Drs. Parker and Behringer correctly point out that in our review1on the potential toxic effects of nitrous oxide we did not cover its contribution to the greenhouse effect; this omission was predicated by considerations of space, direct relevance to patient care, and a lack of relevant data that is less than 20 yr old. Correctly, Drs. Parker and Behringer indicate that nitrous oxide contributes to the greenhouse effect (±0.05%); however, as 99% of the nitrous oxide in the atmosphere originates from industrial and agricultural sources, the total elimination of medical use of nitrous oxide will likely have a negligible effect on the greenhouse effect. What also needs to be considered is the effect of increased use of another greenhouse gas, namely halogenated anesthetics, as a possible replacement for nitrous oxide. Furthermore, we need to understand the totality of the environmental impact of the volatile gases, and that includes its manufacture, transport, and storage. In absence of reliable data on these issues, it would be too simplistic to state that there would be an environmental benefit were we to abandon the anesthetic use of nitrous oxide.

*Imperial College London, Chelsea and Westminster Hospital, London, United Kingdom. m.maze@ic.ac.uk

Sanders RD, Weimann J, Maze M: Biologic effects of nitrous oxide: A mechanistic and toxicologic review. Anesthesiology 2008; 109:707–22