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Improving the awareness of carbon emissions from nitrous oxide use for patients receiving inhalation sedation

R. Lee*¹, BDS BSc MFDS RCS (Eng) AKC
Z. Shehabi², BDS MFDS MSc (SSCD) MSCD MSc (Healthcare management)
¹Dental Core Trainee 1 in General Duties & Oral & Maxillofacial Surgery, The Royal London Dental Hospital, Turner Street, London, E1 1FR
²Clinical Lead and Consultant in Special Care Dentistry, The Royal London Dental Hospital, Turner Street, London, E1 1FR
*Correspondence to: R. Lee
Email: ronan.lee1@nhs.net
Lee R, Shehabi Z. Improving the awareness of carbon emissions from nitrous oxide use for patients receiving inhalation sedation . SAAD Dig. 2025: 41(I): 50-61

Abstract

Background
Nitrous oxide () with oxygen is a safe and effective conscious sedation drug. However, N₂O is a potent greenhouse gas, 300 times more harmful than carbon dioxide. Despite a nationwide push to decommission N₂O from NHS gas manifolds, there is limited data on its justified use in dentistry.

Aims
The project aimed to quantitatively record carbon emissions, time of N₂O use, and evaluate success and justification. The secondary aim was to qualitatively assess N₂O use, stored and to assess clinician's views of N₂O use.

Method
This project collected quantitative data over three retrospective months (May– July 2023), and one prospective month (January 2024), after implementing changes in the Special Care and Paediatric department. Qualitative data were collected from clinicians on N₂O storage and use along with feedback.

Results / discussion
The results highlight the consistent justification for using N₂O for inhalation sedation, as well as its success rate in achieving effective sedation, both retrospectively and prospectively across both departments. Improved recording of N₂O usage time was observed following procedural changes. Prospective data highlighted higher carbon emissions in the paediatric department. Storage spot checks revealed proper procedures, but labelling discrepancies. Clinicians demonstrated safe practice, unanimous support for increased environmental awareness, and strong opposition to decommissioning of N₂O. Recommendations and emphasis on sustainable N₂O practice and environmental awareness were highlighted.

Key learning points

1. This quality improvement project demonstrates consistent justification and effective nitrous oxide (N₂O) use in both paediatric and special care departments
2. The implementation of a new proforma for recording the time of N₂O usage significantly improved the accuracy of carbon emission calculations in both departments
3. There is consensus amongst dental clinicians for increased awareness regarding the environmental impact of N₂O through an expressed commitment to sustainable practice.

Background

Nitrous oxide (N₂O) in combination with oxygen (O₂) is predominantly used as an inhaled gas for dental conscious sedation, defined as, ‘the technique that produces a state of central nervous system depression to enable treatment to be delivered but during which level contact with the patient is maintained throughout the period of sedation’.^1,2 Inhalation sedation (IHS) aims to induce anxiolysis, alleviate fear and improve patient co-operation, ultimately facilitating dental treatment. N₂O is a weak anaesthetic gas, with both sedative and pain relief properties, and is suitable for both adults and children as a safe and effective form of conscious sedation.^2,3 N₂O and O₂ are delivered via a nasal hood placed over the patient’s nose. Once inhaled, N₂O is expired into the environment via the lungs virtually unchanged, with trace amounts metabolised through reduction by gut bacteria.⁴

The success of N₂O is dependent on its appropriate titration, and support by non-pharmacological behavioural management techniques. With proper case selection, N₂O can achieve high success rates ranging between 93.6 to 94.9.%.^5,6 While systematic reviews report a higher success rate amongst adults (99%) than children (91.9%),⁶ some studies suggest a lower success rate, especially for children with more complex needs who otherwise require treatment under general anaesthetic (GA).⁷ Nevertheless, IHS remains an effective and safe option for many patients.

N₂O is used to manage a broad spectrum of children and adults with dental anxiety; gag reflex; medical conditions that compromise patients; prophylactically to prevent medical emergencies; and those who are undergoing long unpleasant procedures.⁸ However, it may not be appropriate for patients with severe respiratory infection / disease, patients with nose blockages, some patients undergoing ocular procedures and pre- / unco- operative patients. Alternative options to IHS include the use of intravenous sedation (IVS) or GA which have their limitations including age restrictions, increased risk of complications and limited access,² furthermore, the use of GA requires more resources (ventilation of theatres, staff, consumables etc) which may result in higher carbon emissions when contrasted with IHS or IVS.⁹

Environmental impact

A caveat of N₂O is that it is a potent greenhouse gas, approximately 300 times more harmful to the environment than carbon dioxide (CO2).¹⁰ Over the past 150 years, the increasing atmospheric concentration of N₂O has contributed to the depletion of the ozone layer and climate change.^11,12 In the National Health Service (NHS), anaesthetic gases including N₂O make up to 5% of the total NHS carbon footprint.⁹ Concerning dentistry, research by Public Health England found N₂O makes up 0.9% of the total NHS dental carbon footprint, and between 80 to 90% of the carbon footprint for a single procedure.¹³ N2O can be supplied either via mobile cylinders or piped connected to a central manifold within a building, the advantages and disadvantages of which can be seen in Table 1. Emerging evidence suggests that N₂O wastage from piped manifold systems is a major contributor to the anaesthetic carbon footprint, estimating an annual wastage of 13.7 million litres of N₂O across the UK and Ireland.¹⁴ At the Royal London Dental Hospital (RLDH), N₂O is supplied in free-standing 1800L Size E Medical grade N2O cylinders supplied by BOC™.

To assess the carbon emissions, a metric used to express the global warming potential of greenhouse gases in terms of their equivalence to CO₂ termed ‘carbon equivalence’ (CO2e) and CO2e mileage can be calculated (see Table 2).¹⁵ When utilising N₂O, an estimated 1.875kg/m3 of N₂O equates to 3.375kg N₂O per cylinder, and roughly 0.3068kg N₂O per patient session. This results in a CO₂e of roughly 91.43, equivalent to driving 30.9 miles.16 BOC™ data from 2019/2020, found that piped manifold N₂O and Entonox® produced up to 27% and 45% of the anaesthetic emissions for NHS England respectively.¹⁷ While there has been a nationwide initiative to decommission N₂O from NHS gas manifolds, limited data exists on the utilisation and justification of N₂O in dentistry.⁹ Recording carbon emissions per N2O treatment session offers an opportunity to quantify N₂O use and implement mitigation strategies to contribute to the Net Zero NHS target by 2040.⁹

Improving clinicians’ awareness of the environmental impact of N₂O, through improved education, increased access to information and stimulating new interest in environmental concerns, can allow for more sustainable N₂O use. In recent years, there has been an increasing focus on sustainability and environmental emissions leading to wider discussions and an effort to improve environmental awareness and introduce waste mitigation strategies. In 2021, the Nitrous Oxide Mitigation Project was launched to recognise the declining use of N₂O in routine anaesthetic practice and its significant environmental impact.¹⁸ The NHS' ‘Delivering a net zero NHS’ report was set out to combat the large NHS carbon footprint to directly control the emissions produced to net zero by 2040, and influence the emissions indirectly produced to net zero by 2045 (see Table 3). Scope 1 emissions are those from activities owned or controlled by organisations; examples of Scope 1 emissions include the emissions produced when N₂O is used during IHS.

Nitrous oxide environmental and occupational exposure

To ensure the safe handling of N₂O, measures aimed at preventing occupational exposures and environmental leaks must be implemented. The health and environmental impact of N₂O is linked to its transportation, filling and general handling.¹⁹ The National Nitrous Oxide Project launched in January 2021 reported an annual system loss of 13,770,000L of N₂O from piped manifold systems, representing roughly 95% of the total N₂O procured.²⁰ N₂O that leaks into the environment can cause iatrogenic effects to clinicians, including dizziness, headaches and nausea, as well as waste into the atmosphere.

Mitigating against N₂O waste involves using portable N₂O cylinders, avoiding wasteful clinical practices, ensuring proper equipment, effective stock management and secure storage.¹⁸ 

Cylinders should be upright with secure valve caps, distinguishing between N₂O (blue) and O₂ (white), stored securely and accurately labelled indicating whether the cylinder is full or in use.¹⁹ When used excess N₂O is removed by scavenging units set at 45 litres/minute to the outside environment to prevent unwanted iatrogenic effects. To further reduce the environmental impact of N₂O, the dental community could consider carbon offsetting initiatives, such as investing in renewable energy or reforestation projects.

Administration

During the administration of N₂O, factors such as the length of time, flow rate and the percentage of maximum N₂O are required to calculate CO₂e, thereby influencing carbon emissions per patient session. While time recorded is vital for CO₂e calculations, it is not a standard practice to document to ensure safe conscious sedation.² When establishing a flow rate of 5-6L/min is required to align with the breathing rate of most patients.²¹ During delivery, N₂O should be titrated to prevent oversedation and unwanted side effects, but there are variations across the literature as to the correct titration regime (see Table 4).

When inhaled into the lungs, the distribution of inspired N₂O throughout the body is influenced by gas tension and solubility. Gases of low solubility will reach tension and equilibrate rapidly in the bloodstream; it is the tension that provides the driving force of inhaled molecules to reach the brain.²⁷ With low solubility, the blood concentration of N₂O reaches equilibrium with alveolar concentration, thereby rapidly transported through the bloodstream to reach the brain inducing sedative effects. Becker and Rosenberg observed the kinetics of N₂O and its onset of effect.²⁷ Extrapolated data shows time against inspired N₂O gas tension as a percentage shown in Table 5. The data highlights the progression of N₂O equilibration, indicating after four minutes the equilibration within the alveoli plateaus. This data suggests longer lengths of time are needed to ensure sufficient N₂O achieves equilibrium and is transported to the brain.

Local trends of N₂O use

Over the last five years, data from BOC™ has shown that N₂O usage at Barts Health NHS Trust (BHNT) has remained high, with the volume of N₂O used by the RLDH presenting as a minute proportion of the total N₂O used (see Figure 1). To provide context, the RLDH utilised 114,400 L of N₂O in 2022, equivalent to driving 175,626 miles or circumnavigating the earth seven times.²⁸ In comparison, the entire BHNT consumed 10,427,430 L in 2022, equating to driving 16,149,274 miles or encircling the earth 648.5 times. However, during the past five years there has been an overall steady decline in N₂O volume used and CO2e at the RLDH (see Figure 2). This may be due to the shift in patient care pathways from secondary care to local provision in primary care, reducing the carbon footprint of associated travel to secondary care hubs.

Aims and objectives

N₂O is a suitable and safer alternative to less resource-intensive and carbon emission-producing procedures such as GA. Demonstrating justified, successful and sustainable use of N₂O is crucial for its continued use in dental conscious sedation. The primary aim of this quality improvement project (QIP) was to quantitatively record and compute CO₂e per patient treatment and improve time recording of N₂O usage to enable accurate carbon emission calculations. Treatment outcomes were assessed and mitigations for N₂O will assess justification and success. The secondary aim was to qualitatively assess how clinicians were using N₂O during procedures, assess if N₂O cylinders are stored appropriately at the RLDH, and get feedback from clinicians on their views of N₂O use.

Driver diagram
A driver diagram was used to help plan this QIP and future QIP / audits, providing theories about what changes will likely lead to our desired aim of providing environmentally sustainable IHS using N₂O (see Figure 3).

Model of improvement
This project hopes to improve the time recording of N₂O usage to enable accurate carbon emission calculations and raise awareness of the environmental impact of N₂O amongst clinicians. The project seeks to highlight whether N₂O use is justified and successful for IHS. The identified improvements include justified and successful treatment, recorded time of N₂O usage, and valid extrapolation of CO₂e. The anticipated outcome is that clinicians will adhere to ‘as low as reasonably possible’ (ALARP) principles when using N₂O, ensure time is recorded for future CO₂e calculations, and ensure IHS is the first line of treatment over more resource-intensive procedures such as GA.

Material and methods

This QIP was approved by the Clinical Effectiveness Team at The Royal London Hospital.
Sample
Preliminary retrospective data collection spanned three months across the Special Care (SC) and Paediatric departments; after implementing procedural changes emphasising N₂O usage time, a subsequent one-month period was used to gather data prospectively to assess change and improvement. During the one month, spot checks of N₂O cylinders and observations of clinicians using N₂O were carried out across both departments. Feedback was then received from clinicians when results were discussed at a cross-department meeting.

Data collection
To comply with the General Data Protection Regulation (GDPR 2018), data were collected via the Trust’s computer system using the hospital database (Cerner Millennium). A data collection proforma was designed and validated (see Table 6). Collected data encompassed patient demographics (age, medical history and American Society of Anaesthesiologists Physical Status score,²⁹ previous N₂O use), treatment justification, alternatives, treatment outcomes, and carbon footprint calculation (N₂O: volume, time of maximum usage, flow rate), the volume of N₂O used, the CO₂e and CO₂e mileage were also recorded. The volume of N₂O was calculated by multiplying the percentage of maximum N₂O by the flow rate and time of maximum N₂O used. CO₂e is then calculated by multiplying this figure by 0.5244, which is then divided by 0.3386 to gain the CO₂e mileage (see Table 2).

Spot checks of N₂O and O₂ were completed across the SC and Paediatric department, a data collection proforma was designed and validated (see Table 7). Spot observations took place across both departments, the form of which can be seen in Table 8. Feedback was asked of clinicians during a cross-department meeting, with an online questionnaire prepared using a Google form and distributed via a QR code allowing clinicians to provide feedback anonymously (see Table 9).

Data analysis
Data was stored and sorted on a secure online hospital hard drive and then analysed using a Microsoft Excel spreadsheet and Microsoft Word program.

Results 

Data were collected retrospectively over three months (1 May to 31 July), and prospectively over one month (1 January to 31 January 2024), the results of which can be viewed in Table 10.

Patient Demographic
Retrospectively, the average age of patients in the paediatric department was 9.2 years, ranging between five and 15 years old. The average age of patients in the SC department was 41.9 years, ranging between 17 and 69 years old. Prospectively, the average age of patients in the paediatric department was 10.6 years, ranging between six and 15 years old. The average age of patients in the SC department was 45.7 years, ranging between 26 and 69 years old.

Justification
Data analysis confirmed consistent justification for the use of IHS using N₂O retrospectively and prospectively in the paediatric department and SC department. Reasons for N₂O use included dental anxiety, needle phobia, medical history, gag reflex, no LA experience, long procedure and previous IV failure.

Success
A high number of treatments were successful; retrospectively the paediatric department saw an 88.9% (n = 40) and SC 100% ( n= 29) success rate; prospectively the paediatric department saw a 93.4% (n = 28) and SC 100% (n = 9) success rate. Procedures that were not successful required either referral for GA, treatment under IVS or monitoring.

Time of N₂O recording
Retrospective data found a deficiency in recording N₂O usage time, observed in only 33.3% (n = 15) of paediatric and 6.9% (n = 3) of SC treatment reducing the accuracy of carbon emission calculations and affecting their validity. After the implementation of a new IHS proforma, prospective data revealed 73.3% (n = 22) of paediatrics and 44.4% (n = 4) of SC treatments having time recorded.

Carbon emissions
Due to the lack of time recorded, the volume of N₂O use, CO₂e and CO₂e mileage could not be recorded retrospectively in both departments. Prospective data showed in the paediatric department, the average volume of N₂O used was 39.2 L, the average CO₂e 20.6 and CO₂e mileage of 60.7 miles. In the SC department, the average volume of N₂O used was 14.7 L, the average CO₂e 7.7 and CO₂e mileage of 22.7 miles.

N₂O storage
Over the one-month period (1 to 31 January 2024), a total of 22 cylinders were spot checked, 14 from the SC and 8 from the Paediatric department, the results of which can be viewed in Table 11. 100% of the N₂O and 100% of the O₂ cylinders were turned off across both departments. However, it was noted two N₂O cylinders and four O₂ cylinders with full / in use labels were missing or incorrectly labelled, and that these extra cylinders were not full on the portable N₂O units.

Observations of clinicians
Over the one-month period (1 to 31 January 2024), a total of seven clinicians were observed between each department the results of which can be seen in Table 12. All pre-operative and post-operative safety checks were completed, during treatment on average the majority of clinicians titrated N₂O for 1 min 5 sec and all provided 3 to 5 minutes of O₂ to prevent desaturation. All clinicians observed kept the maximum amount of N₂O throughout the procedure. 

Feedback from clinicians
During a cross-department meeting, feedback was received from a total of ten clinicians the result of which can be viewed in Table 13. The survey captured a wide range of clinicians from trainees to consultants, with the majority having only zero to five years’ N₂O experience. A spectrum of understanding was noted amongst clinicians, but unanimously agreed on the need for increased awareness regarding the environmental impact. In responses to a hypothetical scenario where BHNT were to decommission N₂O, strong opposition was expressed; clinicians voiced disagreement and disappointment, the potential detriment and indispensability of N₂O to patient care, and one clinician considered it safer than GA.

Discussion

The findings of this QIP present valuable insight into the use of N₂O for IHS in a dental setting. The results indicate consistent justification and high treatment success rates when using N₂O, with significant differences observed between the retrospective and prospective data. The QIP confirms a high success rate for treatment, this outcome underscores the effectiveness of N₂O across both paediatric and SC departments.

Significant findings in the retrospective data found deficiencies and time recording of N₂O usage, impacting carbon emission calculations which ultimately lacked validity. This limitation hindered the comprehensive understanding of the environmental impact associated with each patient session, hence the need for improvement. The findings of the retrospective data were presented at a cross-department meeting, with a new proforma provided to the clinicians to improve the recording of time which can be seen in Figure 4. Encouragingly, the prospective data showed a 543.5% increase and a 120.1% increase in time recorded in both the SC and Paediatric departments respectively. This then enabled valid carbon emission calculations to be evaluated.

From prospective data, the Paediatric department utilises a higher volume of N₂O compared to the SC department. A plausible hypothesis is this increase may be linked to a larger and younger population of patients, for whom IHS is the sole option, in contrast to the SC department, where alternative options such as IVS are aligned with the specific needs of patients treated.

Concerning the storage of N₂O and O₂ cylinders, although the sample size assessed was small, it is reassuring to observe that all cylinders are being properly turned off when stored, reducing occupational and environmental exposure. However, discrepancies were identified including missing or incorrectly labelled full / in use labels with a full N₂O or O₂ cylinder not present on the portable system. This safety concern was raised at a cross- department meeting and brought to the attention of the Senior Dental Nurse responsible for the N2O units, who subsequently purchased new recyclable labels.

During observations, it was noted that safety protocols were followed, and it was encouraging to see that staff members store N₂O appropriately. It was noted, however, that there was a range of titration regimes reflected in the literature (Table 4). Methods to reduce N₂O administration and follow ALARP principles were communicated at a cross-department meeting. Suggestions such as understanding the patient’s anxiety and phobia, as well as determining the specific stage during treatment when maximum N₂O is essential, can lead to a reduction in the overall quantity of required N₂O. An example was given of a patient with needle phobia, who may require a higher concentration of N₂O during local anaesthetic administration but could be provided with a lower concentration for the remainder of treatment. It is the author’s opinion that extending the duration beyond one minute for each titration step is recommended, as evidenced in the literature.²⁷ Regimes such as providing 20% of N₂O for three minutes and increments of 10% every three minutes may provide more time for patients to feel sedated at lower N₂O doses.²⁷ This, coupled with non-pharmacological management strategies, such as hypnosis, virtual reality headsets, lighting and voice control can mitigate the need for higher concentrations of N₂O. The author appreciates that each clinical scenario and environment is different and that the observations carried out were limited to a small number of clinicians, nevertheless, these recommendations are justified in ensuring sustainable N₂O practice.

The consensus amongst clinicians highlighted the need for increased awareness of the environmental impact of N₂O, regardless of the mixed awareness levels amongst the small sample size. It was encouraging to see clinicians employing strategies to reduce N₂O administration, and their strong opposition to the decommissioning of N₂O, with emphasis on its crucial role in patient care. The additional comments reveal a depth of sentiment and underscore the importance of considering clinicians’ perspectives in any future decisions related to N₂O usage. Following the QIP, a poster was designed and distributed across the RLDH to educate clinicians and undergraduate students about promoting sustainable N₂O use for dental IHS, which can be used as a tool for optimising N₂O use in other settings (see Figure 5).

Conclusion

This QIP provides valuable insight into the use of N₂O for IHS in both the SC and Paediatric departments in a dental setting. The results highlight consistent justification and high treatment success rate, affirming the effectiveness of IHS treatment when using N₂O. Retrospective data showed deficiencies in time recording, impacting carbon emission calculations. After improvements were communicated, prospective data showed significant improvements in recorded time, enabling valid calculations, with the Paediatric surpassing the SC department in its use of N₂O attributed to a larger, younger patient population. Environmental leakages were mitigated and other safety discrepancies were addressed promptly. The observations highlighted adherence to safety but identified variations in titration regimes. Recommendations and emphasis on sustainable N₂O practice were communicated as well as the need for increased environmental awareness. The findings of this QIP underscore N₂O's crucial role in treating patients through IHS, urging consideration of clinicians’ perspectives in future decisions regarding decommissioning.

Conflict of interest

The author declares no conflicts of interest.

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