“That’s carbon monoxide!! Mugino, burn the air! If you don’t, we’re done for!!”
Beams of light shot in several directions at once.
That deadly gas was primarily produced by incomplete combustion; an intense oxidation reaction would convert it into relatively safe carbon dioxide. Aneri had colored the toxin red to visualize it, but that coloration was already vanishing.

Toaru Majutsu no Index New Testament vol. 12 ch.4 pt.1