Red fuming nitric acid (RFNA) is a storable oxidizer used as a rocket propellant. It consists of nitric acid, dinitrogen tetroxide and a small amount of water. The color of red fuming nitric acid is due to the dinitrogen tetroxide, which breaks down partially to form nitrogen dioxide. The nitrogen dioxide dissolves until the liquid is saturated, and produces toxic fumes with a suffocating odor. RFNA increases the flammability of combustible materials and is highly exothermic when reacting with water.