From the article, which you can find here:
“Professor J Wesley Boyd works as part of the faculty at the Center for Bioethics and is an associate professor of psychiatry at Harvard Medical School. He spoke to The Mirror about Smith’s upcoming execution, and what the convict could experience as he is suffocated using nitrogen gas.
“87 per cent of the air that we breathe is nitrogen and 21 per cent is oxygen, so nitrogen is by no means a foreign substance for us,” he explained. “When one is executed by nitrogen hypoxia, they are breathing only nitrogen with no oxygen whatsoever, and because they are deprived of oxygen they suffocate.”
The state has maintained that the nitrogen gas, which will be administered through a mask placed over Smith’s face, will cause him to quickly lose consciousness before he dies from a lack of oxygen. However, with this being the first execution using this method, many have compared Smith to a “guinea pig” as much of what’s known about nitrogen hypoxia comes from medical journal records about industrial accidents and suicide attempts, not executions.
Looking at what Smith’s experience will be like, Professor Boyd said: “The short answer is that we don’t know what dying by nitrogen hypoxia will be like because it is novel and untested in humans as a means of execution. As a result, it might be incredibly painful and/or distressing, and to my knowledge, there is no scientific evidence to suggest otherwise.”