As a general rule, 1 hour per millimetre of tissue paraformaldehyde (PFA) must penetrate. This was based on reacting paraformaldehyde and ammonium nitrate in acetic anhydride. The resulting formaldehyde gas from dry heating paraformaldehyde is flammable. Another option is to use an antiseptic filling material containing paraformaldehyde like N2. The phenoxide derivative is susceptible to formylation paraformaldehyde to give salicylaldehyde. Trioxane is often used interchangeably with formaldehyde and with paraformaldehyde. It has been noted that aqueous formaldehyde sometimes produces a better yield than paraformaldehyde. It was produced industrially from paraformaldehyde and a mixture of chlorosulfonic acid and sulfuric acid. Other benefits to paraformaldehyde include long term storage and good tissue penetration. The trimer is 1,3,5-trioxane, and the polymer is called paraformaldehyde. Paraformaldehyde is a poly-acetal. At the endpoint, the first excess of hydroxyl ions catalyzes the depolymerization of paraformaldehyde. Paraformaldehyde is not a fixative; it must be depolymerized to formaldehyde in solution. As a formaldehyde releasing agent, paraformaldehyde is a potential carcinogen. Paraformaldehyde commonly has a slight odor of formaldehyde due to decomposition.