General anesthesia uses volatile halogenated gases and nitrous oxide to knock patients out. Very little is metabolized, so when patients exhale, these gases enter the atmosphere—and may persist for up to 114 years. Nitrous oxide has a climate warming effect 289 times that of carbon dioxide, and desflurane, another anesthesia gas, is 3,714 times more potent. They also damage the ozone layer. If the more than 1 million hip and knee replacements performed in the U.S. in 2009 used general anesthesia, the gases used would equal 3,260,000 pounds of coal burned, or 7,350,000 miles driven in an average passenger car.
An alternative is regional anesthesia, combining intravenous sedatives and anesthetic drugs injected close to a nerve for pain relief.