Volatile organic compound
Volatile organic compounds are organic compounds that have a high vapor pressure at room temperature. They are common and exist in a variety of settings and products, not limited to upholstered furniture, arts and crafts supplies, dry cleaned clothing, and cleaning supplies. VOCs are responsible for the odor of scents and perfumes as well as pollutants. They play an important role in communication between animals and plants, such as attractants for pollinators, protection from predation, and even inter-plant interactions. Some VOCs are dangerous to human health or cause harm to the environment, often despite the odor being perceived as pleasant, such as "new car smell".
Anthropogenic VOCs are regulated by law, especially indoors, where concentrations are the highest. Most VOCs are not acutely toxic, but may have long-term chronic health effects. Some VOCs have been used in pharmaceutical settings, while others are the target of administrative controls because of their recreational use. The high vapor pressure of VOCs correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a trait known as volatility.
Definitions
Diverse definitions of the term VOC are in use. Some examples are presented below.Canada
classifies VOCs as organic compounds that have boiling points roughly in the range of. The emphasis is placed on commonly encountered VOCs that would have an effect on air quality.European Union
The European Union defines a VOC as "any organic compound as well as the fraction of creosote, having at 293.15 K a vapour pressure of 0.01 kPa or more, or having a corresponding volatility under the particular conditions of use;". The VOC Solvents Emissions Directive was the main policy instrument for the reduction of industrial emissions of volatile organic compounds in the European Union. It covers a wide range of solvent-using activities, e.g. printing, surface cleaning, vehicle coating, dry cleaning and manufacture of footwear and pharmaceutical products. The VOC Solvents Emissions Directive requires installations in which such activities are applied to comply either with the emission limit values set out in the Directive or with the requirements of the so-called reduction scheme. Article 13 of The Paints Directive, approved in 2004, amended the original VOC Solvents Emissions Directive and limits the use of organic solvents in decorative paints and varnishes and in vehicle finishing products. The Paints Directive sets out maximum VOC content limit values for paints and varnishes in certain applications. The Solvents Emissions Directive was replaced by the Industrial Emissions Directive from 2013.Switzerland
Organic compounds with a vapor pressure of at least 0.1 mbar at 20 °C or with a boiling point of no more than 240 °C at 1013.25 mbar.China
The People's Republic of China defines a VOC as those compounds that have "originated from automobiles, industrial production and civilian use, burning of all types of fuels, storage and transportation of oils, fitment finish, coating for furniture and machines, cooking oil fume and fine particles ", and similar sources. The Three-Year Action Plan for Winning the Blue Sky Defence War released by the State Council in July 2018 creates an action plan to reduce 2015 VOC emissions 10% by 2020.India
The Central Pollution Control Board of India released the Air Act in 1981, amended in 1987, to address concerns about air pollution in India. While the document does not differentiate between VOCs and other air pollutants, the CPCB monitors "oxides of nitrogen, sulphur dioxide, fine particulate matter and suspended particulate matter ".United States
The definitions of VOCs used for control of precursors of photochemical smog used by the U.S. Environmental Protection Agency and state agencies in the US with independent outdoor air pollution regulations include exemptions for VOCs that are determined to be non-reactive, or of low-reactivity in the smog formation process. Prominent is the VOC regulation issued by the South Coast Air Quality Management District in California and by the California Air Resources Board. However, this specific use of the term VOCs can be misleading, especially when applied to indoor air quality because many chemicals that are not regulated as outdoor air pollution can still be important for indoor air pollution.Following a public hearing in September 1995, California's ARB uses the term "reactive organic gases" to measure organic gases. The CARB revised the definition of "Volatile Organic Compounds" used in their consumer products regulations, based on the committee's findings.
In addition to drinking water, VOCs are regulated in pollutant discharges to surface waters as hazardous waste, but not in non-industrial indoor air. The Occupational Safety and Health Administration regulates VOC exposure in the workplace. Volatile organic compounds that are classified as hazardous materials are regulated by the Pipeline and Hazardous Materials Safety Administration while being transported.
Biologically generated VOCs
Most VOCs in Earth's atmosphere are biogenic, largely emitted by plants.| compound | relative contribution | amount emitted |
| isoprene | 62.2% | 594±34 |
| terpenes | 10.9% | 95±3 |
| pinene isomers | 5.6% | 48.7±0.8 |
| sesquiterpenes | 2.4% | 20±1 |
| methanol | 6.4% | 130±4 |
Biogenic volatile organic compounds encompass VOCs emitted by plants, animals, or microorganisms, and while extremely diverse, are most commonly terpenoids, alcohols, and carbonyls. Not counting methane, biological sources emit an estimated 760 teragrams of carbon per year in the form of VOCs. The majority of VOCs are produced by plants, the main compound being isoprene. Small amounts of VOCs are produced by animals and microbes. Many VOCs are considered secondary metabolites, which often help organisms in defense, such as plant defense against herbivory. The strong odor emitted by many plants consists of green leaf volatiles, a subset of VOCs. Although intended for nearby organisms to detect and respond to, these volatiles can be detected and communicated through wireless electronic transmission, by embedding nanosensors and infrared transmitters into the plant materials themselves.
Emissions are affected by a variety of factors, such as temperature, which determines rates of volatilization and growth, and sunlight, which determines rates of biosynthesis. Emission occurs almost exclusively from the leaves, the stomata in particular. VOCs emitted by terrestrial forests are often oxidized by hydroxyl radicals in the atmosphere; in the absence of NOx pollutants, VOC photochemistry recycles hydroxyl radicals to create a sustainable biosphere–atmosphere balance. Due to recent climate change developments, such as warming and greater UV radiation, BVOC emissions from plants are generally predicted to increase, thus upsetting the biosphere–atmosphere interaction and damaging major ecosystems. A major class of VOCs is the terpene class of compounds, such as myrcene.
Providing a sense of scale, a forest in area, the size of the U.S. state of Pennsylvania, is estimated to emit of terpenes on a typical August day during the growing season. Maize produces the VOC -3-hexen-1-ol and other plant hormones. The taste of bitterness, found in foods such as olives, coffee and dark chocolate is caused by detection of VOCs by taste receptors.
Anthropogenic sources
sources emit about 142 teragrams of carbon per year in the form of VOCs.The major source of man-made VOCs are:
- Fossil fuel use and production, e.g. incompletely combusted fossil fuels or unintended evaporation of fuels. The most prevalent VOC is ethane, a relatively inert compound.
- Solvents used in coatings, paints, and inks. Approximately 12 billion litres of paint are produced annually. Typical solvents include aliphatic hydrocarbons, ethyl acetate, glycol ethers and acetone. Motivated by cost, environmental concerns, and regulation, the paint and coating industries are increasingly shifting toward aqueous solvents.
- Compressed aerosol products, mainly butane and propane, estimated to contribute 1.3 million tonnes of VOC emissions per year globally.
- Biofuel use, e.g., cooking oils in Asia and bioethanol in Brazil.
- Biomass combustion, especially from rain forests. Although combustion principally releases carbon dioxide and water, incomplete combustion affords a variety of VOCs.
Indoor VOCs
Some VOCs are emitted directly indoors, and some are formed through the subsequent chemical reactions. The total concentration of all VOCs indoors can be up to five times higher than that of outdoor levels.
New buildings experience particularly high levels of VOC off-gassing indoors because of the abundant new materials exposed to the indoor air, emitting multiple VOC gases. This off-gassing has a multi-exponential decay trend that is discernible over at least two years, with the most volatile compounds decaying with a time-constant of a few days, and the least volatile compounds decaying with a time-constant of a few years.
New buildings may require intensive ventilation for the first few months, or a bake-out treatment. Existing buildings may be replenished with new VOC sources, such as new furniture, consumer products, and redecoration of indoor surfaces, all of which lead to a continuous background emission of TVOCs, and requiring improved ventilation.
There are strong seasonal variations in indoors VOC emissions, with emission rates increasing in summer. This is largely due to the rate of diffusion of VOC species through materials to the surface, increasing with temperature. This leads to generally higher concentrations of TVOCs indoors in summer.