United Nations experts have called on US authorities not to go ahead with the planned execution of an inmate by nitrogen hypoxia.
Four UN Special Rapporteurs said the method may subject Kenneth Smith to “cruel, inhuman or degrading treatment or even torture”.
Smith, convicted for a murder-for-hire committed in 1988, is scheduled to be executed in the state of Alabama on 25 January.
The method is intended to deprive him of oxygen by using a face mask connected to a cylinder of nitrogen.
Smith, 58, is one of only two people alive in the US to have survived an execution attempt.
Alabama botched his previously scheduled execution by lethal injection in November 2022 when multiple attempts to insert an intravenous line into a vein failed.
“This will be the first attempt at nitrogen hypoxia execution,” the Special Rapporteurs said in a statement.
They said the method could cause “grave suffering” and likely be at odds with the prohibition on torture and other cruel, inhuman or degrading punishment.
“We are concerned that nitrogen hypoxia would result in a painful and humiliating death,” they said.
Smith’s lawyers have said the untested gassing protocol may violate the US Constitution’s ban on “cruel and unusual punishments”.
They have argued a second attempt to execute him by any method is unconstitutional.
Most US executions are carried out using lethal doses of a barbiturate, but some states have struggled to obtain the drugs because of a EU law banning pharmaceutical companies from selling drugs that can be used in executions to prisons.