Volatile organic compound
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. Others believe the concept that a volatile organic compound is any organic that participates in a photoreaction, as found in the EPA's definition, is very broad and vague.  Organics that are not volatile, as described above, can fall into that definition. 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).

# Sources of VOCs
The most common VOC is methane, a greenhouse gas sometimes excluded from analysis of other VOCs using the term non-methane VOCs, or NMVOCs. Major worldwide sources of atmospheric methane include wetlands, ruminants such as cows, energy use, rice agriculture, landfills, and burning biomass such as wood.
Common artificial sources of VOCs include paint thinners, dry cleaning solvents, and some constituents of petroleum fuels (eg. gasoline and natural gas). Trees are also an important biological source of VOC; it is known that they emit large amounts of VOCs, especially isoprene and terpenes. Another significant source of VOC emission is crude oil tanking. Both during offloading and loading of crude oil tankers VOC are released to the atmosphere. Lately there has been an environmental focus on this issue resulting in both VOC handling on newer tankers and also crude oil loading terminals.
Considered a factor in indoor air quality issues such as sick building syndrome, VOCs "are generated by photocopiers, carpets, and furnishings as they are used or when components oxidize.... One irritant, formeldahyde, present in hundreds of office components, including wood and laminated furniture, shelving, and wall covers. It also evaporates from paints, varnishes, and chemicals used for sealing and finishing walls." Tobacco smoke can contribute high levels of VOCs.
VOCs including halogenide and sulfide are emitted through human respiration, and formeldahyde is emitted at a lower rate from the surface of the human body.
Also many VOCs are found in brownfield sites.

# Environmental effects
VOCs are sometimes accidentally released into the environment, where they can damage soil and groundwater. Vapours of VOCs escaping into the air contribute to air pollution.
VOCs are an important outdoor air pollutant. In this field they are often divided into the separate categories of methane (CH4) and non-methane (NMVOCs). Methane is an extremely efficient greenhouse gas which contributes to enhanced global warming.  Other hydrocarbon VOCs are also significant greenhouse gases via their role in creating ozone and in prolonging the life of methane in the atmosphere, although the effect varies depending on local air quality.  Within the NMVOCs, the aromatic compounds benzene, toluene and xylene are suspected carcinogens and may lead to leukemia through prolonged exposure. 1,3-butadiene is another dangerous compound which is often associated with industrial uses.
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. In addition high concentrations of low level ozone can damage crops and buildings.

# Contribution to indoor 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, carpet backing, 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 activities 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.
Air quality with reference to Volatile Organic Compund Emission.

# Terminology and legal definitions
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 Gases
- NMVOC  Non-Methane Volatile Organic Compounds
- ROG  Reactive Organic Gases
- SVOC  Semi-Volatile Organic Compounds
- TOG  Total Organic Gases
- TVOC  Total Volatile Organic Compounds
- 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 precise definitions codified by regulators such as the European Parliament and the US EPA.
Worldwide, legal definitions of the term "VOC" are in many respects, more a matter of policy than a matter of science.  For example, because the US EPA Code of Federal Regulations (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 US standard to determine if a compound is a non-VOC is to compare its reactivity to that of ethane, which was the least reactive compound on the original list.  Unfortunately,  this is a very difficult comparison to make as it is frequently impossible to duplicate the real-world conditions in a laboratory.  To complicate the issue, typical real-world conditions are different from day to day and from place to place.  
However, there is ongoing study on the use of a compound's reactivity as a better tool for pollution control regulation than the "is or isn't" approach currently in use.

## United States definition
The U.S. Environmental Protection Agency definition of VOCs is published in the Code of Federal Regulations. It defines VOCs as "any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions," but also includes a list of dozens of exceptions for compounds "determined to have negligible photochemical reactivity."

## European Union definition
Under European law, the definition of "VOC" is based on evaporation into the atmosphere, rather than reactivity.  For example European Union Directive 2004/42/CE which covers VOC emissions from paints and varnishes defines a VOC as any organic compound having an initial boiling point less than or equal to 250C measured at a standard atmospheric pressure of 101.3 kPa.  Directive 94/63/EC which regulates VOC emissions from storage and distribution of petrol simply defines vapours as any gaseous compound which evaporates from petrol.

## UK coatings classification
The British coatings industry has adopted a VOC labelling scheme for all decorative coatings to inform customers about the levels of organic solvents and other volatile materials present. Coatings manufacturers use standard terminology, text and categories for all products. Information is provided according to five bands, and manufacturers may label products with either a British Coatings Federation text box on the back panel, or a graphical globe symbol, the latter subject to licensing from with B&Q plc. Both styles of labels contain the same text, and warn that VOCs contribute to atmospheric pollution.
The five bands are:
Minimal - VOC content 0% to 0.29%
Low - VOC content 0.3% to 7.99%
Medium - VOC content 8% to 24.99%
High - VOC content 25% to 50%
Very High - VOC content more than 50%
An example of text box labelling for the Minimal band is shown below, while examples of the graphical globe symbols may be seen on websites of some British coatings companies.

# Notes
-  "ESPERE Climate Encyclopedia." (Website.) Atmospheric Chemistry Department, Max Planck Institute for Chemistry. "Industry" section. Retrieved on 2007-09-26.
-  "Climate Change 2001: The Scientific Basis"   Intergovernmental Panel on Climate Change (IPCC), published on United Nations Environmental Programme/GRID-Arendale website. Section 4.2.1.1: "Non-CO2 Kyoto Gases, Methane (CH4)." Retrieved on 2007-09-26.
-  2007-05-02. "Sick Building Syndrome - New Treatments, May 2, 2007" (Website). Library of the National Medical Society. Retrieved on 2007-09-03.
-  "Indoor Air Facts No. 4 (revised) Sick Building Syndrome." (Website). U.S. Environmental Protection Agency. Retrieved on 2007-09-03.
-  Hajima, Tamura, Yoneda Shuji, Fujii Shuji, Yuasa Kazuhiro, Suzuki Miyako. 1999. "Measurement of volatile organic compounds generated from human being." Kuki Seijo to Kontamineshyon Kontororu Kenkyu Taikai Yokoshu, vol. 17, pp. 131-132 (Japanese), via English-language abstract on sciencelinks.jp. Retrieved on 2007-09-03.
-  Air Resources Board: Reactivity Background (Website). Air Resources Board, California Environmental Protection Agency. Retrieved on 2007-09-03.
-  2007-08-30. "Title 40: Protection of Environment, Part 51-Requirements for Preparation, Adoption, and Submittal of Implementation Plans, Subpart FProcedural Requirements, 51.100: Definitions." (Website). GPO Access: Electronic Code of Federal Regulations (e-CFR), United States Government Printing Office. Retrieved on 2007-09-03.
-  "Directive 2004/42/CE of the European Parliament and of the Council" (Website.) EUR-Lex, European Union Publications Office. Retrieved on 2007-09-27.
-  "European Parliament and council directive 94/63/EC of 20 December 1994." (Website.) Estonian Legal Language Centre, Justiits Ministeerium (Justice Ministry of Estonia). Retrieved on 2007-09-27.
-  "What are VOCs." (Website). The British Coatings Federation. Retrieved on 2007-09-03.
-  "VOCs" (Commercial website). ICI Paints. Retrieved on 2007-09-03.