Yesterday, Oklahoma governor Mary Fallin signed into law a bill that approves the use of nitrogen gas for executions in the state. The method, which would effectively asphyxiate death row inmates by forcing them to breathe pure nitrogen through a gas mask, is meant to be the primary alternative to lethal injection.

Fallin and other supporters of the procedure say it’s pain-free and effective, noting that the nitrogen would render inmates unconscious within ten seconds and kill them in minutes. It’s also cheap: state representatives say the method only requires a nitrogen tank and a gas mask

Oklahoma’s primary execution method is still lethal injection, but the state’s procedure is currently under review by the Supreme Court. Earlier this week, Tennessee suspended executions statewide following challenges to its own lethal injection protocol.

The new bill accounts for the possibility that, even if lethal injection is ruled constitutional, the drugs necessary to perform the procedure may still be unavailable. Ohio recently suspended all of its 2015 executions while it attempts to restock its supply of lethal injection drugs.

Last month, the largest group of pharmacists in the US urged its members to refrain from providing prisons with the chemicals used in lethal injections. If Oklahoma cannot use lethal injection for capital punishment, it will turn to the nitrogen method, followed by the electric chair and finally a firing squad if the previous options fail.