agent liquefied inert cantharidin mustard odorless toxic foaming emulsifying Inergen Halon antiknock chlorine liquids solvent solvents gaseous lubricant Butyl fluorinating pyrophoric PhostrEx propane Metol dioxide inhaled odourless liquified vapor hydrofluorocarbons vesicant liquid PDLA drying corrosive naphthalenes polyisobutylene polymerised nitrosourea