Carey Dale Grayson, 50, is set to be executed in Alabama using nitrogen gas, the third such execution since the state introduced the method this year.

Convicted of the 1994 murder of hitchhiker Vickie Deblieux, Grayson is the only one of the four teens involved to face the death penalty, as the others were under 18 at the time.

Nitrogen hypoxia, which causes death by replacing oxygen with pure nitrogen, has faced criticism after previous executions resulted in visible shaking, CNN has reported.

Grayson’s lawyers argue the method is untested and causes “conscious suffocation” and have appealed to delay the execution, but state officials maintain it is constitutional and effective.

Written by B.C. Begley