Formaldehyde

Ball-and-stick Stick Space-filling

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4 atoms, 3 bonds.

IUPAC name: formaldehyde
Molar mass: 30.0260(34) g/mol
Atoms: 4
SMILES: C=O
InChIKey: WSFSSNUMVMOOMR-UHFFFAOYSA-N
PubChem CID: 712

Made of

Structural formula of formaldehyde. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Nobody sells pure formaldehyde. The Hazardous Substances Data Bank (HSDB) explains that the free compound tends to polymerise, linking into chains, so industry ships it dissolved in water instead. At room temperature it is a gas that NIOSH, the US National Institute for Occupational Safety and Health, describes as nearly colourless, with a pungent, suffocating odour. HSDB gives its boiling point as −19.5 °C and its melting point as −92 °C.

ChEBI, a chemical dictionary of biological molecules, calls it an aldehyde and a one-carbon compound. HSDB calls it the simplest aldehyde. On paper it is methanol with two hydrogen atoms removed, the change ChEBI calls a formal oxidation. One atom each of carbon and oxygen and two of hydrogen make up the molecule, and its molar mass is 30.026 g/mol. HSDB notes that the dry gas is fairly stable at 80 to 100 °C but slowly polymerises when cooler. Traces of acids, alkalis or water greatly speed that up.

Structure and bonding

Formaldehyde’s carbon sits in the middle, with the oxygen at one end and the two hydrogens splayed at the other. The Labels button prints C, O, H and H on them. Two sticks join carbon to oxygen as a double bond, and one stick links each hydrogen. Face-on, the four atoms form a flat Y; turned edge-on, the model collapses to a thin line, since all four lie in one plane.

In PubChem’s computed model the H–C–H angle is 115.5° and each O=C–H angle 122.2°, so the Y is narrower between the hydrogens than on the oxygen side. The C=O bond measures 1.225 Å there, against 1.102 Å for each C–H. The Stick view keeps only that Y of bonds, and the Space-filling view fuses the four spheres into one small, flattened lump.

What the viewer draws is the free H₂C=O molecule of the gas. In water, and so in formalin, formaldehyde takes other forms. CAMEO Chemicals, a hazard database, describes it as methylene glycol, CH₂(OH)₂, plus chains HO(CH₂O)_xH averaging about three units. These sit in equilibrium with the free molecule. In the solid, paraformaldehyde, CAMEO puts the chains nearer thirty units long.

Where it turns up

Plants, animals and humans all make small amounts of formaldehyde, notes ATSDR, the US Agency for Toxic Substances and Disease Registry. It is also a byproduct of combustion, the US Environmental Protection Agency (EPA) explains, so it escapes from cigarettes and from unvented gas stoves and kerosene space heaters. The NCI Thesaurus, the reference vocabulary of the National Cancer Institute (NCI), adds smoke from fires and car exhaust.

Indoor air holds more of it than outdoor air, ATSDR reports, and pressed wood is frequently a major source, in the view of an NCI fact sheet. The EPA names resins in hardwood plywood, particleboard and medium-density fibreboard, along with glues, permanent-press fabrics, paints and paper products. Exterior-grade pressed wood gives off less, the NCI explains, because it contains phenol resins rather than urea resins. Once in the air, formaldehyde is broken down within hours, according to ATSDR.

Why it matters

EPA chemical data reports put US production in 2023 at between 2.5 and 4 billion pounds. HSDB records commercial manufacture since 1889, by oxidising methanol over a catalyst. Most of it becomes resins for particleboard, the EPA says, or an intermediate for making other chemicals.

Formaldehyde is also the root of an early plastic. Stephen Lower’s Chem1 text dates the first commercial synthetic moulding plastic to 1907–1909. Leo Baekeland, a Belgian chemist, made it from phenol and formaldehyde, and the common name Bakelite comes from his. In Chem1’s account, the monomers behind such resins carry many reactive sites, and each can become a cross-link. Chem1 describes urea–formaldehyde resins binding wood particles into particleboard and giving cotton and rayon their wrinkle, water and stain resistance. Melamine resins end up as plastic plates, cups and Formica laminates.

NCI Thesaurus lists it as a disinfectant and as a preservative for biological specimens. Its other uses, as the EPA lists them, include fumigants, photography, concrete and plaster additives, and wood preservation.

Properties

Properties of formaldehyde Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 712.
Property	Value	Unit	Source
Molecular formula (Hill order)	CH₂O
Molar mass	30.0260(34) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	1
Hydrogen atoms	2
Oxygen atoms	1
Bonds	3
Single bonds	2
Double bonds	1
PubChem molecular weight	30.026 g/mol	g/mol	PubChem

Safety

Formaldehyde is a flammable, toxic gas and a known human carcinogen. Its EU classification carries the signal word Danger. The listed hazards include fatal if inhaled, severe skin burns and eye damage, allergic skin reactions and cancer. Workplace exposure limits are set out in the NIOSH Pocket Guide card, and the PubChem Laboratory Chemical Safety Summary is the full hazard sheet.

Questions

How can you lower formaldehyde levels at home?

Fresh air from open windows and fans is the easiest way to bring levels down, according to ATSDR, the US Agency for Toxic Substances and Disease Registry. Not smoking and not running unvented heaters indoors also help. So do sealing the unfinished surfaces of manufactured wood and washing new permanent-press clothes before they are worn. The National Cancer Institute adds good ventilation, moderate temperatures and lower humidity.

Is formalin the same thing as formaldehyde?

Formalin is formaldehyde dissolved in water, usually 37 per cent by weight according to HSDB. In that solution the free H₂C=O molecule gives way to other forms: CAMEO Chemicals describes methylene glycol, CH₂(OH)₂, plus short chains. Methanol is often added to keep solid polymers from settling out. NCI Thesaurus describes the formalin that serves as a disinfectant and a preservative for biological specimens as a more dilute solution.

Does the human body make formaldehyde?

Yes. Small amounts form during normal metabolism in most organisms, humans included, as NCI Thesaurus notes. HSDB says the body turns it into formic acid, which leaves in urine or is oxidised to carbon dioxide and water, with a blood half-life of about 1.5 minutes on primate data. ATSDR adds that it is a normal constituent of tissues and fluids, so a blood test cannot show exposure.

When was formaldehyde recognised as a cancer risk?

In 1980, laboratory studies showed that formaldehyde exposure could cause nasal cancer in rats, the National Cancer Institute reports. In 1987 the US Environmental Protection Agency classified it as a probable human carcinogen under unusually high or prolonged exposure. In 2011 the National Toxicology Program named it a known human carcinogen in its 12th Report on Carcinogens, and the International Agency for Research on Cancer also classifies it as a human carcinogen.

Made of the same elements

HCOOH Formic acid 5 atoms, 4 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.