burning nitrate film is highly toxic, containing several poisonous gases and can become lethal if inhaled enough. Many nitrate films have been transferred in recent decades to safety stock, and original nitrate prints are generally stored separately to prevent a nitrate fire from destroying other non-nitrate films; the gas they release also affects the emulsion of safety film. Usually nitrate collections are even split up into several different fireproof rooms to minimize damage to an entire collection should a fire occur in one part. It is normal for a theater today to pass rigorous safety standards and precautions before being certified to run nitrate films; this includes a fireproof projection booth, fire chambers surrounding the feed and take-up reels, and several fire extinguishers built into the projector and aimed at the projector's film gate with a trigger released if the film ignites. Nitrate film is classified as "dangerous goods", which requires licenses for storage and transportation. Nitrate film stock was used in every major film production before about 1951. Many silent films only survived because they were printed to 16 mm film, which did not use a nitrate base. A report published by the United States Library of Congress in September 2013 states that 70 percent of all American silent feature films are lost. Acetate In the literature of photography "acetate" is used as a synonym for several chemicals: cellulose acetate, cellulose triacetate, cellulose diacetate, cellulose acetate propionate, and cellulose acetate butyrate. All have been used separately and in mixtures, to produce film base. Despite the dangers of the nitrate film base being known practically since its development, it was used in virtually all major motion pictures prior to 1952, when Kodak completed a four-year conversion program to the sole manufacturing of acetate base film stocks. Kodak began working with acetate "safety film" as early as 1909, and started selling it in 1910 for 22 mm film. Acetate has always been used with 8 mm and 16 mm formats, as they were originally created for amateur home movie usage, and generally was used for most sub-35 mm formats to minimize risk to the general public. Several formats, such as 17.5 mm, which were often re-slit from 35 mm, were nitrate, however. One of Kodak's reasons for choosing 16 mm instead of 17.5 mm for a standard amateur format width was specifically to prevent nitrate re-slits from being used in home