’s overall emissions in 2014.
National CO2 emissions from Fossil-Fuel Burning, Cement manufacture, and Gas Flaring: 1751-2014. Carbon Dioxide Information Analysis Center, Oak ridge national Laboratory, U.S. Department of energy.
If the US health care sector alone were a country, it would rank 13th in the world for GHG emissions.
In 2012, the Institute of Medicine suggested that the health sector should lead by example by greening itself and reducing its ecological footprint to improve global health and the health of the planet. Anesthesia providers have considerable freedom in making the care plans for patients and it is important to make choices that minimize the environmental impact of anesthetics without affecting the quality of patient care.
The environmental impact of volatile anesthetics, nitrous oxide (N2O), intravenous medication waste, single-use devices, and the energy consumption of the heating, ventilation, and air conditioning (HVAC) systems are discussed here, along with practical suggestions to reduce environmental impact.
Volatile anesthetic agents
In response to the growing hole in the ozone layer above Antarctica that formed as a result of atmospheric GHGs, the Montreal protocol of 1987 aimed to phase out global chlorofluorocarbon use, with hydrofluorocarbons subsequently targeted through the 2016 Kigali amendment.
Anesthetic gases are chlorofluorocarbons (isoflurane) and hydrofluorocarbons (desflurane, sevoflurane), but volatile anesthetic use was not restricted by either protocol because of medical necessity. In 2014, the release of hydrofluorocarbon and chlorofluorocarbon anesthetic gases was equivalent to 3 million tons of CO2, with 80% of the emissions from desflurane alone. GHGs differ in their abilities to trap heat. The effect of this heat trapping over a 100-year period is described using a scale called the global warming potential over 100 years (GWP100), as shown in Table 1. Although other GHGs, such as methane, are emitted in much larger quantities, the environmental impact of volatile anesthetics is significant because volatile anesthetics have much higher GWP100 values. For example, despite small quantities, anesthetic gases represented 2% of the United Kingdom’s acute National Health Service organizations’ carbon footprint in 2012.
Desflurane has far higher GWP100 values than the other commonly used agents sevoflurane and isoflurane. The use of desflurane results in nearly 20 times the global warming impact of using sevoflurane. Thus, the choice of volatile agent has the most impact in determining the carbon footprint of an anesthetic (Box 2). To put it in more practical terms, the environmental impact of volatile anesthetics can be expressed in equivalent miles driven in a car per MAC-hour of anesthesia, as shown in Table 2.
Table 1Atmospheric lifetime and global warming potential of volatile anesthetics compared with other known greenhouse gases
The GHG emissions generated by a 2-hour anesthetic with desflurane (1 L/min fresh gas flow [FGF]) are equivalent to driving a car 608 km (378 miles), roughly the distance from New York City to Akron, Ohio, or from Los Angeles, California, to Phoenix, Arizona. The same anesthetic with sevoflurane (2 L/min FGF) is equivalent to driving 26 km (16 miles).
Table 2Equivalent miles driven per minimum alveolar concentration hour of each volatile anesthetic
After the choice of volatile anesthetic agent, the fresh gas flow (FGF) rate is the next most important determinant of the carbon footprint of a typical anesthetic (Box 3). Any FGF that exceeds the patient’s needs and the system requirements will be delivered directly out the roof of the hospital via the anesthesia machine’s scavenging system. Thus, the importance of low (<2 L/min) flow cannot be overemphasized. Low flow is most easily accomplished during the maintenance phase of anesthesia. There are several considerations when using low-flow delivery that should be addressed, including the production of compound A and carbon monoxide (CO) and ensuring adequate inspired fraction of oxygen.
Choice of volatile anesthetic agent and the rate of FGF are the most important determinants of the carbon footprint of a gas-based anesthetic.
Compound A is formed by the degradation of sevoflurane by CO2 absorbents, most notably those containing the strong bases sodium hydroxide and potassium hydroxide (NaOH and KOH) in desiccated conditions.
Early studies with sevoflurane found a theoretical risk of nephrotoxicity in humans from compound A. However, the literature does not support the evidence of renal injury caused by compound A in humans undergoing anesthesia. Nevertheless, the US Food and Drug Administration (FDA) has included a warning in the package insert for sevoflurane that states: “sevoflurane exposure should not exceed 2 MAC∙hours at flow rates of 1 to <2 L/min. Fresh gas flow rates of <1 L/min are not recommended.”
Although KOH-based CO2 absorbents are no longer available, NaOH-based CO2 absorbents are still in use. More modern CO2 absorbents are calcium hydroxide [Ca(OH)2]–based or lithium hydroxide (LiOH)–based, which interact minimally with sevoflurane but can still produce compound A when dessicated.
Overall, the ability to use CO2 absorbents that do not interact with sevoflurane and the absence of compelling human data that compound A is injurious seem to allow for reasonable doubt regarding FGF limitations with sevoflurane. However, the FDA recommendation mentioned earlier currently still stands and warrants compliance.
CO can be formed by the degradation of any of the volatile agents by desiccated CO2 absorbents that contain the strong bases NaOH and KOH in large quantities, such as Baralyme and soda lime.
Low FGF maintains moisture in the circuit and in the CO2 absorbent, thus decreasing the risk of CO production with these absorbents. More modern absorbents [Ca(OH)2 based or LiOH based] do not produce CO when they interact with volatile agents.
Fraction of inspired oxygen (Fio2) is often set lower in pediatric anesthesia, especially in infants, because of concerns for oxygen toxicity and retinopathy of prematurity. When FGF is also set low, the Fio2 may need to be set higher in order to compensate for oxygen extraction. Diligence to the inspired oxygen concentration is of utmost importance to avoid delivering a hypoxic mixture. In addition, side stream gas analyzers often remove 200 mL/min FGF from the circuit, which must be accounted for when using very low FGF, because not all systems return that volume to the circuit after analysis. Low-flow techniques result in more rapid exhaustion of CO2 absorbent. Contrary to how many anesthesiologists practice, the most efficient use of absorbent results from changing it based on consistently increased inspired CO2 concentration rather than after a predetermined period of time or with the appearance of an indicator.
The carbon footprint of an anesthetic that uses any volatile agent is dramatically higher than an anesthetic that uses only neuraxial, regional, or intravenous agents
(Fig. 1). The GHG emissions that result from using desflurane without N2O (discussed later) are approximately 2600 times the emissions that result from an anesthetic using propofol; roughly 32,000 g of CO2 equivalents (gCO2e) versus roughly 12 gCO2e. Sevoflurane is far less detrimental to the atmosphere than desflurane, but its use still results in approximately 135 times greater emissions as using propofol (roughly 1600 gCO2e versus roughly 12 gCO2e). Although the environmental impact of pharmaceuticals is also concerning, it is drastically less than the environmental impact of volatiles, making total intravenous anesthesia the superior choice.
Fig. 1Life cycle GHG emissions of anesthetics, (A) including waste anesthetic gas emissions of volatile agents and N2O, and (B) excluding waste anesthetic gas emissions to show the lesser impact of manufacturing, transport, packaging, and drug delivery. Note the differing scales. gCO2e, grams of CO2 equivalents; mgmt, management.
(From: Sherman J et al. Life Cycle Greenhouse Gas Emissions of Anesthetic Drugs. Anesth Analg 2012;114:1086 –90; Used with permission from Wolters Kluwer Health, Inc.)
Mask induction of anesthesia is unique to pediatrics and presents an additional challenge to minimizing the venting of volatile anesthetic to the atmosphere. In addition, N2O is often used during mask induction; this is discussed later. High flows are generally used to facilitate rapid changes in inspired sevoflurane during mask induction, and the resulting wasted anesthesia gas is considerable. Flows need not be higher than 5 to 8 L/min initially during mask induction and should be decreased when uptake by the patient has slowed. This slowed uptake is indicated by the expired agent concentration approaching the inspired concentration until nearly balanced and approximating the desired MAC value. Rebreathing increases when FGF decreases, which may decrease the delivered anesthetic concentration, so vigilance is important during this period.
Setting alarm limits for low inspired agent concentration is an easy way to ensure that adequate depth of anesthesia is maintained when FGF is reduced.
Fresh Gas Flow Management During Intubation
When a volatile agent has been initiated before intubation (ie, during induction), it is preferable during intubation to turn off FGF and leave the vaporizer at its set point rather than turn off the vaporizer and leave FGF flowing (Box 4).
Leaving FGF on with the circuit disconnected allows washout of volatile agent from the internal volume of the circuit into the room and requires the reestablishment of a volatile agent in the circuit once the circuit is reconnected. The preferable result of turning off FGF instead is the avoidance of environmental contamination during intubation and the ability to use a lower FGF to maintain circuit concentration after intubation. Although the circuit can be refilled quickly with high FGF, the technique of turning off FGF should be carefully considered in high-risk intubation scenarios. This practice requires mindfulness in resuming FGF after intubation, and individual practitioners must decide their comfort levels with this practice.
Turn off FGF during intubation, leaving the vaporizer at its set point, to avoid washout of volatile agent from the circuit and contamination of the operating room (OR) environment.
Waste Anesthesia Gas Recapture
As mentioned earlier, scavenged waste anesthesia gas (WAG) is typically vented directly out the roof of the hospital. However, several technologies are being developed to capture WAG before its emission to the environment, with the end goals of destruction, warehousing, or purification and reuse.
These techniques use adsorption of the volatile WAG onto either activated charcoal or a selective adsorbent, or condensation of the volatile agent to a liquid. A reversible adsorbent process is used by BlueZone Delta (Toronto) to collect WAG and allow for either destruction or reprocessing. Although no pharmaceutical originating from reprocessed or recycled product has yet to be approved by the FDA, halogenated anesthetics are good candidates for recycling in that they are recovered in their original forms, without additives, and are easily purified.
Clinics care points/suggestions to decrease environmental impact:
•
Eliminate desflurane
•
Use low flow (<2 L/min) during maintenance
•
Turn off FGF during intubation while leaving the vaporizer set to preserve volatile agent concentration in the circuit and avoid washout of volatile agent into the room
•
Use total intravenous anesthesia to eliminate volatile anesthetic emissions whenever possible
Nitrous oxide
N2O is not only a GHG like the volatile anesthetics but it also depletes ozone and persists in the atmosphere for more than 100 years. As a GHG, it has a global warming potential 298 times that of CO2.
N2O has a long tradition of use in pediatric anesthesia both to facilitate mask induction and as a maintenance anesthetic carrier gas. However, mask induction without N2O is not only possible but also advantageous for multiple reasons. Most importantly, use of N2O during induction decreases inspired oxygen concentration, eliminating preoxygenation, which decreases the time from apnea to desaturation, especially in young children.
Because young children are more prone to laryngospasm during induction, and by definition of receiving mask induction usually do not have intravenous access in place, preoxygenation is of utmost importance for safety.
As pediatric anesthesiologists can readily attest, placement of a mask on a child’s face (even before a volatile agent is in the circuit) is often distressing to the child, and N2O offers no benefit for at least 30 seconds after placement of the mask. However, distraction techniques are often effective to help children tolerate both mask placement and delivery of a gradually increasing concentration of sevoflurane. The second gas effect has been mostly studied in adults and with halothane, making application of the principle questionable for pediatric patients who have a larger ratio of alveolar ventilation compared with functional residual capacity and a larger fraction of their cardiac output delivered to the vessel-rich group, both of which serve to speed induction independent of a second gas effect compared with adults.
In clinical practice, mask induction with sevoflurane is not faster with N2O than with 100% oxygen in children.
As an alternative to using N2O during induction, consider using distraction to help the child tolerate mask placement and initiation of volatile anesthetic in the circuit. Screen technology is a powerful distraction tool for children, but storytelling, jokes, or casual conversation are also effective for different age groups. The advantages and disadvantages of N2O during induction are summarized in Table 3.
Table 3Nitrous oxide for mask induction: pros and cons
Advantages
Disadvantages
Euphoria and ambivalence to presence of volatile anesthetic Stable hemodynamics overall (but potential risk of increased pulmonary pressure)
Prohibits preoxygenation, predisposing to rapid desaturation in the event of laryngospasm, bronchospasm, or apnea Significant delay to onset of clinical effect of euphoria Dysphoria common with high inspiratory fraction Does not speed mask induction Increases risk of postoperative nausea and vomiting Increases carbon footprint of the anesthetic
N2O is frequently used during maintenance of anesthesia. Although this allows for a decreased concentration of volatile agent, the overall carbon footprint of the anesthetic is increased when N2O is used for maintenance compared with using volatile anesthetic in air and oxygen.
Clinics care points/suggestions to decrease environmental impact:
•
Avoid N2O
Medication waste
Medication waste is an unavoidable consequence of administering medications during anesthesia care. In anesthesia practice, propofol is the most wasted medication by volume, whereas emergency medications (succinylcholine, atropine, epinephrine, ephedrine, phenylephrine) have the highest waste fractions (percentage of opened medication that is not used and must be wasted).
Virtually all medications drawn up from vials end up in the environment in some form, and thus limiting preparation to only medications that are planned to be used is the best way to decrease medication waste (Box 5).
Virtually all medications drawn up from vials end up in the environment in some form.
Disposal of unused medications varies according to hospital policies as well as state and federal regulations. Controlled substances (all narcotics, benzodiazepines, barbiturates, ketamine, and in some states propofol) are considered hazardous waste pharmaceuticals; their disposal is regulated primarily by the Drug Enforcement Administration (DEA) and the Environmental Protection Agency (EPA) (Boxes 6 and 7). DEA regulations state that controlled substances must be “irretrievable and unusable after disposal.”
US DEA Final Rule for Disposal of Controlled Substances, 79 Fed Reg 53519 (Sep 9, 2014) (to be codified at 21 C.F.R. pts 1300, 1301, 1304, 1305, 1307, & 1317).
The EPA dictates the manner in which controlled substances may be disposed and, in January 2020, banned the sewer system as an acceptable method of hazardous waste pharmaceutical disposal for all but personal and residential purposes.
US EPA Final Rule for Management Standards for Hazardous Waste Pharmaceuticals and Amendment to the P075 Listing for Nicotine, 84 Fed Reg 5816 (Aug 21, 2019) (to be codified at 40 C.F.R. pts 261-265, 268, 270, 273 & 266).
Thus, controlled substances must be disposed of in appropriate containers, in full compliance with both DEA and EPA regulations, which generally requires incineration.
EPA regulations specifically prohibit disposing of controlled substances into the sewer system (eg, wasting medications into the sink or toilet is illegal).
Uncontrolled medications are also preferably disposed of by incineration, thus preventing the eventual release into groundwater that occurs when medications leak through landfills. Neither landfills nor wastewater treatment facilities are designed to prevent pharmaceuticals from entering the environment. This finding is shown by well-documented pharmaceutical contamination of groundwater and surface water.
Because the environmental burden of a pharmaceutical generally correlates with the amount that is dispensed, the most commonly used medications (oral contraceptives, antihypertensives, antibiotics, antiepileptics, mood stabilizers, and over-the-counter analgesics) are those most frequently found in the environment.
Thus, although anesthesia-related medications are a small fraction of overall medication waste, the ubiquitous presence of pharmaceuticals in groundwater is cause for significant concern and serves as an entreaty to avoid unnecessary medication wastage.
Strategies to decrease medication waste include use of prefilled syringes, splitting of vials (especially in pediatric anesthesia) to accommodate smaller dose volumes, and avoiding drawing up medications that may not be used. Third-party vendors’ prefilled syringes of emergency medications decrease waste because, unlike medication drawn up from a vial, an unused prefilled syringe can be returned to stock and has a long shelf life.
These syringes have a much shorter shelf life than commercially available ones but, for frequently used expensive medications (sugammadex, dexmedetomidine), they are preferable to wasting large fractions of a medication vial’s contents. Proper disposal of pharmaceutical waste is expensive, and it is far less expensive to avoid generating pharmaceutical waste than to pay to dispose of it
Clinics care points/suggestions to decrease environmental impact:
•
Use prefilled syringes for emergency medications
•
Request that pharmacy split vials under a sterile hood to decrease waste, which is also beneficial during drug shortages
•
Maintain medications readily available but not opened when possible
•
Use smaller vials for pediatrics (eg, propofol 10 mL vs 20 mL)
•
Follow the controlled substances waste stream to ensure that these medications are not being disposed of by sewer, because this practice is prohibited by law
.
Single-use devices
Health care in the United States uses a mind-boggling number of single-use devices. Anesthesia is no exception, and, with the recent trend of switching to single-use laryngoscopes, there are only a handful of reusable devices used in many practices currently. The switch to single-use devices has resulted from preference for convenience and concern for cross-contamination. However, the environmental impacts have been disastrous. United States hospitals generate approximately 6 million tons of waste annually (Box 8).
Infection control concerns must be addressed in any proposal to decrease use of single-use devices, and the Centers for Disease Control and Prevention (CDC) risk classification and guidelines for disinfection are useful to reference.
Six million tons is equivalent to 25 of the world’s largest cruise ships. US hospitals generate that much waste annually.
Life-cycle analysis (LCA) is a scientific method used to quantify the environmental emissions of a process or product, including natural resource extraction, manufacturing, packaging, transportation, use/reuse, and waste management strategies.
A robust LCA represents the gold standard of a given product’s environmental impact. Although only recently applied to health care, several LCAs are available for anesthesia equipment, including disposable and reusable laryngoscopes,
As more LCAs are published and demands mount on device manufacturers to disclose the environmental impacts of their products, it will become much easier to compare the environmental impacts of reusable and single-use devices.
Many hospitals utilize single-use device reprocessing through third-party vendors. Reprocessing refers to the process of sterilizing, tracking, and repackaging items originally manufactured for single use. The quality standards for this industry are the same as those applied to the original device manufacturers
and additionally require tracking of the number of reprocessing cycles for each individual device to ensure its removal before quality is compromised.
Clinics care points/suggestions to decrease environmental impact:
•
Use supply chain data to determine costs of single-use devices used in a practice and the cost savings associated with switching to reusable devices
•
Implement a reprocessing program to decrease waste, decrease carbon footprint of the operating room (OR), and reduce costs for OR equipment
Heating, ventilation, and air conditioning
Hospitals use an immense amount of energy. The United States Energy Information Administration last quantified hospital energy use in 2007 and found that large hospitals account for less than 1% of all commercial buildings but consume 5.5% of the total delivered energy used by the commercial sector.
and much of that impact is caused by strict requirements for the provision of suitably clean air during surgical procedures. These requirements are set by ANSI/ASHE/ASRAE standard 170-2017
American National Standards Institute/American Society for Healthcare Engineering/American Society of Heating, Refrigeration, and Air Conditioning Engineers Standard 170-2017.
and include requirements for air changes per hour (ACH). ORs are required to maintain 20 ACH (although this rate is also governed by state regulations) during surgical procedures, but only 6 ACH when the OR is not being used, provided that a positive-pressure relationship is maintained between the OR and its adjoining spaces. By setting back the ACH in an OR during off hours, many hospitals have significantly decreased their energy use, carbon footprint, and costs (Box 9).
Decreasing the ACH in the OR during unoccupied times results in a significant decrease in carbon footprint and energy costs for the hospital.
HVAC controls are outside the realm of expertise of most anesthesiologists, and therefore adjustments of OR HVAC controls are a multidisciplinary undertaking. The building infrastructure must allow for computer-controlled adjustments to the HVAC system and real-time monitoring of the pressure relationships of the ORs. Synchronizing the OR setback schedule with the real-time surgery schedule is helpful, and a manual override must be in place to ensure OR ACH are appropriate in unforeseen circumstances. These strict protocols and planning may seem to make OR setbacks too cumbersome a project to initiate, but the cost savings realized by institutions that have successfully implemented setbacks speak to the incentive. Data from Practice Greenhealth (the largest networking organization for sustainable health care in the United States) indicate that member hospitals report a median saving of $2585 per OR per year, and median hospital savings of $33,600 per year attributable to OR setbacks.
Clinics care points/suggestions to decrease environmental impact:
•
Inquire about the HVAC system controls for your OR. If your hospital has a sustainability initiative in place, use that infrastructure to mobilize the relevant stakeholders needed for OR setbacks
Summary
The challenges to human health are many. As of this writing, Sars-CoV-19 has established itself in the forefront of everyone’s mind but climate change, although more insidious and more debated, remains the larger threat. There is high-level buy-in from the global community to minimize the spread of the novel coronavirus, protect those most vulnerable to infection, and to find treatments and vaccines. Humanity has faced the threat of infectious disease before, and, although the end of this pandemic may be years away, the way forward is fairly clear. Climate change presents a much more difficult problem in that it has been created over several decades by humanity’s choices, and, as a result, it will be impossible to effectively mitigate without global cooperation. Ultimately, climate change will affect every person on Earth. A problem of this magnitude must be addressed on every level and from every sector if any headway is to be made on turning the tide before irreversible changes occur. Everyone must each do their part, in both their personal and professional lives, to bring about global change. A desirable side effect of many sustainability efforts is significant cost savings, and this provides the leverage needed to convince departments and larger organizations to pursue sustainability projects.
Sustainability work in hospitals, specifically in ORs, has more impact than the same efforts in other settings because of the hospitals’ and ORs’ significant uses of energy, medications, and supplies. Anesthesia providers can decrease the environmental impact of their clinical care. They can choose to avoid desflurane and N2O because of their high GWP100 status. They can minimize medication waste by only drawing up medications that are needed and by using prefilled syringes. They can use LCA studies to advocate for supplies with lower environmental impact and lower cost instead of submitting to the lure of convenience at the expense of the environment. They can collaborate with other experts in their hospitals to tackle larger projects, such as reducing unnecessary energy use, especially in ORs. As everyone makes changes in their own practices, they positively influence those around them to do the same and, slowly, the collective planetary benefit increases. It took considerable time to get the planet into the dangerous situation it is in, and it is going to take time to “right the ship.” By learning about the carbon footprint of daily choices in anesthesia and adjusting clinical practice to minimize impact, clinicians are playing a part in healing the damage already done to the planet and setting the example for the future of anesthesia care.
Disclosure
The author has nothing to disclose.
References
Watts N.
Amann N.
Arnell N.
et al.
The 2018 report of the Lancet Countdown on health and climate change: shaping the health of nations for centuries to come.
National CO2 emissions from Fossil-Fuel Burning, Cement manufacture, and Gas Flaring: 1751-2014. Carbon Dioxide Information Analysis Center, Oak ridge national Laboratory, U.S. Department of energy.
US DEA Final Rule for Disposal of Controlled Substances, 79 Fed Reg 53519 (Sep 9, 2014) (to be codified at 21 C.F.R. pts 1300, 1301, 1304, 1305, 1307, & 1317).
US EPA Final Rule for Management Standards for Hazardous Waste Pharmaceuticals and Amendment to the P075 Listing for Nicotine, 84 Fed Reg 5816 (Aug 21, 2019) (to be codified at 40 C.F.R. pts 261-265, 268, 270, 273 & 266).
American National Standards Institute/American Society for Healthcare Engineering/American Society of Heating, Refrigeration, and Air Conditioning Engineers Standard 170-2017.
30046-X&id=gr1.jpg){kind=link}