An Alabama prisoner convicted of the 1994 murder of a female hitchhiker is slated to become the third person executed by nitrogen gas.

An Alabama inmate found guilty of murdering a hitchhiker 30 years ago on Thursday became the third person in America to be executed using nitrogen gas, furthering the ongoing debate about whether the method is an inhumane way to carry out death penalty sentences.

The execution is the 22nd in the U.S. this year and the sixth in Alabama, which has put three of the men to death using nitrogen gas.