Volatile organic compounds (VOCs) refers to organic chemical compounds which have significant vapor pressures and which can affect the environment and human health. VOCs are numerous, varied, and ubiquitous. Although VOCs include both man-made and naturally occurring chemical compounds, it is the anthropogenic VOCs that are regulated, especially for indoors where concentrations can be highest. VOCs are typically not acutely toxic but have chronic effects. Because the concentrations are usually low and the symptoms slow to develop, analysis of VOCs and their effects is a demanding area.
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No widely supported definition of a VOC exists.[1]
The definitions of VOCs used for control of precursors of photochemical smog used by EPA and states with their own outdoor air pollution regulations includes exemptions for compounds that are technically only those volatile organic compounds but that are determined to be non-reactive or of low-reactivity in the smog formation process. EPA formerly defined these compounds as Reactive Organic Gases (ROG) but changed the terminology to VOC for simplicity's sake. However, this specific use of the term VOCs can be misleading, specifically when applied to indoor air quality because many chemicals that are not regulated for purposes of controlling outdoor air pollution but that are important from an indoor air quality perspective are still found in products that are labeled as to VOC content according to the requirements of ambient air pollution regulation.
Health Canada classes 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 which would have an effect on air quality.[2]
A VOC is 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 and can do damage to visual or audible senses.[3]
VOCs (or specific subsets of the VOCs) are legally defined in the various laws and codes under which they are regulated. Other definitions may be found from government agencies investigating or advising about VOCs.[4] The United States Environmental Protection Agency regulates VOCs in the air, water, and land. The Safe Drinking Water Act implementation even includes a short list labeled VOCs in connection with contaminants which are organic and volatile.[5] The EPA also publishes testing methods for chemical compounds, some of which refer to VOCs.[6] In addition to drinking water, VOCs are regulated in discharges to waters (sewage treatment and stormwater disposal), as hazardous waste,[7] but not in non industrial indoor air.[8] The United States Department of Labor and its Occupational Safety and Health Administration (OSHA) regulate VOC exposure in the workplace. Volatile organic compounds which are hazardous material would be regulated by the Pipeline and Hazardous Materials Safety Administration while being transported.
The majority of VOCs arise from plants.[9] One indication of this flux is the strong odor emitted by many plants. The 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. A major class of VOCs are terpenes, such as myrcene.[10] Providing a sense of scale, a forest 62,000 km2 in area (the U.S. state of Pennsylvania) is estimated to emit 3,400,000 kilograms of terpenes on a typical August day during the growing season.[11] 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.[12]
A major source of man-made VOCs are solvents, especially paints and protective coatings. Solvents are required to spread a protective or decorative film. Approximately 12 billion liters 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.[13]
Chlorofluorocarbons, which are banned or highly regulated, were widely used cleaning products and refrigerants. Tetrachloroethene is used widely in dry cleaning and by industry. Industrial use of fossil fuels produces VOCs either directly as products (e.g. gasoline) or indirectly as byproducts (e.g. automobile exhaust).
Many building materials such as paints, adhesives, wall boards, and ceiling tiles slowly emit formaldehyde, which irritates the mucous membranes and can make a person irritated and uncomfortable.[14] Formaldehyde emissions from wood are in the range of 0.02 – 0.04 ppm. Relative humidity within an indoor environment can also affect the emissions of formaldehyde. High relative humidity and high temperatures allow more vaporization of formaldehyde from wood-materials.[15] There are also many sources of VOCs in office buildings, which include new furnishings, wall coverings, and office equipment such as photocopy machines which can off-gas VOCs into the air.[14]
MTBE was banned in the US around 2004 in order to limit further contamination of drinking water aquifers.
Since people today spend most of their time at home or in an office, long-term exposure to VOCs in the indoor environment can contribute to sick building syndrome.[16] In offices, VOC results from new furnishings, wall coverings, and office equipment such as photocopy machines, which can off-gas VOCs into the air.[14][17] Good ventilation and air conditioning systems are helpful at reducing VOC emissions in the indoor environment.[14] Studies also show that relative leukemia and lymphoma can increase through prolonged exposure of VOCs in the indoor environment.[18]
The United States Environmental Protection Agency (EPA) has found concentrations of VOCs in indoor air commonly to be 2 to 5 times greater than in outdoor air and sometimes far greater. During certain activities indoor levels of VOCs may reach 1,000 times that of the outside air.[19] 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.[20] 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.[21] In addition to new buildings, we also use many consumer products that emit VOC compounds, therefore the total concentration of VOC levels is much greater within the indoor environment.[21]
VOC concentration in an indoor environment is three to four times higher than the VOC concentrations during the summer.[22] 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[23]
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% non polar 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). See also these standards: ISO 16000-6, ISO 13999-2, VDI 4300-6, German AgBB evaluating scheme, German DIBt approval scheme, GEV testing method for the EMICODE. Overview over VOC emissions rating schemes, Healthy Buildings Conference 2009 [1].
Respiratory, allergic, or immune effects in infants or children are associated with man-made VOCs and other indoor or outdoor air pollutants.[24]
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.[25][26] Unspecified VOCs are important in the creation of smog.[27]
The exhaled human breath contains a few hundred volatile organic compounds and is used in breath analysis to serve as a VOC biomarker to test for diseases such as lung cancer.[28] One study has shown that "volatile organic compounds ... are mainly blood borne and therefore enable monitoring of different processes in the body."[29] 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.[30][31] Research is still in the process to determine whether VOCs in the body are contributed by cellular processes or by the cancerous tumors in the lung or other organs.