When most of us think of greenhouse gases, we imagine carbon dioxide or methane spewing into the atmosphere above. It's not often we consider general anesthesia being part of the problem – but it is, and it's something that happens thousands of times every day, at hospitals all around the world.

The gases involved in putting patients to sleep, like nitrous oxide and desflurane, are not always well understood in terms of their effects on the body, but in the atmosphere, we know some of them can stick around for up to 114 years, damaging the ozone layer and further amplifying climate change.

That said, almost no definitive data exists on their global contribution. In the United States, studies suggest the healthcare system is responsible for between 5 to 10 percent of pollutant emissions, and anesthetic gases are thought to make up half an operating theatre's carbon footprint.

Now, a new study has found that switching to local anesthetic where possible could help cut global greenhouse emissions by a significant amount.

Using a hospital in New York as a real-world example, researchers calculated what happens to a facility's emissions when surgeons carry out as many hip and knee replacements as possible under regional anesthetic, rather than using local.

In 2019, the Hospital for Special Surgery in New York conducted over 10,000 of these procedures, and that year, just 4 percent were done under general anesthetic, compared to the previous average of 75 percent.

That switch alone, the authors say, 'saved' the equivalent of nearly 12,000 kilograms of coal burned (27,000 lbs) or over 97,365 kilometres of driving (60,500 miles). To put it a different way, that's over three million smartphones fully charged.

When general anesthetic gases are delivered to a patient, less than 5 percent are metabolised by the body. The rest is vented directly up into the atmosphere. Once outside, the global warming potential of a halogenated anesthetic is up to 2,000 times greater than CO2.

Nitrous oxide alone, which has been regularly used for 150 years, has been estimated by some to contribute 3 percent of total emissions in the US today.

These gases have extremely high global warming potential, too. Using desflurane for just one hour, in fact, is equivalent to up to 750 kilometres of driving (470 miles).

"Unlike prior benefits described with the use of regional anesthesia, the benefits in this case are truly global in nature," the authors of the new study write.

"However, the decision to use a specific anesthetic method should primarily be based on the optimal technique for that individual patient."

Of course, not all surgeries can be done under regional anesthetic, which usually on intravenous sedatives, rather than gas, and not all general anesthetics are gas either.

Nonetheless, in cases where local can be used, evidence suggests it is much more effective at pain relief compared to general, has fewer unpleasant side effects, shortens hospital stays, and may even be preferred by patients.

"Thus," the authors write, "increasing the use of regional anesthesia is potentially good for the climate, improves the quality of care (at least for hip and knee replacements), and may allow individual practitioners to take personal responsibility in the fight against global warming."

It should be noted that the research is based on only one hospital in a country where surgery is quite common. What's more, it doesn't take into account the entire production and transportation emissions required for anesthetic.

Still, if the results are expanded nationwide, the potential seems clear.

In 2009, more than a million hip and knee surgeries were carried out in the US, and if every one of these was done under general (obviously, an unrealistic standard) that would result in 112,000 kilograms of desflurane and 9,000 kg of nitrous oxide released into the atmosphere.

Given the urgency of cutting greenhouse gas emissions wherever we can, it's definitely something to consider.

The study was published in The BMJ.