Although formaldehyde is a gas at room temperature, it is readily soluble in water, and it is most commonly sold as a 37% solution in water called by trade names such as formalin or formol. In water, formaldehyde polymerizes, and formalin actually contains very little formaldehyde in the form of HCHO monomer. Usually, these solutions contain a few percent methanol to limit the extent of polymerization.
Formaldehyde reversibly polymerizes to produce its cyclic trimer, 1,3,5-trioxane or the linear polymer polyoxymethylene. Formation of these substances makes formaldehyde's gas behavior differ substantially from the ideal gas law, especially at high pressure or low temperature.
Formaldehyde is readily oxidized by atmospheric oxygen to form formic acid. Formaldehyde solutions must be kept tightly sealed to prevent this from happening in storage.
Industrially, formaldehyde is produced by the catalytic oxidation of methanol. The most commonly catalysts are silver metal and a mixture of an iron oxide with molybdenum. In the more commonly used iron oxide system, methanol and oxygen react at 400°C to produce formaldehyde according to the chemical equation
The silver-based catalyst is usually operated at a higher temperature, about 650°C. On it, two chemical reactions simultaneously produce formaldehyde: the one shown above, and the dehydrogenation reaction
Formaldehyde kills most bacteria, and so a solution of formaldehyde in water is commonly used as a disinfectant or to preserve biological specimens. It is also used as a preservative in vaccinations.
Most formaldehyde, however, is used in the production of polymers and other chemicals. When combined with phenol, urea, or melamine, formaldehyde produces a hard thermoset resin. These resins are commonly used in permanent adhesives, such as those used in plywood or carpeting. They are also foamed to make insulation, or cast into molded products. Production of formaldehyde resins accounts for more than half of formaldehyde consumption.
Formaldehyde is also used to make numerous other chemicals. Many of these are polyfunctional alcohols such as pentaerythritol, which is used to make paints and explosives. Other formaldehyde derivatives include diphenylmethane diisocyanate, an important component in polyurethane paints and foams, and hexamethylene tetramine, which is used in phenol-formaldehyde resins and to make the explosive RDX.
Because formaldehyde resins are used in many construction materials, including plywood, carpet, and spray-on insulating foams, and because these resins slowly give off formaldehyde over time, formaldehyde is one of the more common indoor air pollutants. At concentrations above 0.1 ppm in air, inhaled formaldehyde can irritate the eyes and mucous membranes, resulting in watery eyes, headache, a burning sensation in the throat, and difficulty breathing.
Large formaldehyde exposures, for example from drinking formaldehyde solutions, are potentially lethal. Formaldehyde is converted to formic acid in the body, leading to a rise in blood acidity, rapid, shallow breathing, hypothermia, and coma or death. People who have ingested formaldehyde require immediate medical attention.
In the body, formaldehyde can cause proteins to irreversibly bind to DNA. Laboratory animals exposed to large doses of inhaled formaldehyde over their lifetimes have developed more cancers of the nose and throat than are usual, as have workers in particle-board sawmills. However, some studies suggest that smaller concentrations of formaldehyde like those encountered in most buildings have no carcinogenic effects. Formaldehyde is classifed as a probable human carcinogen.
