Cellulose nitrate was once the standard film variety used by the motion picture industry — up until about the 1940s. That substance is unstable and highly flammable; cellulose nitrate fires are difficult to extinguish and produce dangerous gases, including corrosive nitrogen oxides and carbon monoxide. Many important films produced on cellulose nitrate have been forever lost due to degradative oxidation . More recent film stock employs cellulose acetate, the so-called "safety film." Unfortunately, even this improved film is subject to degradation, though it can be preserved for more than a century if stored under cold, dry conditions.