Nitrogen tetroxide (N2O4) was a chemical which was used extensively as a propellant for reaction control systems on Earth spacecraft at the beginning of Human space travel. It was apparently still used in some alien spacecraft even in the 24th century, Kazon shuttles being one example. In 2372, the presence of nitrogen tetroxide fumes in the cabin of Tierna's shuttle seemed to confirm his claim to have been attacked, as Harry Kim found it unlikely that he would want to breathe poison gas for hours while waiting for rescue. (VOY: "Basics, Part I")