"In accidents, when people have been exposed to high levels of nitrogen and little air in an enclosed space, they have died quickly. In some cases co-workers who rushed in to rescue them also collapsed and died. Nitrogen itself is not poisonous, but someone who inhales it, with no air, will pass out quickly, probably in less than a minute, and die soon after — from lack of oxygen. The same is true of other physiologically inert gases, including helium and argon, which kill only by replacing oxygen.... Death from nitrogen is thought to be painless. It should prevent the condition that causes feelings of suffocation: the buildup of carbon dioxide from not being able to exhale. Humans are highly sensitive to carbon dioxide — too much brings on the panicky feeling of not being able to breathe. Somewhat surprisingly, the lack of oxygen doesn’t trigger that same reflex. Someone breathing pure nitrogen can still exhale carbon dioxide and therefore should not have the sensation of smothering. Before passing out, a person may feel lightheaded, dizzy or maybe even a bit euphoric, and vision may dim...."

From "States Turn to an Unproven Method of Execution: Nitrogen Gas" (NYT).