It may also contain potassium aluminum sulfate as a hardener and boric acid as a buffer. Fixer solutions slowly release sulfur dioxide gas as they age. However, when these solutions are contaminated with acid from the stop bath, the gas sulfur dioxide is released at a more rapid rate. Sodium sulfite decomposes to produce sulfur dioxide, when heated or allowed to stand for a long time in water or acid. Sodium thiosulfate upon heating or a long-standing solution can also decompose to form highly toxic sulfur dioxide gas.
Many asthmatics are particularly sensitive to sulfur dioxide.
Sodium bisulfite also decomposes to form sulfur dioxide if it makes contact with boric acid or acetic acid. Boric acid is moderately toxic unless the skin is abraided or burned, then it can be highly toxic. Ensure good ventilation of the darkroom. Wear gloves and goggles. Cover all solutions when not in use to prevent evaporation or release of toxic vapors and gases.