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Volatile organic compound
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Volatile organic compounds (VOCs) are organic chemical compounds that have high enough vapor pressures under normal conditions to significantly vaporize and enter the atmosphere. A wide range of carbon-based molecules, such as aldehydes, ketones, and other light hydrocarbons are VOCs. 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.
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Encyclopedia
Volatile organic compounds (VOCs) are organic chemical compounds that have high enough vapor pressures under normal conditions to significantly vaporize and enter the atmosphere. A wide range of carbon-based molecules, such as aldehydes, ketones, and other light hydrocarbons are VOCs. 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 United States Environmental Protection Agency(EPA) defines a VOC as any organic compound that participates in a photoreaction; others believe this definition is very broad and vague as organics that are not volatile in the sense that they vaporize under normal conditions can be considered volatile by this EPA definition. The term may refer both to well characterized organic compounds and to mixtures of variable composition.
Definitions
There is no clear and widely supported definition of a VOC. VOC is a term used more in relation to air quality and environmental studies. From a chemistry viewpoint “Volatile Organic Compound” can mean any organic compound (all chemical compounds containing carbon with exceptions) that is volatile (evaporating or vaporizing readily under normal conditions). This is a very broad set of chemicals. Definitions vary depending on the particular context. There are many other widely used terms that are a subclass of VOCs (see below).
Canada
Health Canada classes VOCs as organic compounds that have boiling points roughly in the range of 50 to 250 °C (120 to 480 °F). The emphasis is placed on commonly encountered VOCs which would have an effect on air quality.
European Union
A VOC is any organic compound having an initial boiling point less than or equal to 250 °C measured at a standard atmospheric pressure of 101.3 kPa.
United States
A VOC has high vapor pressure and low water solubility. The US EPA’s Terms of Environment defines a VOC as any organic compound that participates in atmospheric photochemical reactions except those designated by the EPA as having negligible photochemical reactivity. Under the Code of Federal Regulations it is similarily defined 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.
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 B&Q plc. Both styles of labels contain the same text, and warn that VOCs contribute to atmospheric pollution.
The five bands are:
| Name | Range |
|---|
| Minimal | 0% = VOC content = 0.29% | | Low | 0.3% = VOC content = 7.99% | | Medium | 8% = VOC content = 24.99% | | High | 25% = VOC content = 50% | | Very High | 50% < VOC content |
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.
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:
| Abbreviation | Term |
|---|
| 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.
Chemical properties
Applications
Sources of VOCs
VOCs can be found indoors and outdoors.
The most common VOC is methane, a greenhouse gas sometimes excluded from analysis of other VOCs using the term non-methane VOCs, or NMVOCs. Other terms used in pollution regulations include NMOG - "non-methane organic gas", and NMHC - "non-methane hydrocarbons".
Major worldwide sources of atmospheric methane include wetlands, ruminants such as cows, energy use, rice agriculture, landfills, and burning biomass such as wood. Methane is the primary component of natural gas.
Common artificial VOCs include paint thinners, dry cleaning solvents, semiconductor cleaner, 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 improved VOC handling on newer tankers, and 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, formaldehyde, 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 formaldehyde is emitted at a lower rate from the surface of the human body.
Also many VOCs are found in brownfield sites.
Environment
VOCs are sometimes accidentally released into the environment, where they can damage soil and groundwater. Vapors of VOCs escaping into the ambient air contribute to indoor air pollution and outdoor 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, a carcinogen , and suspected carcinogens toluene and xylene, may lead to leukemia through prolonged exposure.
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 cleaning and personal hygiene products, paint strippers and wood preservatives, may contribute to sick building syndrome and other effects of exposure to indoor air pollution because of their abundance indoors and their high vapor pressure. VOC's are often used in paint, carpet backing, plastics, and cosmetics. They also occur in and are released from most the common indoor materials from natural sources such as trees, animals, and plants as well as from synthetic sources such as petroleum derivatives. The United States Environmental Protection Agency (EPA) has found concentrations of VOCs in indoor air commonly to be 2 to 5 times greater than in outdoor air and sometimes far greater. During certain activities indoor levels of VOCs may reach 1,000 times that of the outside air. In fact, VOCs with only indoor sources may not be detectable in outdoor air except very close to the indoor environment from which they originate. Not all organic compounds are volatile; many plastics (polymers) and other large molecules may not have significant vapor pressure at normal temperatures.
The definitions of VOCs used for control of precursors of photochemical smog used by EPA and states with their own outdoor air pollution regulations includes exemptions for compounds that are technically only those volatile organic compounds but that are determined to be non-reactive or of low-reactivity in the smog formation process. EPA formerly defined these compounds as Reactive Organic Gases (ROG) but changed the terminology to VOC for simplicity's sake. However, this specific use of the term VOCs can be misleading, specifically when applied to indoor air quality because many chemicals that are not regulated for purposes of controlling outdoor air pollution but that are important from an indoor air quality perspective are still found in products that are labeled as to VOC content according to the requirements of ambient air pollution regulation.
In recent years many common materials and products used indoors have been developed and are labeled by their manufacturers as "low VOC" or "zero VOC content" and other similar terms. While some of these products may actually have low VOC content in the broader definition of VOC relevant to indoor air, some products so labeled may actually have larger VOC content but the VOCs contained in them may be exempt from the EPA's definition. For more information on VOCs, visit EPA's indoor air quality web pages at http://www.epa.gov/iaq/voc.html.
See also
External links
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