Simplest member of aldehyde family, formula CH2O. Formaldehyde is very soluble in water; solutions of formaldehyde (formalin) are used in biological labs to preserve samples. Formaldehyde is fairly toxic; health concerns are associated with its emission from foam, cavity insulation, or new plastic materials such as upholstery, carpet, etc. Formaldehyde is present in the atmosphere as an intermediate in the oxidation of methane and many other hydrocarbons; its photolysis is a major source of free radicals in the atmosphere.