Four states still authorize gas chamber executions: Arizona, California, Missouri, and Wyoming. In every one of them, hydrogen cyanide gas is a backup or conditional option rather than the default, and no state has carried out a gas chamber execution since Arizona put Walter LaGrand to death in March 1999.
The Four States That Still Authorize Lethal Gas
The statutes differ in important ways, and the differences determine whether the method could realistically ever be used again.
Arizona. Anyone sentenced to death for a crime committed before November 23, 1992 may choose lethal gas instead of lethal injection. The choice must be made in writing at least twenty days before the execution date. Without a written election, the state uses lethal injection by default.1Arizona Legislature. Arizona Code 13-757 – Method of Infliction of Sentence of Death The eligible population shrinks every year.
California. State law lets any person under a death sentence elect lethal gas over lethal injection. The election must be submitted in writing within ten days after the warden serves the execution warrant; otherwise, lethal injection applies.2California Legislative Information. California Penal Code 3604 – Death Penalty Execution Method An executive moratorium has suspended all executions in California since 2019, so the provision is dormant in practice.
Missouri. The statute authorizes both lethal injection and lethal gas, but it is ambiguous about who chooses. Missouri has used lethal injection exclusively for decades.
Wyoming. Lethal gas activates only if lethal injection is held unconstitutional. Inmates have no right to elect it.3Justia. Wyoming Code 7-13-904 – Method of Execution
Older summaries sometimes list Mississippi as a lethal gas state. That is out of date. Mississippi’s current statute authorizes lethal injection, nitrogen hypoxia, electrocution, and firing squad — not hydrogen cyanide gas.
Legislatures keep these provisions on the books for a practical reason. If lethal injection drugs become unavailable or a court strikes down the primary method, the state retains a legal pathway to carry out existing death sentences without passing new legislation. Whether any state could actually operate a gas chamber on short notice is a separate question.
When an Inmate Can Actually Choose the Gas Chamber
The right to elect lethal gas is narrow where it exists at all. Arizona confines the choice to people sentenced for crimes committed before November 23, 1992.1Arizona Legislature. Arizona Code 13-757 – Method of Infliction of Sentence of Death California imposes no date restriction but requires a written election within ten days of receiving an execution warrant.2California Legislative Information. California Penal Code 3604 – Death Penalty Execution Method Wyoming’s statute never gives the inmate a choice. Missouri’s statute leaves the question open.
Election deadlines in capital states more generally range from thirty days after the state supreme court affirms the sentence to fourteen or twenty days before the execution itself. In almost every scheme, failure to elect in time means lethal injection.
Why No State Has Used the Gas Chamber Since 1999
The decisive legal attack came in Fierro v. Gomez. In 1996, the Ninth Circuit upheld a district court finding that California’s gas chamber protocol was cruel and unusual. The trial court found that inmates “do not become immediately unconscious upon the first breath of lethal gas,” that consciousness likely persisted for at least fifteen seconds to a minute, and that there was a “substantial likelihood” of waxing and waning awareness for several additional minutes. During that stretch, inmates experience intense pain from cellular oxygen deprivation, which the court compared to a major heart attack or being held underwater.
The Supreme Court vacated the ruling in Gomez v. Fierro, but on narrow grounds. California had amended Penal Code § 3604 to let inmates choose lethal injection, and the Court sent the case back for reconsideration in that new light.4Legal Information Institute. Gomez v Fierro, 519 US 918 The Court has never ruled directly on whether hydrogen cyanide gas violates the Eighth Amendment, and it has “repeatedly denied certiorari in challenges to lower court determinations that execution by lethal gas does not violate the Eighth Amendment.”5U.S. Department of Justice. Restoring and Strengthening the Federal Death Penalty
Any future challenge would be governed by Bucklew v. Precythe (2019). That standard requires the inmate to identify a feasible, readily available alternative that would significantly reduce a substantial risk of severe pain, and to show that the state has refused to adopt it without a legitimate reason.6Supreme Court of the United States. Bucklew v Precythe, 587 US 119 Given the documented pain of cyanide gas and the ready availability of lethal injection as an alternative, a challenger would have strong factual arguments.
How a Gas Chamber Execution Works
The chamber itself is a sealed steel vessel, typically hexagonal or cylindrical, with industrial rubber gaskets on the doors and reinforced glass for witnesses. A metal chair is bolted to the floor with restraints for arms, legs, and torso. Beneath the chair sits a reservoir of sulfuric acid diluted with distilled water. A lever operated from an adjacent room drops cyanide pellets into the acid, and the reaction releases hydrogen cyanide gas.
Hydrogen cyanide kills by blocking cytochrome oxidase, the enzyme cells need to use oxygen. Oxygen remains in the blood, but cells throughout the body, especially in the brain, can no longer process it.7National Center for Biotechnology Information. Hydrogen Cyanide – Acute Exposure Guideline Levels The result is sometimes called cellular suffocation. The condemned person is typically told to breathe deeply once the gas begins to rise. Early breaths trigger a surge in heart rate and respiration. Muscle spasms, gasping, and convulsions follow. The Fierro findings put consciousness at fifteen seconds to a minute, with awareness probably continuing intermittently for several minutes beyond that.
Walter LaGrand’s 1999 execution in Arizona reportedly took eighteen minutes. Witnesses described agonized choking and gagging throughout. LaGrand had chosen the gas chamber himself over lethal injection, in what many observers read as a deliberate attempt to spotlight the method’s brutality.
After death is pronounced, a ventilation system draws the gas through chemical scrubbers that neutralize the hydrogen cyanide before venting. Staff wearing oxygen tanks and protective equipment enter and spray the chamber and the body with ammonia, which reacts with residual cyanide. Arizona’s protocol instructs the removal team to ruffle the deceased’s hair to release any gas trapped against the scalp. Only after air-quality sensors confirm safe conditions can the body be moved.
Arizona’s 2020 Refurbishment and the Obstacles to Reuse
In 2020, Arizona quietly began refurbishing its gas chamber after two decades of disuse. The state bought a solid brick of potassium cyanide for $1,530 along with sodium hydroxide pellets and sulfuric acid, spending over $2,000 on chemicals. Staff tested the seals with water and a smoke grenade, held a candle flame near the sealed windows and door to check for leaks, and ran role-play drills with guards simulating resistant inmates. By December 2020 the chamber was declared operationally ready. The refurbishment drew widespread attention in part because hydrogen cyanide gas was used by the Nazis under the trade name Zyklon B. Arizona ultimately did not carry out any gas chamber executions and has used lethal injection for the executions it has performed since.
Practical obstacles still weigh against future use. The equipment requires regular maintenance and testing. Chemical suppliers have grown reluctant to sell execution chemicals and precursors, which was one of the reasons states started looking for alternatives to lethal injection in the first place. Staff need extensive training in hazardous-material handling, and the post-execution decontamination is far more complex than lethal injection. Federal workplace safety rules add a layer of compliance obligations on top of all of that.8Occupational Safety and Health Administration. Process Safety Management of Highly Hazardous Chemicals – 1910.119
Nitrogen Hypoxia Is a Different Method
Hydrogen cyanide gas and nitrogen hypoxia are often grouped together as “gas” executions, but they are not the same thing. Nitrogen hypoxia works by replacing breathable air with pure nitrogen through a mask, causing death through oxygen deprivation rather than chemical poisoning. There is no acid bath, no cyanide, and no hazardous decontamination afterward.
Alabama carried out the first nitrogen hypoxia execution on January 25, 2024, putting Kenneth Smith to death. Alabama, Arkansas, Louisiana, Mississippi, and Oklahoma have authorized the method. None of those authorizations revived or added hydrogen cyanide gas; they are a separate track. Federal courts have so far upheld nitrogen hypoxia against Eighth Amendment challenges, and the Supreme Court has declined to intervene.9Legal Information Institute. Boyd v Hamm
The hydrogen cyanide gas chamber sits in a legal twilight: authorized in four states, operational in perhaps one, and carrying a record of documented suffering that makes any future use an almost certain invitation to constitutional challenge.