Formaldehyde

Formaldehyde, also called methanal, is the simplest aldehyde. It is a colorless gas with a sharp, irritating odor. The molecule is Y-shaped, with a carbon–oxygen bond and carbon–hydrogen bonds; its H–C–H bond angle is 117°, close to the 120° of a trigonal planar molecule. In its gaseous state, it is stable up to about 150 °C, but when condensed it polymerizes. At normal temperatures, it exists in several interchangeable forms: the monomeric gas; a white solid trimer called 1,3,5-trioxane, which dissolves in organic solvents without breaking down; a white solid polymer called paraformaldehyde, which slowly releases the monomer; and methanediol, which in water forms a saturated solution of about 40% formaldehyde by volume (37% by mass), known as formalin. Commercial formalin typically contains 10–12% methanol as a stabilizer to prevent oxidation and polymerization, along with trace metallic impurities. The name “formaldehyde” was first used as a generic trademark in 1893, following the earlier trade name “formalin.”

The compound is produced globally at an estimated 26 million tons per year (2024). Its main use is in making industrial resins for particle board, coatings, plastics, pulp, paper, synthetic fibers, and textiles. A roughly 5% solution in water serves as a disinfectant and fumigant in industrial, agricultural, and healthcare settings, while a 37% solution preserves tissue samples in laboratories. Formaldehyde is also a precursor to many other materials and chemicals.

Naturally, formaldehyde arises from the breakdown of serine, dimethylglycine, and lipids. Demethylase enzymes convert N-methyl groups into formaldehyde. In the atmosphere, it enters through both primary emissions—from vegetation, biomass burning, vehicles, industry, and coal combustion—and secondary formation, mostly from the oxidation of methane and other volatile organic compounds (VOCs). Over land, biogenic and anthropogenic VOCs are the main source; in remote and marine areas, methane oxidation dominates. Isoprene from vegetation is a key natural precursor. Formaldehyde is a short-lived but reactive carbonyl that helps sustain the atmosphere’s oxidative capacity by producing odd-hydrogen radicals, and it is an important precursor to ozone. It is removed from the troposphere mainly by photodissociation, either into molecular products or into radicals (H and HCO), which then react with oxygen to form hydroperoxyl radicals. In the presence of NOx and sunlight, it contributes to tropospheric ozone and photochemical smog. More than 80% of environmental formaldehyde comes from processes in the upper atmosphere, and it is also an intermediate in the combustion of methane and other carbon compounds from sources like forest fires, automobile exhaust, and tobacco smoke. Formaldehyde has been detected in outer space.

Satellite instruments—such as OMI on Aura, GOME-2 on MetOp, TROPOMI on Sentinel-5P, and TEMPO on Intelsat 40e—monitor tropospheric formaldehyde using ultraviolet–visible spectroscopy, producing near-global maps with daily coverage and kilometer-scale resolution, as well as regional hourly maps over North America. The close link between isoprene and formaldehyde allows scientists to estimate regional and global isoprene emissions from satellite data. Formaldehyde is classified as a Group 1 carcinogen and can irritate the respiratory system and skin.