Formaldehyde (/fɔːrˈmældɪhaɪd/ⓘfor-MAL-di-hide,US also/fər-/ⓘfər-) (systematic namemethanal) is anorganic compound with thechemical formulaCH2O and structureH−CHO, more preciselyH2C=O. The compound is a pungent, colourless gas thatpolymerises spontaneously intoparaformaldehyde. It is stored as aqueous solutions (formalin), which consists mainly of the hydrate CH2(OH)2. It is the simplest of thealdehydes (R−CHO). As a precursor to many other materials and chemical compounds, in 2006 the global production of formaldehyde was estimated at 12 million tons per year.[14] It is mainly used in the production of industrialresins, e.g., forparticle board andcoatings.
Formaldehyde also occurs naturally. It is derived from the degradation ofserine,dimethylglycine, andlipids.Demethylases act by converting N-methyl groups to formaldehyde.[15]
Formaldehyde is more complicated than many simple carbon compounds in that it adopts several diverse forms. These compounds can often be used interchangeably and can be interconverted.[citation needed]
Molecular formaldehyde. A colorless gas with a characteristic pungent, irritating odor. It is stable at about 150 °C, but it polymerizes when condensed to a liquid.
1,3,5-Trioxane, with the formula (CH2O)3. It is a white solid that dissolves without degradation in organic solvents. It is atrimer of molecular formaldehyde.
Paraformaldehyde, with the formula HO(CH2O)nH. It is a white solid that is insoluble in most solvents.
Methanediol, with the formula CH2(OH)2. This compound also exists in equilibrium with variousoligomers (short polymers), depending on the concentration and temperature. A saturated water solution, of about 40% formaldehyde by volume or 37% by mass, is called "100% formalin".
A small amount ofstabilizer, such asmethanol, is usually added to suppressoxidation andpolymerization. A typical commercial-grade formalin may contain 10–12% methanol in addition to various metallic impurities.
"Formaldehyde" was first used as ageneric trademark in 1893 following a previous trade name, "formalin".[17]
Main forms of formaldehyde
Monomeric formaldehyde (subject of this article)
Trioxane is a stable cyclic trimer of formaldehyde.
Paraformaldehyde is a common form of formaldehyde for industrial applications.
Methanediol, the predominant species in dilute aqueous solutions of formaldehyde
Processes in the upperatmosphere contribute more than 80% of the total formaldehyde in the environment.[24] Formaldehyde is an intermediate in the oxidation (orcombustion) ofmethane, as well as of other carbon compounds, e.g. inforest fires,automobile exhaust, andtobacco smoke. When produced in the atmosphere by the action of sunlight andoxygen onatmospheric methane and otherhydrocarbons, it becomes part ofsmog. Formaldehyde has also been detected in outer space.
Formaldehyde and itsadducts are ubiquitous in nature. Food may contain formaldehyde at levels 1–100 mg/kg.[25] Formaldehyde, formed in the metabolism of the amino acidsserine andthreonine, is found in the bloodstream of humans and other primates at concentrations of approximately 50 micromolar.[26] Experiments in which animals are exposed to an atmosphere containing isotopically labeled formaldehyde have demonstrated that even in deliberately exposed animals, the majority of formaldehyde-DNA adducts found in non-respiratory tissues are derived from endogenously produced formaldehyde.[27]
Formaldehyde does not accumulate in the environment, because it is broken down within a few hours by sunlight or by bacteria present in soil or water. Humans metabolize formaldehyde quickly, converting it toformic acid.[28][29] It nonetheless presentssignificant health concerns, as acontaminant.
Formaldehyde appears to be a useful probe in astrochemistry due to prominence of the 110←111 and 211←212K-doublet transitions. It was the first polyatomicorganic molecule detected in theinterstellar medium.[30] Since its initial detection in 1969, it has been observed in many regions of thegalaxy. Because of the widespread interest in interstellar formaldehyde, it has been extensively studied, yielding new extragalactic sources.[31] A proposed mechanism for the formation is the hydrogenation of CO ice:[32]
Formaldehyde was discovered in 1859 by the Russian chemistAleksandr Butlerov (1828–1886) when he attempted to synthesize methanediol ("methylene glycol") fromiodomethane andsilver oxalate.[35] In his paper, Butlerov referred to formaldehyde as "dioxymethylen" (methylene dioxide) because his empirical formula for it was incorrect, as atomic weights were not precisely determined until theKarlsruhe Congress.
The compound was identified as an aldehyde byAugust Wilhelm von Hofmann, who first announced its production by passing methanol vapor in air over hot platinum wire.[36][37] With modifications, Hofmann's method remains the basis of the present day industrial route.
Solution routes to formaldehyde also entail oxidation of methanol oriodomethane.[38]
The silver-based catalyst usually operates at a higher temperature, about 650 °C. Two chemical reactions on it simultaneously produce formaldehyde: that shown above and thedehydrogenation reaction:
CH3OH → CH2O + H2
In principle, formaldehyde could be generated by oxidation ofmethane, but this route is not industrially viable because the methanol is more easily oxidized than methane.[41]
Formaldehyde is produced via several enzyme-catalyzed routes.[42] Living beings, including humans, produce formaldehyde as part of their metabolism. Formaldehyde is key to several bodily functions (e.g.epigenetics[26]), but its amount must also be tightly controlled to avoid self-poisoning.[43]
Formaldehyde is a building block in the synthesis of many other compounds of specialised and industrial significance. It exhibits most of the chemical properties of other aldehydes but is more reactive.[45]
Monomeric CH2O is a gas and is rarely encountered in the laboratory. Aqueous formaldehyde, unlike some other small aldehydes (which need specific conditions to oligomerize throughaldol condensation) oligomerizes spontaneously at a common state. The trimer 1,3,5-trioxane,(CH2O)3, is a typical oligomer. Many cyclicoligomers of other sizes have been isolated. Similarly, formaldehyde hydrates to give the geminal diolmethanediol, which condenses further to form hydroxy-terminated oligomers HO(CH2O)nH. The polymer is calledparaformaldehyde. The higher concentration of formaldehyde—the more equilibrium shifts towards polymerization. Diluting with water or increasing the solution temperature, as well as adding alcohols (such as methanol or ethanol) lowers that tendency.
Gaseous formaldehyde polymerizes at active sites on vessel walls, but the mechanism of the reaction is unknown.[46] Small amounts ofhydrogen chloride,boron trifluoride, orstannic chloride present in gaseous formaldehyde provide the catalytic effect and make the polymerization rapid.[47]
Formaldehyde forms cross-links by first combining with a protein to formmethylol, which loses a water molecule to form aSchiff base.[48] The Schiff base can then react with DNA or protein to create a cross-linked product.[48] This reaction is the basis for the most common process ofchemical fixation.
Formaldehyde is readilyoxidized by atmospheric oxygen intoformic acid. For this reason, commercial formaldehyde is typically contaminated with formic acid. Formaldehyde can be hydrogenated intomethanol.
When conducted in the presence of hydrogen chloride, the product is the chloromethyl compound, as described in theBlanc chloromethylation. If the arene is electron-rich, as in phenols, elaborate condensations ensue. With 4-substituted phenols one obtainscalixarenes.[50] Phenol results in polymers.
Two steps in formation of urea-formaldehyde resin, which is widely used in the production of particle board
When condensed withphenol,urea, ormelamine, formaldehyde produces, respectively, hard thermoset phenol formaldehyde resin, urea formaldehyde resin, and melamine resin. These polymers are permanent adhesives used inplywood andcarpeting. They are also foamed to makeinsulation, orcast into moulded products. Production of formaldehyde resins accounts for more than half of formaldehyde consumption.
Formaldehyde is also a precursor to polyfunctionalalcohols such aspentaerythritol, which is used to makepaints andexplosives. Other formaldehyde derivatives include methylene diphenyl diisocyanate, an important component inpolyurethane paints and foams, andhexamine, which is used in phenol-formaldehyde resins as well as the explosiveRDX.
Condensation with acetaldehyde affordspentaerythritol, a chemical necessary in synthesizingPETN, a high explosive:[52]
An aqueous solution of formaldehyde can be useful as a disinfectant as it kills mostbacteria and fungi (including their spores). It is used as an additive in vaccine manufacturing to inactivate toxins and pathogens.[53]Formaldehyde releasers are used as biocides in personal care products such as cosmetics. Although present at levels not normally considered harmful, they are known to cause allergiccontact dermatitis in certain sensitised individuals.[54]
Formaldehyde is also approved for use in the manufacture of animal feeds in the US. It is an antimicrobial agent used to maintain complete animal feeds or feed ingredientsSalmonella negative for up to 21 days.[57]
Injecting agiant squid specimen with formalin for preservation
Formaldehyde preserves orfixes tissue or cells. The process involvescross-linking of primaryamino groups. The European Union has banned the use of formaldehyde as abiocide (includingembalming) under theBiocidal Products Directive (98/8/EC) due to its carcinogenic properties.[58][59] Countries with a strong tradition of embalming corpses, such as Ireland and other colder-weather countries, have raised concerns. Despite reports to the contrary,[60] no decision on the inclusion of formaldehyde on Annex I of the Biocidal Products Directive for product-type 22 (embalming and taxidermist fluids) had been made as of September 2009[update].[61]
Formaldehyde-based crosslinking is exploited inChIP-on-chip orChIP-sequencing genomics experiments, where DNA-binding proteins are cross-linked to their cognate binding sites on the chromosome and analyzed to determine what genes are regulated by the proteins. Formaldehyde is also used as a denaturing agent inRNA gelelectrophoresis, preventing RNA from forming secondary structures. A solution of 4% formaldehyde fixes pathology tissue specimens at about one mm per hour at room temperature.
In photography, formaldehyde is used in low concentrations for the processC-41 (color negative film) stabilizer in the final wash step,[62] as well as in theprocess E-6 pre-bleach step, to make it unnecessary in the final wash. Due to improvements in dye coupler chemistry, more modern (2006 or later) E-6 and C-41 films do not need formaldehyde, as their dyes are already stable.
In view of its widespread use, toxicity, and volatility, formaldehyde poses a significant danger to human health.[63][64] In 2011, the USNational Toxicology Program described formaldehyde as "known to be a human carcinogen".[65][66][67]
Concerns are associated with chronic (long-term) exposure by inhalation as may happen fromthermal orchemical decomposition of formaldehyde-based resins and the production of formaldehyde resulting from thecombustion of a variety of organic compounds (for example, exhaust gases). As formaldehyde resins are used in manyconstruction materials, it is one of the more commonindoor air pollutants.[68][69] At concentrations above 0.1 ppm in air, formaldehyde can irritate the eyes andmucous membranes.[70] Formaldehyde inhaled at this concentration may cause headaches, a burning sensation in the throat, and difficulty breathing, and can trigger or aggravate asthma symptoms.[71][72]
TheCDC considers formaldehyde as a systemic poison. Formaldehyde poisoning can cause permanent changes in thenervous system's functions.[73]
A 1988 Canadian study of houses withurea-formaldehyde foam insulation found that formaldehyde levels as low as 0.046 ppm were positively correlated with eye and nasal irritation.[74]A 2009 review of studies has shown a strong association between exposure to formaldehyde and the development of childhoodasthma.[75]
A theory was proposed for the carcinogenesis of formaldehyde in 1978.[76] In 1987 theUnited States Environmental Protection Agency (EPA) classified it as aprobable human carcinogen, and after more studies theWHOInternational Agency for Research on Cancer (IARC) in 1995 also classified it as aprobable human carcinogen. Further information and evaluation of all known data led the IARC to reclassify formaldehyde as aknown human carcinogen[77] associated with nasal sinus cancer andnasopharyngeal cancer.[78] Studies in 2009 and 2010 have also shown a positive correlation between exposure to formaldehyde and the development ofleukemia, particularlymyeloid leukemia.[79][80] Nasopharyngeal and sinonasal cancers are relatively rare, with a combined annual incidence in the United States of < 4,000 cases.[81][82] About 30,000 cases of myeloid leukemia occur in the United States each year.[83][84] Some evidence suggests that workplace exposure to formaldehyde contributes to sinonasal cancers.[85] Professionals exposed to formaldehyde in their occupation, such as funeral industry workers andembalmers, showed an increased risk of leukemia and brain cancer compared with the general population.[86] Other factors are important in determining individual risk for the development of leukemia or nasopharyngeal cancer.[85][87][88] In yeast, formaldehyde is found to perturb pathways for DNA repair and cell cycle.[89]
In the residential environment, formaldehyde exposure comes from a number of routes; formaldehyde can be emitted by treatedwood products, such asplywood orparticle board, but it is produced by paints,varnishes, floor finishes, andcigarette smoking as well.[90] In July 2016, the U.S. EPA released a prepublication version of its final rule on Formaldehyde Emission Standards for Composite Wood Products.[91] These new rules impact manufacturers, importers, distributors, and retailers of products containing composite wood, including fiberboard, particleboard, and various laminated products, who must comply with more stringent record-keeping and labeling requirements.[92]
The U.S. EPA allows no more than 0.016 ppm formaldehyde in the air in new buildings constructed for that agency.[93][failed verification] A U.S. EPA study found a new home measured 0.076 ppm when brand new and 0.045 ppm after 30 days.[94] TheFederal Emergency Management Agency (FEMA) has also announced limits on the formaldehyde levels in trailers purchased by that agency.[95] The EPA recommends the use of "exterior-grade" pressed-wood products with phenol instead of urea resin to limit formaldehyde exposure, since pressed-wood products containing formaldehyde resins are often a significant source of formaldehyde in homes.[78]
The eyes are most sensitive to formaldehyde exposure: The lowest level at which many people can begin to smell formaldehyde ranges between 0.05 and 1 ppm. The maximum concentration value at the workplace is 0.3 ppm.[96][need quotation to verify] In controlled chamber studies, individuals begin to sense eye irritation at about 0.5 ppm; 5 to 20 percent report eye irritation at 0.5 to 1 ppm; and greater certainty for sensory irritation occurred at 1 ppm and above. While some agencies have used a level as low as 0.1 ppm as a threshold for irritation, the expert panel found that a level of 0.3 ppm would protect against nearly all irritation. In fact, the expert panel found that a level of 1.0 ppm would avoid eye irritation—the most sensitive endpoint—in 75–95% of all people exposed.[97]
Someair purifiers include filtering technology that is supposed to lower indoor formaldehyde concentration.
Formaldehyde levels in building environments are affected by a number of factors. These include the potency of formaldehyde-emitting products present, the ratio of the surface area of emitting materials to volume of space, environmental factors, product age, interactions with other materials, and ventilation conditions. Formaldehyde emits from a variety of construction materials, furnishings, and consumer products. The three products that emit the highest concentrations aremedium density fiberboard, hardwood plywood, and particle board. Environmental factors such as temperature and relative humidity can elevate levels because formaldehyde has a highvapor pressure. Formaldehyde levels from building materials are the highest when a building first opens because materials would have less time to off-gas. Formaldehyde levels decrease over time as the sources suppress.
Inoperating rooms, formaldehyde is produced as a byproduct of electrosurgery and is present in surgical smoke, exposing surgeons and healthcare workers to potentially unsafe concentrations.[98]
Formaldehyde levels in air can be sampled and tested in several ways, including impinger, treated sorbent, and passive monitors.[99] TheNational Institute for Occupational Safety and Health (NIOSH) has measurement methods numbered 2016, 2541, 3500, and 3800.[100]
In June 2011, the twelfth edition of theNational Toxicology Program (NTP) Report on Carcinogens (RoC) changed the listing status of formaldehyde from "reasonably anticipated to be a human carcinogen" to "known to be a human carcinogen."[65][66][67] Concurrently, aNational Academy of Sciences (NAS) committee was convened and issued an independent review of the draft U.S.EPA IRIS assessment of formaldehyde, providing a comprehensive health effects assessment and quantitative estimates of human risks of adverse effects.[101]
For most people, irritation from formaldehyde is temporary and reversible, although formaldehyde can cause allergies and is part of the standard patch test series. In 2005–06, it was the seventh-most-prevalentallergen inpatch tests (9.0%).[102] People with formaldehyde allergy are advised to avoidformaldehyde releasers as well (e.g.,Quaternium-15,imidazolidinyl urea, anddiazolidinyl urea).[103] People who suffer allergic reactions to formaldehyde tend to display lesions on the skin in the areas that have had direct contact with the substance, such as the neck or thighs (often due to formaldehyde released from permanent press finished clothing) ordermatitis on the face (typically from cosmetics).[54] Formaldehyde has been banned in cosmetics in bothSweden[104] andJapan.[105]
In humans, ingestion of as little as 30 millilitres (1.0 US fl oz) of 37% formaldehyde solution can cause death. Other symptoms associated with ingesting such a solution include gastrointestinal damage (vomiting, abdominal pain), and systematic damage (dizziness).[73] Testing for formaldehyde is by blood and/or urine bygas chromatography–mass spectrometry. Other methods to detect formaldehyde include infrared detection, gas detector tubes, gas detectors using electrochemical sensors, andhigh-performance liquid chromatography (HPLC). HPLC is the most sensitive.[106]
The fifteenth edition (2021) of the U.S. National Toxicology Program Report on Carcinogens notes that currently in the U.S. “The general population can be exposed to formaldehyde primarily from breathing indoor or outdoor air, from tobacco smoke, from use of cosmetic products containing formaldehyde, and, to a more limited extent, from ingestion of food and water.” Affected water includes groundwater, surface water, and bottled water. It also notes that occupational exposure can be significant.[107]
Formaldehyde in food can be present naturally, added as an inadvertent contaminant, or intentionally added as a preservative, disinfectant, orbacteriostatic agent. Cooking and smoking food can also result in formaldehyde being produced in food. Foods that theU.S. National Toxicology Program has reported to have higher levels compared to other foods are fish, seafood, and smoked ham. It also notes formaldehyde in food generally occurs in a bound form and that formaldehyde is unstable in anaqueous solution.[108]
Scandals have broken in both the2005 Indonesia food scare and2007 Vietnam food scare regarding the addition of formaldehyde to foods to extend shelf life. In 2011, after a four-year absence, Indonesian authorities found foods with formaldehyde being sold in markets in a number of regions across the country.[109] In August 2011, at least at twoCarrefour supermarkets, the CentralJakarta Livestock and Fishery Sub-Department foundcendol containing 10parts per million of formaldehyde.[110] In 2014, the owner of two noodle factories inBogor, Indonesia, was arrested for using formaldehyde in noodles. 50 kg of formaldehyde was confiscated.[111] Foods known to be contaminated included noodles, salted fish, and tofu. Chicken and beer were also rumored to be contaminated. In some places, such as China, manufacturers still use formaldehyde illegally as a preservative in foods, which exposes people to formaldehyde ingestion.[112]
In 2011 inNakhon Ratchasima, Thailand, truckloads of rotten chicken were treated with formaldehyde for sale in which "a large network", including 11 slaughterhouses run by a criminal gang, were implicated.[113] In 2012, 1 billion rupiah (almost US$100,000) of fish imported fromPakistan toBatam, Indonesia, were found laced with formaldehyde.[114]
Formalin contamination of foods has been reported inBangladesh, with stores and supermarkets selling fruits, fishes, and vegetables that have been treated with formalin to keep them fresh.[115] However, in 2015, aFormalin Control Bill was passed in theParliament of Bangladesh with a provision of life-term imprisonment as the maximum punishment as well as a maximum fine of 2,000,000BDT but not less than 500,000BDT for importing, producing, or hoarding formalin without a license.[116]
In the early 1900s, formaldehyde was frequently added by US milk plants to milk bottles as a method of pasteurization due to the lack of knowledge and concern[117] regarding formaldehyde's toxicity.[118][119]
Formaldehyde is banned from use in certain applications (preservatives for liquid-cooling and processing systems,slimicides, metalworking-fluid preservatives, and antifouling products) under the Biocidal Products Directive.[121][122] In the EU, the maximum allowed concentration of formaldehyde in finished products is 0.2%, and any product that exceeds 0.05% has to include a warning that the product contains formaldehyde.[54]
In the United States,Congress passed a bill July 7, 2010, regarding the use of formaldehyde in hardwoodplywood,particle board, andmedium density fiberboard. The bill limited the allowable amount of formaldehyde emissions from these wood products to 0.09 ppm, and required companies to meet this standard by January 2013.[123] The final U.S. EPA rule specified maximum emissions of "0.05 ppm formaldehyde for hardwood plywood, 0.09 ppm formaldehyde for particleboard, 0.11 ppm formaldehyde for medium-density fiberboard, and 0.13 ppm formaldehyde for thin medium-density fiberboard."[124]
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