An Alabama death row inmate is expected to become the first person in the US to be executed with nitrogen gas, after losing last-minute appeals.
The US Supreme Court and a lower appeals court declined to block what Kenneth Eugene Smith’s lawyers called a “cruel and unusual” punishment.
Opponents say using nitrogen could cause unnecessary suffering, and a leak could harm people present in the room.
Smith, 58, was convicted in 1989 of murdering Elizabeth Sennett.
Alabama has 30 hours to carry out the execution, which involves pumping nitrogen gas through a mask, from Thursday at 0600 GMT (0100 ET).
He told the BBC earlier this week that the wait felt like torture.
Smith would be the first person to be put to death by this method in the US and, according to the Death Penalty Information Center, anywhere in the world.
Lawyers for the inmate, who has been on death row since 1996, told the BBC on Wednesday night that they were lodging another appeal to the nation’s top court in the hope of a last minute reprieve.
Breathing pure nitrogen without oxygen causes the cells to break down and leads to death. Alabama said in a court filing that they expect him to lose consciousness within seconds and die in a matter of minutes.
But its use has been denounced by some medical professionals, who warn it could cause a range of catastrophic mishaps, ranging from violent convulsions to survival in a vegetative state.
Alabama and two other US states have approved the use of nitrogen hypoxia as an alternative method of execution because the drugs used in lethal injections have become more difficult to find, contributing to a fall in the use of the death penalty nationally.
Alabama already tried to execute Smith by lethal injection two years ago but were unable to raise a vein before the state’s death warrant expired.
Smith was one of two men convicted of murdering 45-year-old Sennett in a $1,000 (£790) killing-for-hire in March 1988.
She was beaten with a fireplace implement and stabbed in the chest and neck, and her death was staged to look like a home invasion and burglary.
Her husband, a debt-ridden preacher, had orchestrated the scheme to collect insurance money. He killed himself as investigators closed in.
Smith’s fellow hitman, John Forrest Parker, was executed in 2010. (BBC)