Climate effect of inhaled anaesthetics.

Almost all areas of human activity have the potential to influence climate. The administration of inhalation anaesthetics is one such activity. However, since the first dedicated study in 1989 of the possible role of halogenated anaesthetics, they have received only sporadic interest. Two recent papers, published in this issue of the British Journal of Anaesthesia and in Anesthesia and Analgesia earlier this year, have refocused attention on isoflurane, desflurane, and sevoflurane. To be clear, such anaesthetics currently make only a very minor contribution to climate change. Using an estimate of the amount currently in the atmosphere (around a part per trillion), these three gases contribute only 0.02% of the climate effect that results from the increases in carbon dioxide due to human activity. Nevertheless, the issue is, all other considerations being equal, which of the anaesthetics is most ‘climate friendly’? Although this question is easy to pose, it is not easy to answer—how do we compare the climate effect of gas A with gas B? I expect that there are similar issues when comparing the clinical effectiveness of different anaesthetics. In assessing the climate effect of a gas, there are two distinct issues. The first is more straightforward and requires basic physical characteristics of a gas to be determined, via laboratory measurements and calculations. The key quantities are the atmospheric lifetime of the gas and the effectiveness of the gas at absorbing and emitting infrared radiation (the impact per unit concentration is characterized by a quantity called ‘radiative efficiency’)—this effect on infrared radiation results in a molecule contributing to the greenhouse effect. The two recent papers 3 provide important updates for these quantities (albeit with some significant disagreement, especially for sevoflurane). The second issue is much trickier. The aim is to place the climate effect of emissions of gases with differing lifetimes and radiative efficiencies on some common scale. So, for example, does a gas with a long lifetime but a low radiative efficiency have a larger or smaller effect on climate than a gas with a short lifetime and a high radiative efficiency? There is no unique answer to this question: it depends on the way the climate effect is quantified and it depends on a number of value-laden decisions. Because carbon dioxide is the largest contributor to human-induced climate change, it is conventional to use climate metrics to place emissions of gases on a CO2equivalent scale. By using these metrics, a kilogram emission of a gas can be said to be somehow equivalent to the emission of X kilograms of CO2. The Kyoto Protocol to the United Nations Framework Convention on Climate Change, which committed nations to targets for their CO2-equivalent emissions, is a multigas treaty. It covered emissions of greenhouse gases including CO2, methane, nitrous oxide, and groups of fluorinated compounds. To make the Protocol operational, a metric was required so that non-CO2 emissions could be given a CO2 equivalence. The chosen metric was the global warming potential (GWP). The GWP has been used in the recent studies on anaesthetics 3 with clear results that, per kilogram emitted into the atmosphere, desflurane has the largest effect and isoflurane the least. According to Ryan and Nielsen, the effect of desflurane is significantly accentuated by the fact that more of it is required to deliver the same clinical effect. These authors go a step further by considering the carrier gas that is used. The significance here is that nitrous oxide is also a greenhouse gas. In the case of mixtures of Volume 105, Number 6, December 2010