Quantifying the volume of nitrous oxide mix wasted through stock rotation in a manifold supply system

Nitrous oxide accounts for more healthcare-associated carbon-equivalent emissions than all other anaesthetic gases combined. Prior research has demonstrated significant waste of pure nitrous oxide. However, waste from nitrous oxide mix ('gas and air') remains poorly characterised. We conducted a single-site service evaluation of waste due to returns of unused nitrous oxide mix. A manifold-based system of eight G-size cylinders was assessed across four exchanges in two non-consecutive weeks. Cylinders were weighed following removal from the manifold. The residual mass of gas was calculated by subtracting the tare weight from the total mass. Results were analysed using descriptive statistics. Carbon-equivalent greenhouse gas emissions were estimated using 100-year global warming potential (273). The mean residual mass per cylinder was 1.25 kg (SD 0.53; range 0-2.1), representing 14.8% of original contents. Across four exchanges, 38.9 kg of nitrous oxide mix was returned unused, equivalent to 6.17 tonnes CO₂. Waste is embedded in the routine practice of maintaining an uninterrupted supply of nitrous oxide mix. There is substantial variance, both between and within cylinder bank changes, which may be due to leaks or gas handling practices. Multi-site evaluations are needed to develop a broader understanding of waste, and why it occurs.