Volatile organic compounds are organic chemical compounds that have high enough vapor pressures under normal conditions to significantly vaporize and enter the atmosphere. A wide range of carbon-based molecules, such as aldehydes, ketones, and other light hydrocarbons are VOCs. The term often is used in a legal or regulatory context and in such cases the precise definition is a matter of law. These definitions can be contradictory and may contain "loopholes"; e.g. exceptions, exemptions, and exclusions. The United States Environmental Protection Agency defines a VOC as any organic compound that participates in a photoreaction; others believe this definition is very broad and vague as organics that are not volatile in the sense that they vaporize under normal conditions can be considered volatile by this EPA definition. The term may refer both to well characterized organic compounds and to mixtures of variable composition.
The most common VOC is methane, a greenhouse gas sometimes excluded from analysis of other VOCs using the term non-methane VOCs. Major worldwide sources of atmospheric methane include wetlands, ruminants such as cows, energy use, rice agriculture, landfills, and burning biomass such as wood. Methane is the primary component of natural gas.
Common artificial VOCs include paint thinners, dry cleaning solvents,semiconductor cleaner, and some constituents of petroleum fuels. Trees are also an important biological source of VOC; it is known that they emit large amounts of VOCs, especially isoprene and terpenes. Another significant source of VOC emission is crude oil tanking. Both during offloading and loading of crude oil tankers VOCs are released to the atmosphere. Recently, there has been an increase in environmental focus on this issue resulting in improved VOC handling on newer tankers, and crude oil loading terminals.