Volatile organic compound
Volatile organic compounds, or VOCs, are organic chemical compounds whose composition makes it possible for them to evaporate under normal indoor atmospheric conditions of temperature and pressure. These hydrocarbon gases are a major class of pollutant. They can be generated by combustion engines, paints and printing inks, gasoline pumps, cleaning solvents, chemical or metallurgical processes. In a major city, solvent fumes can average 500 tons per day.
VOCs are sometimes categorized by the ease they will be emitted. For example, the World Health Organization (WHO) categorizes indoor organic pollutants as:
- Very volatile organic compounds (VVOCs): Propane butane, methyl chloride, etc.
- Volatile organic compounds (VOCs): Formaldehyde, d-Limonene, toluene, acetone, ethanol (ethyl alcohol) 2-propanol (isopropyl alcohol), hexanal
- Semi-volatile organic compounds (SVOCs): Pesticides (DDT, chlordane, plasticizers (phthalates), fire retardants (PCBs, PBB))
The higher the volatility (lower the boiling point), the more likely the compound will be emitted from a product or surface into the air. Very volatile organic compounds are so volatile that they are difficult to measure and are found almost entirely as gases in the air rather than in materials or on surfaces. The least volatile compounds found in air constitute a far smaller fraction of the total present indoors while the majority will be in solids or liquids that contain them or on surfaces including dust, furnishings and building materials.
n the United States, emissions of VOCs to the outdoors are regulated by EPA mostly to prevent the formation of ozone, a constituent of photochemical smog. Many VOCs form ground-level ozone by “reacting” with sources of oxygen molecules such as nitrogen oxides (NOx), and carbon monoxide (CO) in the atmosphere in the presence of sunlight. However, only some VOCs are considered “reactive” enough to be of concern. VOCs that are non-reactive or of negligible reactivity to form ozone under these conditions are exempted from the definition of VOCs used by EPA in its regulation. Since first establishing the list of exempt compounds in 1977, the EPA has added several to the list, and frequently has several petitions for additional compounds undergoing review. In addition, some states have their own definitions and lists of exempted compounds.
These exemptions for regulated VOCs in the US have caused many misunderstandings. For example a paint that is listed as 'zero VOC' may still contain acetone. See below for a table of some regulated and exempted compounds.
|Some US regulated VOCS||Some US exempted VOCS|
|Halomethanes||methyl chloride||Aliphatic hydrocarbons||propane, butane, hexane|
|Chlorinated ethanes||1,1,1-trichloroethanol||Alcohols||methanol, ethanol|
|Aromatic solvents||benzene, toluene, ethylbenzene||Ketones||acetone|
|Polynuclear aromatics||naphthalene||Nitrogen containing||ammonia, nitrogen dioxide|
|Chlorobenzenes||1,2-dichlorobenzene||Sulfur containing||hydrogen sulfide, sulfur dioxide|
|Phthalate esters||plasticizers||Commercial usage||methylene chloride (paint stripper), perchloroethylene (dry cleaning fluid)|
|Others (semi-volatile organics)||pesticides (DDT, chlordane), fire retardants (PCBs, PBB)||Others||carbon monoxide, cabon dioxide, carbonic acid, carbonate|
Synonyms and Related Terms
Resources and Citations
- P.Hatchfield, Pollutants in the Museum Environment, Archetype Press, London, 2002.
- EPA: Volatile Organic Compounds
- World Health Organization, 1989. "Indoor air quality: organic pollutants." Report on a WHO Meeting, Berlin, 23-27 August 1987. EURO Reports and Studies 111. Copenhagen, World Health Organization Regional Office for Europe.