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," i.e., exceptions, exemptions, and exclusions. The United States Environmental Protection Agency (EPA) defines a VOC as any organic compound that participates in a photoreaction. Some find the EPA definition too broad and vague, since organics that are not volatile in the sense that they vaporize under normal conditions can be considered volatile by this definition. The term may refer both to well-characterized organic compounds and to mixtures of variable composition.
Sources of VOCs
The most common VOC is methane, a greenhouse gas sometimes distinguished from other VOCs by calling the others non-methane VOCs, or NMVOCs. Other terms used in pollution regulations include non-methane organic gas, or NMOG, and non-methane hydrocarbons, or NMHC.
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, and some constituents of petroleum fuels (e.g., gasoline and natural gas). Trees are also an important biological source of VOCs; it is known that they emit large amounts of VOCs, especially isoprene and terpenes. Another significant source of VOC emission is crude oil tanking. During offloading and loading of crude oil tankers, VOCs 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 can be emitted from photocopiers, carpets, and furnishings. Formaldehyde, found in wood and laminate furniture, shelving, paints, varnishes, and other chemicals, is a major irritant. Tobacco smoke can also contribute to 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.
Environmental Effects
VOCs are sometimes accidentally released into the environment, where they can damage soil and groundwater. Vapors 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 that contributes to 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.
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 radiation, 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 vapor pressure. VOCs are often used in paint, carpet backing, plastics, and cosmetics. The US EPA has found concentrations of VOCs in indoor air to be 2 to 5 times greater than in outdoor air. During certain activities, the indoor level 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.
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 that 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 whether 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 since 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 that points to the use of a compound’s reactivity as a better basis for determining pollution control regulation than the approach currently in use.
United States Definition
The US EPA definition of VOCs is published in the Code of Federal Regulations. It defines a VOC 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 250 °C 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 labeling 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 being 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
- 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%
External Links
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Volatile organic compound."
