Volatile organic compound

Volatile organic compound (VOC) are organic chemicals that have a high vapour pressure at room temperature. High vapor pressure 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.[1]

VOC's are responsible for the odor, scents, and perfumes as well as pollutants. VOCs play an important role in communication between animals and plants, e.g. attractants for pollinators,[2] protection from predation,[3] and even inter-plant interactions.[4] Some VOCs are dangerous to human health or cause harm to the environment. 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.

Definitions

Diverse definitions of the term VOC are in use.

Canada

Health Canada classifies VOCs as organic compounds that have boiling points roughly in the range of 50 to 250 °C (122 to 482 °F). The emphasis is placed on commonly encountered VOCs that would have an effect on air quality.[5]

European Union

The European Union defines a VOC as "any organic compound having an initial boiling point less than or equal to 250 °C (482 °F) measured at a standard atmospheric pressure of 101.3 kPa." The VOC Solvents Emissions Directive is the main policy instrument for the reduction of industrial emissions of volatile organic compounds (VOCs) 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.[6][7]

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 (PM 2.5)," and similar sources.[8] 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.[9]

India

The Central Pollution Control Board of India released the Air (Prevention and Control of Pollution) Act in 1981, amended in 1987, to address concerns about air pollution in India.[10] While the document does not differentiate between VOCs and other air pollutants, the CPCB monitors "oxides of nitrogen (NOx), sulphur dioxide (SO2), fine particulate matter (PM10) and suspended particulate matter (SPM)."[11]

United States
Thermal oxidizers provide an air pollution abatement option for VOCs from industrial air flows.[12] A thermal oxidizer is an EPA-approved device to treat VOCs.

The definitions of VOCs used for control of precursors of photochemical smog used by the U.S. Environmental Protection Agency (EPA) 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 (ARB).[13] 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.

California's ARB uses the term "reactive organic gases" (ROG) to measure organic gases after public hearing in September 1995. The ARB revised the definition of "Volatile Organic Compounds" used in the consumer products regulations, based on their committee's findings.[14]

In addition to drinking water, VOCs are regulated in pollutant discharges to surface waters (both directly and via sewage treatment plants)[15] as hazardous waste,[16] but not in non-industrial indoor air.[17] The Occupational Safety and Health Administration (OSHA) 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
Limonene, a common biogenic VOC, is emitted especially by pine trees.

Most VOCs in earth's atmosphere are biogenic, largely emitted by plants.[1]

Major biogenic VOCs[18]
compound relative contribution amount emitted (Tg/y)
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 (BVOCs) encompass VOCs emitted by plants, animals, or microorganisms, and while extremely diverse, are most commonly terpenoids, alcohols, and carbonyls (methane and carbon monoxide are generally not considered).[19] Not counting methane, biological sources emit an estimated 760 teragrams of carbon per year in the form of VOCs.[18] The majority of VOCs are produced by plants, the main compound being isoprene. Small amounts of VOCs are produced by animals and microbes.[20] 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.

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.[21] Due to recent climate change developments, such as warming and greater UV radiation, BVOC emissions are generally predicted to increase, thus upsetting the biosphere-atmosphere interaction and damaging major ecosystems.[22] A major class of VOCs is terpenes, such as myrcene.[23] Providing a sense of scale, a forest 62,000 km2 in area (the US state of Pennsylvania) is estimated to emit 3,400,000 kilograms of terpenes on a typical August day during the growing season.[24] Induction of genes producing volatile organic compounds, and subsequent increase in volatile terpenes, has been achieved in maize using (Z)-3-hexen-1-ol and other plant hormones.[25]

Paints and coatings are a major anthropogenic sources of VOCs.

Anthropogenic sources
The handling of petroleum-based fuels is a major source of VOCs.

Anthropogenic sources emit about 142 teragrams (1.42 x1011 kg) of carbon per year in the form of VOCs.[26]

The major source of man-made VOCs are:[27]

  • 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 paints are produced annually. Typical solvents are 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.[28]
  • Biofuel use, e.g., cooking oils in Asia and bioethanol in Brazil.
  • Biomass combustion, especially from rain forests. Although in principle combustion gives carbon dioxide and water, incomplete combustion affords a variety of VOCs.

Indoor VOCs

EPA has found concentrations of VOCs in indoor air to be 2 to 5 times greater than in outdoor air and sometimes far greater.[17] During certain activities indoor levels of VOCs may reach 1,000 times that of the outside air. Studies have shown that individual VOC emissions by themselves are not that high in an indoor environment, but the indoor total VOC (TVOC) concentrations can be up to five times higher than the VOC outdoor levels.[29] New buildings especially, contribute to the highest level of VOC off-gassing in an indoor environment because of the abundant new materials generating VOC particles at the same time in such a short time period.[30] In addition to new buildings, many consumer products emit VOCs, therefore the total concentration of VOC levels is much greater within the indoor environment.[30]

VOC concentration in an indoor environment during winter is three to four times higher than the VOC concentrations during the summer.[31] High indoor VOC levels are attributed to the low rates of air exchange between the indoor and outdoor environment as a result of tight-shut windows and the increasing use of humidifiers.[32]

Indoor air quality measurements

Measurement of VOCs from the indoor air is done with sorption tubes e. g. Tenax (for VOCs and SVOCs) or DNPH-cartridges (for carbonyl-compounds) or air detector. The VOCs adsorb on these materials and are afterwards desorbed either thermally (Tenax) or by elution (DNPH) and then analyzed by GC-MS/FID or HPLC. Reference gas mixtures are required for quality control of these VOC-measurements.[33] Furthermore, VOC emitting products used indoors, e. g. building products and furniture, are investigated in emission test chambers under controlled climatic conditions.[34] For quality control of these measurements round robin tests are carried out, therefore reproducibly emitting reference materials are ideally required.[33]

Regulation of indoor VOC emissions

In most countries, a separate definition of VOCs is used with regard to indoor air quality that comprises each organic chemical compound that can be measured as follows: adsorption from air on Tenax TA, thermal desorption, gas chromatographic separation over a 100% nonpolar column (dimethylpolysiloxane). VOC (volatile organic compounds) are all compounds that appear in the gas chromatogram between and including n-hexane and n-hexadecane. Compounds appearing earlier are called VVOC (very volatile organic compounds); compounds appearing later are called SVOC (semi-volatile organic compounds).

France, Germany, and Belgium have enacted regulations to limit VOC emissions from commercial products, and industry has developed numerous voluntary ecolabels and rating systems, such as EMICODE,[35] M1,[36] Blue Angel[37] and Indoor Air Comfort[38] In the United States, several standards exist; California Standard CDPH Section 01350[39] is the most common one. These regulations and standards changed the marketplace, leading to an increasing number of low-emitting products.

Health risks

Respiratory, allergic, or immune effects in infants or children are associated with man-made VOCs and other indoor or outdoor air pollutants.[40]

Some VOCs, such as styrene and limonene, can react with nitrogen oxides or with ozone to produce new oxidation products and secondary aerosols, which can cause sensory irritation symptoms.[41] VOCs contribute to the formation of Tropospheric ozone and smog.[42][43]

Health effects include eye, nose, and throat irritation; headaches, loss of coordination, nausea; and damage to the liver, kidney, and central nervous system.[44] Some organics can cause cancer in animals; some are suspected or known to cause cancer in humans. Key signs or symptoms associated with exposure to VOCs include conjunctival irritation, nose and throat discomfort, headache, allergic skin reaction, dyspnea, declines in serum cholinesterase levels, nausea, vomiting, nose bleeding, fatigue, dizziness.[45]

The ability of organic chemicals to cause health effects varies greatly from those that are highly toxic, to those with no known health effects. As with other pollutants, the extent and nature of the health effect will depend on many factors including level of exposure and length of time exposed. Eye and respiratory tract irritation, headaches, dizziness, visual disorders, and memory impairment are among the immediate symptoms that some people have experienced soon after exposure to some organics. At present, not much is known about what health effects occur from the levels of organics usually found in homes.[46]

Limit values for VOC emissions

Limit values for VOC emissions into indoor air are published by AgBB,[47] AFSSET, California Department of Public Health, and others. These regulations have prompted several companies in the paint and adhesive industries to adapt with VOC level reductions their products. VOC labels and certification programs may not properly assess all of the VOCs emitted from the product, including some chemical compounds that may be relevant for indoor air quality.[48] Each ounce of colorant added to tint paint may contain between 5 and 20 grams of VOCs. A dark color, however, could require 5-15 ounces of colorant, adding up to 300 or more grams of VOCs per gallon of paint.[49]

Analytical methods

Sampling

Obtaining samples for analysis is challenging. VOC's, even when at dangerous levels, are dilute, so preconcentration is typically required. Many components of the atmosphere are mutually incompatible, e.g. ozone and organic compounds, peroxyacyl nitrates and many organic compounds. Furthermore, collection of VOCs by condensation in cold traps also accummulates a large amount of water, which generally must be removed selectively, depending on the analytical techniques to be employed.[27] Solid-phase microextraction (SPME) techniques are used to collect VOCs at low concentrations for analysis.[50] As applied to breath analysis, the following modalities are employed for sampling: gas sampling bags, syringes, evacuated steel and glass containers.[51]

Principle and measurement methods

In the U.S., standard methods have been established by the National Institute for Occupational Safety and Health (NIOSH) and another by U.S. OSHA. Each method uses a single component solvent; butanol and hexane cannot be sampled, however, on the same sample matrix using the NIOSH or OSHA method.[52]

VOCs are quantified and identified by two broad techniques. The major technique is gas chromatography (GC) . GC instruments allow the separation of gaseous components. When coupled to a flame ionization detector (FID) GCs can detect hydrocarbons at the parts per trillion levels. Using electron capture detectors, GCs are also effective for organohalide such as chlorocarbons.

The second major technique associated with VOC analysis is mass spectrometry, which is usually coupled with GC, giving the hyphenated technique of GC-MS.

Direct injection mass spectrometry techniques are frequently utilized for the rapid detection and accurate quantification of VOCs.[53] PTR-MS is among the methods that have been used most extensively for the on-line analysis of biogenic and antropogenic VOCs.[54] PTR-MS instruments based on time-of-flight mass spectrometry have been reported to reach detection limits of 20 pptv after 100 ms and 750 ppqv after 1 min. measurement (signal integration) time. The mass resolution of these devices is between 7000 and 10,500 m/Δm, thus it is possible to separate most common isobaric VOCs and quantify them independently.[55]

Chemical fingerprinting and breath analysis

The exhaled human breath contains a few thousand volatile organic compounds and is used in breath biopsy to serve as a VOC biomarker to test for diseases,[51] such as lung cancer.[56] One study has shown that "volatile organic compounds ... are mainly blood borne and therefore enable monitoring of different processes in the body."[57] And it appears that VOC compounds in the body "may be either produced by metabolic processes or inhaled/absorbed from exogenous sources" such as environmental tobacco smoke.[56][58]

Metrology for VOC measurements

To achieve comparability of VOC measurements, reference standards traceable to SI-units are required. For a number of VOCs gaseous reference standards are available from specialty gas suppliers or national metrology institutes, either in the form of cylinders or dynamic generation methods. However, for many VOCs, such as oxygenated VOCs, monoterpenes, or formaldehyde, no standards are available at the appropriate amount of fraction due to the chemical reactivity or adsorption of these molecules. Currently, several national metrology institutes are working on the lacking standard gas mixtures at trace level concentration, minimising adsorption processes, and improving the zero gas.[33] The final scopes are for the traceability and the long-term stability of the standard gases to be in accordance with the data quality objectives (DQO, maximum uncertainty of 20% in this case) required by the WMO/GAW program.[59]

See also

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