Volatile organic compounds (VOCs) are organic chemical compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere. (The term VOC is also occasionally used as an abbreviation, especially in biological contexts, for "volatile organic carbon".) A wide range of carbon-based molecules, such as aldehydes, ketones, and hydrocarbons are VOC's. The term often is used in a legal or regulatory context and in such cases the precise definition is a matter of law. These definitions can be contradictory and may contain "loopholes"; e.g. exceptions, exemptions, and exclusions. The term may refer both to well characterized organic compounds and to mixtures of variable composition. Most often the definition used is one from the United States Environmental Protection Agency (see below).

Common examples of VOCs include paint thinners, dry cleaning solvents, and some constituents of petroleum fuels (eg. gasoline and natural gas). VOCs are sometimes accidentally released into the environment, where they can become soil and groundwater contaminants. Vapours of VOCs escaping into the air contribute to air pollution.

Many VOCs found around the house, such as paint strippers and wood preservatives, contribute to sick building syndrome because of their high vapour pressure. VOC's are often used in paint, plastics, and cosmetics. The United States Environmental Protection Agency (EPA) has found concentrations of VOCs in indoor air to be 2 to 5 times greater than in outdoor air. During certain activites indoor levels of VOCs may reach 1,000 times that of the outside air. Not all organic compounds are volatile; many plastics (polymers) and other large molecules may not have significant vapor pressure at normal temperatures.

VOCs are an important outdoor air pollutant. In this field they are often divided up into the separate catagories of methane (CH4) and non-methane (NMVOCs). Methane is an extremely efficient greenhouse gas which may contribute to enhanced global warming. Significant biological sources of methane are termites, cows (ruminants) and cultivation (estimated emissions 15, 75 and 100 million tons per year respectively). Within the NMVOCs, benzene is a suspected carcinogen and may lead to leukaemia through prolonged exposure. 1,3-butadiene is another dangerous compound which is often associated with industrial uses. It is also known that trees emit huge amounts of VOCs.

Some VOCs also react with nitrogen oxides in the air in the presence of sunlight to form ozone. Although ozone is beneficial in the upper atmosphere because it absorbs UV thus protecting humans, plants, and animals from exposure to dangerous solar radiation, it poses a health threat in the lower atmosphere by causing respiratory problems.

There are a number of different ways to collectively refer to those chemical compounds that participate in photochemical reactions. That is, those that react with other pollutants, in the presence of sunlight, to form tropospheric ozone.

Some of the more common terms are:

• NMHC - Non-Methane Hydrocarbons
• NMOG - Non-Methane Organic Gasses
• ROG - Reactive Organic Gasses
• TOG - Total Organic Gasses
• VOC - Volatile Organic Compounds

While all these terms are used, it is not always clear which pollutants are included in each term. The term "VOC" has the advantage of having a precise definition codified by the US EPA in the Code of Federal Regulations (CFR).

It is worth noting that the CFR definition is, in many respects, more a matter of policy than a matter of science. Because the CFR has characterized a compound as having "negligible photochemical reactivity" it does not necessarily imply that it is, at any particular time, less reactive than those compounds which are not on the list. Since first establishing the list of exempt compounds in 1977, the EPA has added several to the list, and frequently has several petitions undergoing review.

The traditional standard to determine if a compound is a non-VOC is to compare its reactivity to that of ethane, which was the most reactive compound on the original list. Unfortunately, this is a very difficult comparison do make as it is frequently impossible to duplicate the real-world conditions in a laboratory. To make matters more complex, typical real-world conditions are different from day to day and from place to place. However, there is ongoing activity studying the use of a compound's reactivity as a better tool for pollution control regulation than the "all or nothing", Is or Isn't, approach currently in use. (See Maximum Incremental Reactivity, MIR.)

40 CFR Part 51.100(s) gives this definition as follows:

(s) Volatile organic compounds (VOC) means any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions.

(1) This includes any such organic compound other than the following, which have been determined to have negligible photochemical reactivity: methane; ethane; methylene chloride (dichloromethane); 1,1,1-trichloroethane (methyl chloroform); 1,1,2-trichloro-1,2,2-trifluoroethane (CFC–113); trichlorofluoromethane (CFC–11); dichlorodifluoromethane (CFC–12); chlorodifluoromethane (HCFC–22); trifluoromethane (HFC–23); 1,2-dichloro 1,1,2,2-tetrafluoroethane (CFC–114); chloropentafluoroethane (CFC–115); 1,1,1-trifluoro 2,2-dichloroethane (HCFC–123); 1,1,1,2-tetrafluoroethane (HFC–134a); 1,1-dichloro 1-fluoroethane (HCFC–141b); 1-chloro 1,1-difluoroethane (HCFC–142b); 2-chloro-1,1,1,2-tetrafluoroethane (HCFC–124); pentafluoroethane (HFC–125); 1,1,2,2-tetrafluoroethane (HFC–134); 1,1,1-trifluoroethane (HFC–143a); 1,1-difluoroethane (HFC–152a); parachlorobenzotrifluoride (PCBTF); cyclic, branched, or linear completely methylated siloxanes; acetone; perchloroethylene (tetrachloroethylene); 3,3-dichloro-1,1,1,2,2-pentafluoropropane (HCFC–225ca); 1,3-dichloro-1,1,2,2,3-pentafluoropropane (HCFC–225cb); 1,1,1,2,3,4,4,5,5,5-decafluoropentane (HFC 43–10mee); difluoromethane (HFC–32); ethylfluoride (HFC–161); 1,1,1,3,3,3-hexafluoropropane (HFC–236fa); 1,1,2,2,3-pentafluoropropane (HFC–245ca); 1,1,2,3,3-pentafluoropropane (HFC–245ea); 1,1,1,2,3-pentafluoropropane (HFC–245eb); 1,1,1,3,3-pentafluoropropane (HFC–245fa); 1,1,1,2,3,3-hexafluoropropane (HFC–236ea); 1,1,1,3,3-pentafluorobutane (HFC–365mfc); chlorofluoromethane (HCFC–31); 1 chloro-1-fluoroethane (HCFC–151a); 1,2-dichloro-1,1,2-trifluoroethane (HCFC–123a); 1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxy-butane (C4F9OCH3); 2-(difluoromethoxymethyl)-1,1,1,2,3,3,3-heptafluoropropane ((CF3)2CFCF2OCH3); 1-ethoxy-1,1,2,2,3,3,4,4,4-nonafluorobutane (C4F9OC2H5); 2-(ethoxydifluoromethyl)-1,1,1,2,3,3,3-heptafluoropropane ((CF3)2CFCF2OC2H5); methyl acetate and perfluorocarbon compounds which fall into these classes:

(i) Cyclic, branched, or linear, completely fluorinated alkanes;

(ii) Cyclic, branched, or linear, completely fluorinated ethers with no unsaturations;

(iii) Cyclic, branched, or linear, completely fluorinated tertiary amines with no unsaturations; and

(iv) Sulfur containing perfluorocarbons with no unsaturations and with sulfur bonds only to carbon and fluorine.

note: refer to the current CFR for the up-to-date definition.

See also

• Solvent
• Dutch standards
• Organic compound
• Photochemical smog

External links

• Sources of Indoor Air Pollution (http://www.epa.gov/iaq/voc.html)
• EPA NE: Ground-level Ozone (Smog) Information (http://www.epa.gov/region01/airquality)