As a product of normal metabolism, formaldehyde has been documented to be naturally present in many common food items, including fruits and vegetables, meats, fish, crustacea and dried mushrooms etc., at a wide range of levels (Table 1). In some seafood species, formaldehyde is a natural breakdown product of a chemical known as trimethylamine oxide (TMAO) that exists in their bodies. Trimethylamine oxide breaks down into formaldehyde and dimethylamine in equal parts after the animal dies. The level of formaldehyde can accumulate in certain marine fish during frozen storage and crustacea after death. Its levels were reported to be up to 400 mg/kg in Bombay-duck after cold storage. The detection of dimethylamine in Bombay-duck was used to distinguish whether formaldehyde had been added deliberately.