Formaldehyde

# Overview
Formaldehyde is a chemical compound with the formula H2CO. It is the simplest aldehydean organic compound containing a terminal carbonyl group: it consists
-f exactly one carbonyl. It was first synthesized by the Russian chemist Aleksandr Butlerov (1828-1886), but was conclusively identified by August Wilhelm von Hofmann. Formaldehyde exists in several forms aside from H2CO: the cyclic trimer trioxane and the polymer paraformaldehyde. It exists in water as the hydrate H2C(OH)2. Aqueous solutions of formaldehyde are referred to as formalin. "100%" formalin consists of a saturated solution of formaldehyde (roughly 40% by mass) in water, with a small amount of stabilizer, usually methanol to limit oxidation and polymerization.
Formaldehyde is an intermediate in the oxidation (or combustion) of methane as well as other carbon compounds, e.g. forest fires, in automobile exhaust, and in tobacco smoke. It is produced in the atmosphere by the action of sunlight and oxygen on atmospheric methane and other hydrocarbons; thus, it becomes part of smog. Small amounts of formaldehyde are produced as a metabolic byproduct in most organisms, including humans.

# Production
Formaldehyde is produced industrially by the catalytic oxidation of methanol. The most common catalysts are silver metal or a mixture of an iron oxide with molybdenum and vanadium. In the more commonly used FORMOX process methanol and oxygen react at ca 250-400 C in presence of iron oxide in combination with molybdenum and/or vanadium to produce formaldehyde according to the chemical equation
The silver-based catalyst is usually operated at a higher temperature, about 650 C. Two chemical reactions on it simultaneously produce formaldehyde: that shown above and the dehydrogenation reaction
Formaldehyde is readily oxidized by atmospheric oxygen to form formic acid. Formic acid is found in ppm levels in commercial formaldehyde.
Formalin can be produced on a smaller scale using a whole range of other methods including conversion from ethanol instead of the normally-fed methanol feedstock. Such methods are of less commercial importance.

# Organic chemistry
Formaldehyde is a central building block in the synthesis of many other compounds. It exhibits most of the chemical properties of other aldehydes but is more reactive. Formaldehyde is a good electrophile, participating in electrophilic aromatic substitution reactions with aromatic compounds, and can undergo electrophilic addition reactions with alkenes. Formaldehyde undergoes a Cannizzaro reaction in the presence of  basic catalysts to produce formic acid and methanol.
Condensation with acetaldehyde affords pentaerythritol. Condensation with phenols gives phenol-formaldehyde resins. With 4-substituted phenols one obtains calixarenes.
When combined with hydrogen sulfide it forms trithiane.

# Biology
Formaldehyde solutions are used as a fixative for microscopy and histology. Pure solutions that are free of the oxidation product formic acid and of the stabilizer methanol are often produced by depolymerization of paraformaldehyde in hot water.
Formaldehyde (and its oligomers and hydrates) are rarely encountered in living organisms. Methanogenesis, which can start from many C1 sources, proceeds via the equivalent of formaldehyde, but this one-carbon species is masked as a methylene group carried by methanopterin. The formaldehyde is the primary cause of the methanol's toxicity, since methanol is metabolised into toxic formaldehyde by alcohol dehydrogenase.

# Applications

## As a disinfectant and biocide
An aqueous solution of formaldehyde can be useful as a disinfectant as it kills most bacteria and fungi (including their spores). It is also used as a preservative in vaccines. Formaldehyde solutions are applied topically in medicine to dry the skin, such as in the treatment of warts. Many aquarists use formaldehyde as a treatment for the parasite ichthyophthirius.
Formaldehyde preserves or fixes tissue or cells by irreversibly cross-linking primary amino groups in proteins with other nearby nitrogen atoms in protein or DNA through a -CH2- linkage. This is exploited in ChIP-on-chip transcriptomics experiments.
Formaldehyde is also used as a detergent in RNA gel electrophoresis, preventing RNA from forming secondary structures.
Formaldehyde is converted to formic acid in the body, leading to a rise in blood acidity (acidosis).

## Industry
Formaldehyde is a common building block for the synthesis of more complex compounds and materials.
Most formaldehyde is used in the production of polymers and other chemicals. When reacted with phenol, urea, or melamine formaldehyde produces, respectively, hard thermoset phenol formaldehyde resin, urea formaldehyde resin, and melamine resin. These resins are commonly used in permanent adhesives such as those used in plywood or carpeting. It is used as the wet-strength resin added to sanitary paper products such as (listed in increasing concentrations injected into the paper machine headstock chest) facial tissue, table napkins, and roll towels. They are also foamed to make insulation, or cast into moulded products. Production of formaldehyde resins accounts for more than half of formaldehyde consumption.
Many of these are polyfunctional alcohols such as pentaerythritol, which is used to make paints and explosives. Other formaldehyde derivatives include methylene diphenyl diisocyanate, an important component in polyurethane paints and foams, and hexamine, which is used in phenol-formaldehyde resins as well as the explosive RDX.
Formaldehyde is still used in low concentrations for process C-41 (color negative film) stabilizer in the final wash step, as well as in the process E-6 pre-bleach step, to obviate the need for it in the final wash.
Formaldehyde is used to produce glues used in the manufacture of particleboard, plywood, veneers, and other wood products, as well as spray-on insulating foams.
Formaldehyde, along with 18 M (concentrated) sulfuric acid (the entire solution often called the Marquis reagent), is used as an MDMA "testing kit" by such groups as Dancesafe as well as MDMA consumers. The solution alone cannot verify the presence of MDMA but reacts with many other chemicals that the MDMA tablet itself may be adulterated with. The reaction itself produces colors that correlate with these components.
The textile industry uses formaldehyde-based resins as finishers to make fabrics crease-resistant.

## Embalming
Formaldehyde-based solutions are used in embalming to disinfect and temporarily preserve human remains. It is the ability of formaldehyde to fix the tissue that produces the tell-tale firmness of flesh in an embalmed body. Whereas other heavier aldehydes produce a similar firming action none approaches the completeness of formaldehyde. Several European countries restrict the use of formaldehyde, including the import of formaldehyde-treated products and embalming, and the European Union is considering a complete ban on formaldehyde usage (including embalming), subject to a review of List 4B of the Technical Annex to the Report from the Commission to the European Parliament and the Council on the Evaluation of the Active Substances of Plant Protection Products by the European Commission Services. Countries with a strong tradition of embalming corpses, such as Ireland and other colder-weather countries, have raised concerns. The European Union decided on September 22, 2007 to ban Formaldehyde use throughout Europe due to its carcinogenic properties.

# Safety
Occupational exposure to formaldehyde by inhalation is mainly from three types of sources: thermal or chemical decomposition of formaldehyde-based resins, formaldehyde emission from aqueous solutions (for example, embalming fluids), and the production of formaldehyde resulting from the combustion of a variety of organic compounds (for example, exhaust gases). Formaldehyde can be toxic, allergenic, and carcinogenic. Because formaldehyde resins are used in many construction materials it is one of the more common indoor air pollutants. At concentrations above 0.1 ppm in air formaldehyde can irritate the eyes and mucous membranes, resulting in watery eyes. Formaldehyde inhaled at this concentration may cause headaches, a burning sensation in the throat, and difficulty breathing, as well as triggering or aggravating asthma symptoms.
Formaldehyde is classified as a probable human carcinogen by the U.S. Environmental Protection Agency. The International Agency for Research on Cancer (IARC) has determined that there is "sufficient evidence" that occupational exposure to formaldehyde causes nasopharyngeal cancer in humans. 
The United States Environmental Protection Agency USEPA allows no more than 0.016 ppm formaldehyde in the air in new buildings constructed for that agency.
Formaldehyde can cause allergies and is part of the standard patch test series. People with formaldehyde allergy are advised to avoid formaldehyde releasers as well (e.g., Quaternium-15, imidazolidinyl urea, and diazolidinyl urea). Formaldehyde has been banned in cosmetics in both Sweden and Japan.

## FEMA Trailer Formaldehyde Exposures
The Federal Emergency Management Agency (FEMA) provided travel trailers and mobile homes starting in 2006 for habitation by Gulf Coast residents displaced by Hurricane Katrina and Hurricane Rita. Some of the people who moved into the trailers complained of breathing difficulties, nosebleeds, and persistent headaches. Formaldehyde exposure can cause burning eyes and nose, coughing, difficulty breathing, headaches, and has been shown to be carcinogenic, causing nasal and nasopharyngeal cancer and possibly leukemia. Formaldehyde-catalyzed resins are used in the manufacture of engineered wood products such as particle board, medium-density fibreboard (MDF), plywood, and oriented strand board (OSB), all of which have applications in site-built homes, mobile homes, and travel trailers.
The United States Centers For Disease Control and Prevention (CDC) performed indoor air quality testing for formaldehyde  in some of the units. On Thursday, February 14, 2008 the CDC announced that potentially hazardous levels of formaldehyde were found in many of the travel trailers and mobile homes provided by the agency. The CDC's preliminary evaluation of a scientifically established random sample of 519 travel trailers and mobile homes tested between Dec. 21, 2007 and Jan. 23, 2008 (2+ years after manufacture) showed average levels of formaldehyde in all units of about 77 parts per billion (ppb). Long-term exposure to levels in this range can be linked to an increased risk of cancer and, at levels above this range, there can also be a risk of respiratory illness. These levels are higher than expected in indoor air, where levels are commonly in the range of 10-20 ppb, and are higher than the Agency for Toxic Substance Disease Registry (ATSDR, division of the CDC) Minimal Risk Level (MRL) of 8 ppb . Levels measured ranged from 3 ppb to 590 ppb.
The Federal Emergency Management Agency, which requested the testing by the CDC, said it would work aggressively to relocate all residents of the temporary housing as soon as possible. Lawsuits are being filed against FEMA as a result of the exposures.