Volatile organic compound
Volatile organic compounds (VOCs) are organic compounds that have a high vapor pressure at room temperature.[1] 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.[2]
VOCs are responsible for the odor of scents and
Definitions
Diverse definitions of the term VOC are in use. Some examples are presented below.
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.[6]
European Union
The
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.[10] 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.[11]
India
The
United States
The definitions of VOCs used for control of precursors of
Following a public hearing in September 1995, California's ARB uses the term "reactive organic gases" (ROG) 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.[16]
In addition to
Biologically generated VOCs
-
coniferous forests.
Most VOCs in Earth's atmosphere are biogenic, largely emitted by plants.[2]
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 |
Emissions are affected by a variety of factors, such as temperature, which determines rates of volatilization and growth, and sunlight, which determines rates of
Providing a sense of scale, a forest 62,000 square kilometres (24,000 sq mi) in area, the size of the US state of Pennsylvania, is estimated to emit 3,400,000 kilograms (7,500,000 lb) of terpenes on a typical August day during the growing season.[27] Researchers investigating mechanisms of induction of genes producing volatile organic compounds, and the subsequent increase in volatile terpenes, has been achieved in maize using (Z)-3-hexen-1-ol and other plant hormones.[28]
Anthropogenic sources
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Pollution |
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Anthropogenic sources emit about 142 teragrams (1.42 × 1011 kg) of carbon per year in the form of VOCs.[29]
The major source of man-made VOCs are:[30]
- 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.
- glycol ethers, and acetone. Motivated by cost, environmental concerns, and regulation, the paint and coating industries are increasingly shifting toward aqueous solvents.[31]
- Compressed aerosol products, mainly butane and propane, estimated to contribute 1.3 billion tonnes of VOC emissions per year globally.[32]
- bioethanolin Brazil.
- Biomass combustion, especially from rain forests. Although combustion principally releases carbon dioxide and water, incomplete combustion affords a variety of VOCs.
Indoor VOCs
Concentrations of VOCs in
New buildings experience particularly high levels of VOC off-gassing indoors because of the abundant new materials (building materials, fittings, surface coverings and treatments such as glues, paints and sealants) exposed to the indoor air, emitting multiple VOC gases.[34] 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.[35]
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.[34]
Numerous studies[35] show 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. Most studies have shown that this leads to generally higher concentrations of TVOCs indoors in summer.[35]
Indoor air quality measurements
Measurement of VOCs from the indoor air is done with sorption tubes e. g.
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).
Health risks
Some VOCs, such as
Health effects include eye, nose, and
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.[53]
Ingestion
While null in comparison to the concentrations found in indoor air, benzene, toluene, and methyl tert-butyl ether (MTBE) were found in samples of human milk and increase the concentrations of VOCs that we are exposed to throughout the day.[54] A study notes the difference between VOCs in alveolar breath and inspired air suggesting that VOCs are ingested, metabolized, and excreted via the extra-pulmonary pathway.[55] VOCs are also ingested by drinking water in varying concentrations. Some VOC concentrations were over the EPA's National Primary Drinking Water Regulations and China's National Drinking Water Standards set by the Ministry of Ecology and Environment.[56]
Dermal absorption
The presence of VOCs in the air and in groundwater has prompted more studies. Several studies have been performed to measure the effects of dermal absorption of specific VOCs. Dermal exposure to VOCs like formaldehyde and toluene downregulate antimicrobial peptides on the skin like cathelicidin LL-37, human β-defensin 2 and 3.[57] Xylene and formaldehyde worsen allergic inflammation in animal models.[58] Toluene also increases the dysregulation of filaggrin: a key protein in dermal regulation.[59] this was confirmed by immunofluorescence to confirm protein loss and western blotting to confirm mRNA loss. These experiments were done on human skin samples. Toluene exposure also decreased the water in the trans-epidermal layer allowing for vulnerability in the skin's layers.[57][60]
Limit values for VOC emissions
Limit values for VOC emissions into indoor air are published by
VOCs in healthcare settings
VOCs are also found in hospital and health care environments. In these settings, these chemicals are widely used for cleaning, disinfection, and hygiene of the different areas.[64] Thus, health professionals such as nurses, doctors, sanitation staff, etc., may present with adverse health effects such as asthma; however, further evaluation is required to determine the exact levels and determinants that influence the exposure to these compounds.[64][65][66]
Studies have shown that the concentration levels of different VOCs such as halogenated and aromatic hydrocarbons differ substantially between areas of the same hospital. However, one of these studies reported that
In relation to exposure to VOCs by cleaning and hygiene personnel, a study conducted in 4 hospitals in the United States established that sterilization and disinfection workers are linked to exposures to
Other healthcare environments such as
Analytical methods
Sampling
Obtaining samples for analysis is challenging. VOCs, 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 accumulates a large amount of water, which generally must be removed selectively, depending on the analytical techniques to be employed.[30] Solid-phase microextraction (SPME) techniques are used to collect VOCs at low concentrations for analysis.[72] As applied to breath analysis, the following modalities are employed for sampling: gas sampling bags, syringes, evacuated steel and glass containers.[73]
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.[74]
VOCs are quantified and identified by two broad techniques. The major technique is
The second major technique associated with VOC analysis is mass spectrometry, which is usually coupled with GC, giving the hyphenated technique of GC-MS.[75]
Direct injection mass spectrometry techniques are frequently utilized for the rapid detection and accurate quantification of VOCs.[76] PTR-MS is among the methods that have been used most extensively for the on-line analysis of biogenic and anthropogenic VOCs.[77] 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.[78]
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
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.[36] 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.[82]
See also
- Aroma compound
- Criteria air contaminants
- Fugitive emissions
- Non-methane volatile organic compound
- Organic compound
- Trichloroethene
- Vapor intrusion
- VOC contamination of groundwater
- Volatile Organic Compounds Protocol
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External links
- Volatile Organic Compounds (VOCs) web site of the Chemicals Control Branch of Environment Canada
- EPA New England: Ground-level Ozone (Smog) Information
- VOC emissions and calculations
- Examples of product labels with low VOC emission criteria
- KEY-VOCS: Metrology for VOC indicators in air pollution and climate change, a European Metrology Research Project.
- VOCs in Paints