Methanal, better known as formaldehyde, is the simplest aldehyde: a sharp-smelling gas that dissolves readily in water to form formalin. It is used as a disinfectant, tissue fixative, and industrial preservative, but it is not an approved food additive. Food-safety agencies watch for its illegal use, such as preserving fish, milk, tofu, or noodles in some markets, because formaldehyde is a known irritant and carcinogen. Laboratories detect it in suspect foods with colorimetric and chromatographic methods, and its presence triggers seizure and enforcement action. Regulatory limits for formaldehyde in foods address its occurrence as a natural background compound in some products as well as deliberate adulteration.