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Panel: Steve Risotto
Media: Sarah Scruggs

Definition of a Volatile Organic Compound

EPA has defined “VOC” very broadly. In effect, “any volatile compound of carbon” is classified as a VOC for regulatory purposes, unless specifically exempted. See 40 C.F.R. 51.100 (s). The list of VOC-exempt compounds is discussed further below.

EPA’s VOC regulations, however, do not always apply to all compounds that meet the very broad definition found at 40 C.F.R. 51.100(s). For example, for regulations involving paints and coatings, there is a specific test method, known as Test Method 24, that generally determines what is to be treated as a VOC. See 40 C.F.R. part 60, Appendix A. Test Method 24 is a collection of ASTM test methods that collectively define the VOC content of a coating formulation. Generally, any compound that is “picked up” by these test methods is considered a VOC for purposes of regulating coating formulations.

Individual states may have their own VOC definitions, including their own list of exemptions. Although state definitions (including exemptions) are generally the same as the EPA definition, a solvent user should be aware of the precise definition that applies in his or her state.

VOC-Exempt Solvents

EPA regulations include a list of compounds that are explicitly exempted from regulation as VOCs, even though they are “compounds of carbon.” These include a short list of compounds such as carbon monoxide and carbon dioxide that historically have not been regulated as VOCs, and a longer list of compounds that EPA has classified as “negligibly reactive.” Negligibly reactive compounds are compounds that, based on scientific studies, have been found “not to contribute appreciably to ozone formation.” This list of compounds (often referred to as “VOC-exempt compounds”) is established and modified by regulation. The list of exempt compounds is found at 40 C.F.R. 51.100(s). EPA has proposed to add t-butyl acetate (TBAC) to the exempt list, and final action is expected before the end of 2004.

Although there are relatively few VOC-exempt solvents at this time, where such solvents can be used, they provide product formulators with additional flexibility in designing solvent blends that meet regulatory requirements. Product formulators, however, obviously must consider the full range of characteristics of all products and components, not just whether they are VOC-exempt. Careful selection of all components, including VOC and VOC-exempt materials, will help formulators to develop effective, efficient, and economical products that meet applicable regulatory requirements.

Regulation Based on Relative Photochemical Reactivity

One significant new development in the regulation of volatile organic compounds involves the use of “relative reactivity” to rank a chemical’s potential to contribute to the formation of ground-level ozone. “Relative Reactivity” recognizes that individual VOCs are not equal in their potential to contribute to the formation of ozone. Rather, the potential contribution that each VOC makes to ozone formation depends on its photochemical reactivity: the higher the reactivity, the greater the potential contribution to ozone formation.

Recognizing that different VOCs behave differently, the California Air Resources Board has developed a compliance option for aerosol coating products that permits the product to contain more total VOCs than would otherwise be allowable provided the overall photochemical reactivity of the product does not exceed a specified relative reactivity. The regulations became effective June 1, 2002 for general coatings, such as flat paint and primers, and January 1, 2003 for makers of specialty coatings, such as marine paints, automotive body products, and shellac sealers. See Cal. Code Regs. Tit. 17, Art. 3, §§ 94521-94524 and 94526 (Aerosol Coating Products). The regulations can be accessed here. The relative reactivity values adopted by California are based on research by Dr. William Carter at the University of California – Riverside. 

An organization of state and local air administrators (State and Territorial Air Pollution Program Administrators/Association of Local Air Pollution Control Officials, “STAPPA/ ALAPCO”) has offered a generic version of a California rule to reduce VOCs from paints and other architectural coatings. EPA is evaluating whether a similar approach should be adopted at the federal level.


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