Adrexan vapor is a highly combustible gas, which burns at extremely high temperatures. It can be absorbed by metal and biological material alike. It's appearance is similar to dense fog or steam, white and expanding. The vapor is kept in high pressure containers, where it is isolated from sources of combustion, such as electricity. In this form, the vapor is quite cold, as remarked by Chiana. Once combusted, though, Adrexan vapor burns at a temperature intense enough to severely harm a full grown Leviathan.
When Moya was infected with Karack metallites, Zhaan was forced to make Adrexan vapor, which was absorbed by Moya and subsequently ignited, destroying both the metallites and a large portion (one ninth) of the Leviathan. ("Liars, Guns and Money - With Friends Like These...")