Introduction

‘First do no harm' is one of the earliest lessons that clinicians learn. It reminds us to protect our patients by considering the potential harm of medical interventions and to weigh these harms against the expected benefit. As we increasingly understand the detrimental impact that healthcare has on our environment, and the reciprocal effect of the environment on human health, we have an individual responsibility to apply ‘first do no harm' principles more holistically by considering the environmental impact of our healthcare services.

Climate change is one of the most significant threats to human health of the twenty-first century.1 Rising global temperatures risk rolling back decades of health gains by impacting food and water security; altering patterns of disease; causing more extreme weather events; and influencing populations and migration.1

Positive action on climate change and advocacy for sustainable practices are urgently required to address this health emergency. Global organisations such as the United Nations (UN) and World Health Organisation prioritise urgent action on climate change through policy such as the UN Sustainable Development Goals.2 In 2019, the UK became the first major economy to enshrine a commitment to achieving carbon neutrality by 2050 into law.3 To achieve this goal, wide-ranging measures are needed to decarbonise all sectors of the UK economy, including healthcare.

Healthcare is a resource-intensive industry, and 5% of the UK's total national carbon emissions result from NHS activities.4 The NHS has a responsibility to deliver a service which is resilient and continues to deliver high-quality care for all within the limits of financial, social and environmental resources into the future.5 Sustainable practices support this objective. Direct health benefits are also gained as ill health and health inequality share driving factors with climate change. For example, air pollution disproportionally affects deprived and vulnerable communities and there is evidence that up to one-third of new asthma cases might be avoided by reducing air pollution.6

The NHS aims to achieve carbon neutrality by 2040 and become the world's first net-zero health service.4 It is therefore imperative for every part of the NHS and its 13 million staff to work together to realise this ambitious goal.4

The impact of dental anxiety on climate change

Dental services contribute 3% of the overall carbon footprint of the NHS in England,7 a figure that is high given the size of the sector. The major contributors to the carbon footprint of dental services are travel (64%), procurement (19%) and energy use (15.3%).8 However, some dental anxiety management techniques have a disproportionately high impact on the environment relative to their usage.

Nitrous oxide is commonly used to manage dental anxiety but is a potent greenhouse gas. When nitrous oxide is used during dental treatment, it increases the carbon footprint of that procedure 4.7 to 9.5 times. Carbon dioxide equivalents (CO2e) and global warming potential (GWP100) denote the warming potential of a greenhouse gas relative to carbon dioxide (GWP100 = 1) and nitrous oxide has a GWP100 298 times higher than carbon dioxide. It can add up to 342 kg CO2e to dental treatment, depending on the length of the procedure.7 This is equivalent to driving an average petrol car 849 miles.9

Despite its environmental burden, inhalation sedation with nitrous oxide (IS) is an invaluable anxiety management tool for dentists: 36% of the UK adult population have moderate dental anxiety and 12% report extreme anxiety or phobia.10 IS is an effective and low-cost technique with a wide margin of safety which facilitates access to dental treatment for anxious patients who would otherwise be unable to accept care under local anaesthetic. It has particular benefit for those with medical co-morbidities which make techniques such as intravenous sedation or general anaesthetic risky or not feasible. IS plays a significant role in the UK in reducing health inequality and limits the need for general anaesthesia which is extremely financially and environmentally costly.11 This is particularly true for children under 12 years of age, where alternative techniques, such as intravenous sedation, are not available for management of dental anxiety outside of a specialist setting.12

Currently, there are no alternative gases available to dentists in the UK to provide conscious sedation. National bodies such as the Society for the Advancement of Anaesthesia in Dentistry and the Dental Sedation Teachers Group rightly advocate for the continued use of nitrous oxide but with consideration for the environmental harm.13 However, there is no guidance available for implementing sustainable practices into our sedation services. While research progresses to identify more environmentally sustainable alternatives to nitrous oxide, it is important that we understand how we can optimise the care we provide to mitigate its continued use for now. This paper will discuss evidence-based methods sedationists can use to reduce the carbon footprint of their services and make a significant contribution to our net-zero goals for the NHS.

Limiting the environmental impact of conscious sedation services

Efficient use of nitrous oxide

Reducing nitrous oxide use by improving system efficiencies is a surprisingly straightforward and effective way to limit the amount of gas we use without impacting service levels. More efficient changes include optimising gas delivery to the lungs and by ensuring patients are optimally sedated with titrated sedation for as short a time as is necessary.14

The delivery of nitrous oxide can vary by setting. In services with multiple users of the gas it is often supplied via piped systems from centralised storage, in contrast to primary care settings, where smaller, in-surgery cylinders are more common. Where nitrous oxide is delivered piped from central manifold systems, leakage of gas before it reaches the patient is a significant problem. Up to March 2021, 16 NHS hospital trusts had audited their piped nitrous oxide supply and estimated a loss of 13.7 million litres of gas per annum. This represented 95% of their total collective annual volume.15 Other trusts have identified that 70-98% of their total purchased nitrous oxide is wasted through leakage or other supply inefficiencies before reaching patients.15

There are effective ways of identifying and managing leaks. The Nitrous Oxide Project16 is a national UK initiative with the goal of reducing the environmental impact of nitrous oxide use in hospitals. Their guidelines advise providers to begin by quantifying gas loss by undertaking a waste review, then involve clinicians and estates teams in multidisciplinary planning to reduce leaks and inefficiencies.

Reviewing how piped systems are used clinically can often identify ageing or superfluous pipe and manifold infrastructure which can be decommissioned to improve system efficiency and reduce potential leakage. Regular leak and pressure testing of systems is also recommended but may not be possible where supplies are shared with acute services and cannot be shut down to facilitate testing. Installing flow valves to manifolds is often more feasible and allows qualitative monitoring of leaks by matching manifold gas flows to clinical usage.

Replacing central piped systems with local cylinders of nitrous oxide offers benefits to dental sedationists. Cylinders deliver nitrous oxide up to 74% more efficiently than piped systems16 and back-up cylinders provide protection against system failures. Portable cylinders and scavenging systems also allow the flexible provision of IS within clinics, which can reduce the amount of equipment needed and maintenance costs. Mobile cylinders mean IS units can be moved around the dental surgery which can help to facilitate treatment for patients with mobility or other special care requirements. Cylinders also offer financial efficiencies: one NHS trust found switching from a centrally piped system to nitrous oxide portable cylinders resulted in a net cost saving of £1,681.70 per year, and a CO2e saving of 54,236 kg; the equivalent to 156,209 miles driven in an average car.17

It is important to remember that gas can also be wasted after delivery but before it reaches the patients' lungs. Ensuring that nasal hoods are correctly sized and well-fitting reduces opportunity for leakage, as does reminding patients to breathe exclusively through the nasal hood (Fig. 1). Carrying out dental treatment under rubber dam both discourages mouth breathing and improves exhaled gas scavenging.

Fig. 1
figure 1

a, b, c, d) Ensuring a well-fitting nasal hood increases sedation efficiency and prevents unnecessary gas wastage

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Taking time to understand individual patient needs has the dual benefit of improving quality of care and reducing the carbon impact of sedation services. Understanding the driving factors behind an individual's anxiety allows titration of total dose of nitrous oxide and optimum timing of gas delivery. For example, higher percentages of nitrous oxide might be delivered during administration of local anaesthetic for a needle-phobic patient, which can then be reduced or removed as treatment progresses.

These small changes to practice ensure that the maximum health benefit is obtained from each unit of nitrous oxide used relative to the detrimental impact on the environment.

Catalysing scavenged nitrous oxide

It is important to prevent the nitrous oxide we use from reaching the atmosphere where possible. The nasal hoods used to deliver IS during dental treatment often allow very efficient scavenging of exhaled gases, with minimal nitrous oxide escaping into the clinical environment. However, typically, scavenging systems then vent gases directly to the outside atmosphere without processing.

Nitrous oxide ‘cracking' is an emerging technology being trialled in some maternity services in the UK.18 This process involves catalysing exhaled nitrous oxide to break it down into non-greenhouse gases nitrogen and oxygen. Central destruction units (CDUs) are large units which can be retrofitted to existing gas scavenging systems whereas mobile destruction units (MDU) are portable and act as independent scavenging units (Fig. 2).

Fig. 2
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Mobile destruction unit. Image kindly provided by Medclair AB, Stockholm, Sweden

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CDUs and MDUs can destroy 99% of the nitrous oxide passed through the units,19 but these systems are not currently in widespread use in the UK. Some NHS trusts are considering implementing CDUs where centralised scavenging systems are used, and there are a few small scale trials of MDUs in dental settings underway, but with as yet unpublished results. These systems may become commonplace as implementation research progresses but careful lifecycle analysis should be undertaken to ensure there are no unintended consequences of adopting this technology. In maternity settings, destruction units have been demonstrated to be effective at reducing occupational exposure to nitrous oxide for healthcare staff,18 which may be what ultimately drives progression of this technology, but it is not clear whether benefit would translate to dental settings where effective scavenging systems are already in place.

Non-pharmacological anxiety management

Anxiety pathways are a stepwise method of introducing patients to increasingly intensive techniques for managing their dental anxiety. Pathways involve simple behaviour management techniques initially, followed by more interventive techniques, such as hypnosis or cognitive behavioural therapy (CBT), and progressing to inhalation or intravenous conscious sedation or general anaesthetic if required. This ladder allows a tailored approach to anxiety management, resulting in patients accessing the least interventive method that is sufficient to meet their anxiolysis needs. Guidelines for sustainable dental care recommend prioritising behaviour management techniques over more resource-intensive pharmacological interventions where possible and clinically appropriate.20

Anxiety management techniques, such as rapport building, environmental change, enhancing control and management of physical arousal, are important tools which can be employed in any setting and are effective for most patients with low to moderate levels of dental anxiety. These techniques have very low environmental impact as no additional resources are required other than time and clinician understanding.

CBT is a brief, short-term talking therapy, which is increasingly recognised as a valuable tool for managing a range of psychological disorders as recommended by the National Institute for Health and Clinical Excellence for the management of specific phobias.21

There is a well-developed evidence-base for the efficacy and effectiveness of CBT in long-term management of dental phobia.22 CBT-based interventions can be delivered by any specifically trained dental professional with proven effectiveness.23 Increasingly, CBT services for dental anxiety are led by specially trained dental nurses and treatment courses typically consist of six to ten sessions which can be delivered by telephone.24 CBT can be used to support anxious individuals to accept dental treatment under local anaesthetic instead of escalating to conscious sedation or general anaesthesia.

Reducing the net impact of services to mitigate nitrous oxide use

As it is not possible to completely phase out the use of nitrous oxide until a more environmentally friendly and safe alternative is available, it is important that we act to reduce the climate impact of sedation services in other ways. Simple alterations to the way services are designed and delivered can have a huge impact on its carbon footprint without compromising on service quality or quantity.

Travel

Patient and staff travel are the biggest contributors to the carbon footprint of most dental procedures.7 Reducing the distance travelled by patients to access services reduces emissions and reduces health inequality by improving access to care and limiting health burden for patients. Designing services which deliver lean patient pathways can limit the amount of travel required of patients and staff. Lean pathways aim to optimise health benefit by maximising high-value and high-quality care from each patient contact.25 One way of doing this is to incorporate technology into services to gather information before face-to-face appointments.

Telephone and video consultations in dentistry proliferated during the COVID-19 pandemic when usual services were not able to operate.26 This practice of ‘telemedicine' reduces the carbon footprint of healthcare,27 improves health equality28 and is increasingly recognised as time-efficient, effective and safe.29 Telephone and video consultations should be used where practical to maximise the benefit of face-to-face consultations. For example, collecting medical history information and providing pre-operative sedation instructions in advance allows triage to appropriate services and reduces wasted appointments. Providing logistical support for staff to carry out telephone consultations from home reduces travel and frees clinical space for delivering interventive care.

Combining visits where possible can minimise the impact of travel on the overall carbon footprint for a course of treatment. Family members could be offered consecutive appointments and if users require access to multiple medical services, these appointments could be scheduled for the same day. Allowing patients to access care from multiple specialties through multi-disciplinary clinics is cost-effective for healthcare providers, allows more time and is travel efficient for patients and staff, and improves treatment outcomes.

Where travel is necessary, the mode of transport has a significant impact on the emissions generated by the journey. Promoting active travel or public transport for appointments could be as simple as encouraging carbon-neutral travel to clinics in appointment letters or providing information on cycle storage or bus routes.

Small efficiencies

It is important not to underestimate the combined power of individuals making sustainable everyday choices. The Clinical guidelines for environmental sustainability in dentistry20 highlight the cumulative effect of small changes to practice on the total carbon footprint of a service. Simple interventions, such as proper segregation of clinical and non-clinical waste, recycling, reducing paper waste and eliminating single-use plastics where possible, have the highest impact relative to effort. Installing low-energy or movement-activated lighting is also straightforward, reduces energy use and saves more than the value of installation over its lifetime.30 Investing in staff training for best practice in ‘green dentistry' is worthwhile, as changing attitudes has the most long-term impact on day-to-day activities.

Patient choices

One innovative way of promoting sustainable healthcare choices is to consider discussing the climate impact of treatment options with patients during decision-making. The general public are accustomed to weighing environmental impact when making consumer choices and there is increasing evidence that they wish to do the same for healthcare decisions. A recent study found participants were willing to sacrifice their time and convenience for more sustainable dental treatment and would be happy to pay more for dental services if this reduced the environmental impact.31,32 It might be important to discuss the environmental impact of anxiety management measures with patients to support them in making an informed choice which aligns with their values. Clearly the climate impact of treatment is only a small part of a shared decision, where clinical effectiveness and the experience and expertise of the consulting clinician play an important role. However, an awareness of climate impact is likely to be increasingly important to healthcare consumers who may wish to offset the climate impact of their treatment choices by non-clinical means.

Future developments - alternatives to nitrous oxide

There is a cohort of patients for whom nitrous oxide has particular value as they are not suitable for routine management with single-drug intravenous sedation due to age or medical co-morbidities. These groups would benefit from sedation techniques with a similar effect and safety profile to nitrous oxide but with reduced environmental impact.

Methoxyflurane

Methoxyflurane is an anaesthetic gas which can have a similar analgesic and hypnotic effect to nitrous oxide but with a much lower greenhouse effect (methoxyflurane has a GWP100 of 4 compared with GWP100 of 298 for nitrous oxide). Methoxyflurane is no longer used for general anaesthesia but is licenced in Australia and New Zealand for analgesia in acute medical settings and is used in a similar way to nitrous oxide in the UK. The Australian Dental Board regulates use of methoxyflurane for conscious sedation by dentists who have appropriate airway management and life support skills.33

There is very limited research on the efficacy and safety of using methoxyflurane for conscious sedation in dentistry. A study in 1975 found that it was effective and produced fewer symptoms of nausea and vomiting than nitrous oxide, but that nitrous oxide resulted in less uncooperative behaviour.34 A more recent study demonstrated a proprietary methoxyflurane inhaler (Penthrox) provided a sedative effect comparable to nitrous oxide when used during removal of third molars under local anaesthetic in a cohort of healthy young adults.35

However, there are some limitations to the potential of methoxyflurane as a replacement for nitrous oxide in dental settings in the UK. The pharmacological and side-effect profile mean that more caution is necessary for use in medically complex or high body mass index (BMI) patients where nitrous oxide often has utility. The effects of methoxyflurane can be prolonged for patients with high BMI due to a propensity for the drug to diffuse into adipose tissues. This drug reservoir can take several days to be slowly released and has the potential to result in prolonged respiratory depression.33 It is also possible for methoxyflurane to cause renal impairment36 and repeated exposure may cause hepatoxicity,33 both of which may make it unsuitable for routine use in medically complex cohorts.

Sevoflurane

Sevoflurane is an alternative anaesthetic gas which also has a lower GWP100 than nitrous oxide and emerging application in conscious sedation. Although currently not widely employed, advantages of sevoflurane include low blood solubility, fast induction and short recovery time, with relatively minimal effect on respiratory functions or protective airway reflexes. Due to the high potency of the gas it can be delivered via a nasal cannulae rather than a nasal hood with benefits to operator field of view.

A significant disadvantage of sevoflurane is the potential to cause malignant hyperthermia in susceptible individuals, and appropriate scavenging and ventilation are also required, as the gas is potent and can spread to the clinical environment. As sevoflurane is not commonly used for dental conscious sedation, proprietary delivery and scavenging systems are not available. Neither are there sufficient safety and efficacy studies to support use of sevoflurane by dentists in the UK.

Despite these concerns, both methoxyflurane and sevoflurane have future potential as more environmentally sustainable gases to facilitate conscious IS. Further safety and efficacy research specific to the operator-sedationist delivery style in dentistry would be of benefit.

Conclusion

Climate change represents an urgent threat to our way of life and without action, we face significant reductions to our standard of living.5 All healthcare decisions must weigh the potential harms of treatment against expected benefits. Management of dental anxiety using inhaled nitrous oxide has a high environmental cost but offers significant health and equality benefits for anxious dental patients. While research progresses to identify a suitable sustainable replacement for nitrous oxide, there are strategies we can employ to limit the impact of sedation services on the environment without compromising on the quality of care provided (Box 1).

In our personal lives, many of us are making lifestyle changes which recognise the importance of living sustainably and minimising our individual impact on the environment. As healthcare providers, we are in a privileged position to contribute to decarbonisation and act as advocates for change. The choices we make professionally have the potential to make a substantially higher contribution to the net-zero goals of society than we can have through sustainable choices in our home lives.