volatile organic compound volatile organic compound volatile organic compounds (vocs) are organic compounds that have a high vapor 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. vocs are responsible for the odor of scents and perfumes as well as pollutants. vocs play an important role in communication between animals and plants, e.g. attractants for pollinators, protection from predation, and even inter-plant interactions. 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. some vocs have been used in pharmacy, others are target of administrative controls because of their recreational use. 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. 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 (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. the solvents emissions directive was replaced by the industrial emissions directive from 2013. 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. 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 (prevention and control of pollution) 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 (nox), sulphur dioxide (so2), fine particulate matter (pm10) and suspended particulate matter (spm)". united states thermal oxidizers provide an air pollution abatement option for vocs from industrial air flows. 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 (carb). 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" (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. in addition to drinking water, vocs are regulated in pollutant discharges to surface waters (both directly and via sewage treatment plants) as hazardous waste, but not in non-industrial indoor air. 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 into the atmosphere primarily by trees which grow in coniferous forests. most vocs in earth's atmosphere are biogenic, largely emitted by plants. major biogenic vocs 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). 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 62,000 km2 in area, the size of the us state of pennsylvania, is estimated to emit 3,400,000 kilograms of terpenes on a typical august day during the growing season. 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. anthropogenic sources part of a series onpollutionpaints and coatings are major anthropogenic sources of vocs. air acid rain air quality index atmospheric dispersion modeling chlorofluorocarbon exhaust gas haze indoor air internal combustion engine global dimming global distillation ozone depletion particulates persistent organic pollutant smog aerosol soot volatile organic compound biological biological hazard genetic introduced species (invasive species) digital information electromagnetic light ecological overillumination radio spectrum natural ozone radium and radon in the environment volcanic ash wildfire noise transportation land water air rail sustainable transport urban sonar marine mammals and sonar industrial military abstract noise control radiation actinides bioremediation depleted uranium nuclear fission nuclear fallout plutonium poisoning radioactivity uranium electromagnetic radiation and health radioactive waste soil agricultural herbicides manure waste pesticides land degradation bioremediation defecation electrical resistance heating soil guideline values phytoremediation solid waste biodegradable waste brown waste electronic waste battery recycling food waste green waste hazardous waste biomedical waste chemical waste construction waste lead poisoning mercury poisoning toxic waste international waste industrial waste lead smelting litter mining coal mining gold mining surface mining deep sea mining mining waste uranium mining municipal solid waste garbage nanomaterials plastic microplastics packaging waste post-consumer waste waste management landfill thermal treatment space satellite thermal urban heat island visual air travel advertising clutter overhead power lines traffic signs vandalism war chemical warfare herbicidal warfare (agent orange) nuclear holocaust (nuclear fallout - nuclear famine - nuclear winter) scorched earth unexploded ordnance war and environmental law water agricultural wastewater diseases eutrophication firewater freshwater groundwater hypoxia industrial wastewater marine debris monitoring nonpoint source see misc section nutrient ocean acidification oil spill pharmaceuticals freshwater salinization septic tanks sewage septic tanks pit latrine shipping stagnation sulfur water surface runoff turbidity urban runoff water quality misc point source area source legacy pollution lists diseases law by country most polluted cities least polluted cities by pm2.5 treaties categories by country environment portal ecology portal the handling of petroleum-based fuels is a major source of vocs. anthropogenic sources emit about 142 teragrams (1.42 × 1011 kg) 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 billion 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 concentrations of vocs in indoor air may be 2 to 5 times greater than in outdoor air, sometimes far greater. during certain activities, indoor levels of vocs may reach 1,000 times that of the outside air. studies have shown that emissions of individual voc species are not that high in an indoor environment, but the total concentration of all vocs (tvoc) 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 (building materials, fittings, surface coverings and treatments such as glues, paints and sealants) 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 strategy. 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, requiring improved ventilation. numerous studies 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. 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. furthermore, voc emitting products used indoors, e.g. building products and furniture, are investigated in emission test chambers under controlled climatic conditions. for quality control of these measurements round robin tests are carried out, therefore reproducibly emitting reference materials are ideally required. other methods have used silcosteel canisters with constant flow inlets to collect samples over several days. these methods are not limited by the adsorbing properties of materials like tenax. 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