Sustainability in dental sedation – nitrous oxide waste mitigation

Key points

• Nitrous oxide is a greenhouse gas with 298 times the global warming potential (GWP) of carbon dioxide and is also a major contributor to ozone depletion [1].
• Nitrous oxide has a lifetime of 114 years, remaining in the atmosphere and impacting on the environment into the future [1].
• Waste and losses of nitrous oxide can occur through system leaks, excess supply, poor stock management, theft and residual content left in cylinders after use.
• Gas suppliers are required to empty returned unused and part-full nitrous oxide cylinders before refilling and currently this is released into the atmosphere [2].
• Releasing the contents of one unused size E cylinder into the environment produces emissions equivalent to ~1 tonne of CO₂, which is approximately the amount produced by the use of a small car for a year.*
• There are feasible actions that can be taken to reduce nitrous oxide emissions from dental sedation provision without compromising patient care. All members of staff involved in the use of inhalation sedation equipment, irrespective of their role, can contribute to these actions.

*Assuming: a size E cylinder contains 1800 litres when full; the density of N₂O is 1.964 g/l; the GWP of N₂O is 298 relative to CO₂; emissions of a small car are 120g CO₂/km; annual mileage is 6000 miles (9656 km).

 

Mitigation of nitrous oxide emissions

Reducing the environmental impact of anaesthetic gases is a priority for the NHS in Scotland and England as part of their net zero strategies, with reduction in waste and effective capture and destruction as key steps [3][4]. Catalytic cracking technologies which convert captured nitrous oxide to non-greenhouse gases, nitrogen and oxygen, are under investigation but not yet widely implemented in the UK.

Although scavenging systems reduce nitrous oxide levels in the surgery, reducing the risk to staff from occupational exposure, the nitrous oxide is externally vented into the wider environment and is not captured or destroyed.

Urgent action to identify and reduce waste is the essential first step in reducing nitrous oxide emissions. Analysis of nitrous oxide supply and clinical use at NHS acute sites identified very substantial losses (up to 83-100%) from piped systems and this has informed the UK wide Nitrous Oxide Project and the NHS Scotland Nitrous Oxide Mitigation Implementation Plan [5][6].

Nitrous oxide waste can occur whether supplied via a central manifold and piped system or directly from cylinders in the surgery. The following action points aim to support mitigating nitrous oxide waste in dental facilities as a first step in reducing the environmental impact of dental anxiety management. The actions are intended to cover a range of dental inhalation sedation settings and nitrous oxide supply systems and some may not be applicable in every case.

All members of staff involved in aspects of the supply, management and operation of inhalation sedation equipment should be made aware of the sustainability issues and involved in actions to mitigate losses and waste.

 

As advised in the SDCEP Conscious Sedation in Dentistry guidance, consider all relevant anxiety management techniques including non-pharmacological behaviour management techniques.

If inhalation sedation with nitrous oxide/oxygen is indicated, use the minimum effective dose of nitrous oxide, at an appropriate flow rate for the patient.

Check equipment (including breathing circuit tubing) for leaks and faults.

• Note that substantial leaks can occur if the Bodok seal between the mobile inhalation sedation machine and cylinder is missing or perished.

Use a correctly fitting patient nasal hood, with a working flapper valve in place.

• Leakage from around the mask increases nitrous oxide usage during the sedation episode and increases staff exposure.

Turn off cylinders in the surgery/disconnect pipes from wall outlets after use.

When recording details of the sedation episode in the patient record/sedation log, include the percentage nitrous oxide at sedation endpoint, flow rate and length of time administered, to allow assessment of clinical use (see below for review of supply versus use).

Record the date of receipt, size, expiry date and serial number of each cylinder or tank supplied (see Nitrous Oxide Stock Record template).

• Delivery requests, supplier invoices, pharmacy records or manifold logbooks might provide supply details.

Store nitrous oxide cylinders safely, away from sources of ignition, and securely to protect from theft. 

When changing cylinders, apply good stock management principles e.g., select those with the shortest remaining shelf-life, to avoid unused cylinders reaching their expiry date (see Nitrous Oxide Stock Record template for an example stock control system).

• Although the shelf-life of nitrous oxide for medical use from when the cylinder is filled is approximately 36 months [7], be aware that the remaining time may be less when stock is received. 

Review stock regularly to ensure that while adequate supplies are available for predicted use, there is not excess stock (see Nitrous Oxide Stock Record template).

• Incorporating the monitoring of nitrous oxide stock and expiry dates into other routine checks (e.g. of emergency drugs) might be helpful. 

Consider implementing a protocol for optimal clinical use of cylinders, to ensure that residual gas is kept to a minimum [8].

• For example, when the pressure gauge on a size E nitrous oxide cylinder starts to fall from the initial level (typically 745 psi), the cylinder could still contain approximately 250 litres [9], which could provide sedation for around 90 minutes with 40% nitrous oxide at a combined flow rate of 7 litres/min.

Consider reviewing nitrous oxide supply versus clinical use in your setting, to identify and mitigate possible sources of waste or losses.*

• Identify relevant staff for provision of information and collaboration to avoid duplication of effort. This might include your medical gas supplier, individuals responsible for ordering, returning and setting up cylinders, clinicians providing inhalation sedation, local sustainability champion, NHS health board nitrous oxide mitigation group etc.
• Develop and put into practice an action plan to address any issues identified and repeat the review after an appropriate time period to measure the effectiveness of the actions.

* Supply of nitrous oxide can be determined using the Nitrous Oxide Stock Record template. Clinical use can be estimated using the Nitrous Oxide Clinical Usage Review template or the Sedation Logbook with nitrous oxide use calculator. 

Other resources

The Nitrous Oxide Project provides tools and guidance for assessing piped manifold systems.

NHS Education for Scotland (via TURAS Learn) provides information about Quality Improvement (QI) approaches that may be applicable for mitigating nitrous oxide emissions.

The PSM Quality Improvement Activity topic provides information about quality improvement in dental practice.