His lithium hydroxide CO2 scrubber broke, allowing the lethal gas to build up to dangerous levels. Lithium hydroxide is a corrosive alkali hydroxide, a white hygroscopic crystalline material that is used in filters to purify air. Cassidy didn’t notice any apparent effect, but the spacewalk came to a sudden ending after Houston noticed that things weren’t right at all inside his suit.
His words after getting into the ISS airlock: “I’m just going to sit here and wait for Dave and enjoy the view.”
Balls of titanium, I tell you. [Aviation Week]