Attorneys for Kenneth Eugene Smith, 58, filed an amended lawsuit challenging the proposed new execution method as a potential violation of the Constitution’s ban on cruel and unusual punishment. They asked a federal judge to issue a preliminary injunction to block the execution from going forward next year.

“There is sparse research on how long a human must be exposed to 100% pure nitrogen to cause death, what happens if a human is exposed to less than 100% pure nitrogen for a prolonged period of time, or on the pain or sensations that a human exposed to nitrogen might experience,” his attorneys wrote in the amended lawsuit filed Monday.

They noted in the filing that the American Veterinary Medical Association wrote in 2020 euthanasia guidelines that nitrogen hypoxia is an acceptable method of euthanasia for pigs but not other mammals because it could create an “anoxic environment that is distressing for some species.”