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Frequently Asked Questions About Solvents


What is a solvent?
What is ground level ozone?
What is a VOC?
What are VOC-exempt solvents?
What is photochemical reactivity?
What are hazardous air pollutants (HAPs)?
What is a MACT standard?
What does “residual risk” refer to?

What is a solvent?

Solvents are chemical substances that can dissolve, suspend or extract other materials usually without chemically changing either the solvents or the other materials. Solvents can be organic, meaning the solvent contains carbon as part of its makeup, or inorganic, meaning the solvent does not contain carbon. For example, “rubbing” alcohol is an organic solvent and water is an inorganic solvent. Hydrocarbon and oxygenated solvents are examples of types of organic solvents that can effectively dissolve many materials.

Modern solvents make it happen.®

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What is ground level ozone?

Ground-level ozone is the main component of urban smog. It is formed by the photochemical reaction of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the atmosphere in the presence of heat and sunlight. There are several significant sources of VOC emissions, including natural or "biogenic" sources such as trees and vegetation, and man-made sources such as vehicle emissions, petroleum refining and distribution, and combustion sources. VOC emissions also come from the use of organic solvents, most of which are classified as VOCs. In order to reduce ozone levels in areas that do not meet federal standards (non-attainment areas), the Clean Air Act regulates man-made emissions of both VOCs and NOx.

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What is a VOC?

EPA has defined "VOC" broadly to include in effect "any volatile compound of carbon" that is not specifically exempted. See 40 C.F.R.51.100 (s). 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 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.

We have more information available on VOCs.

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What are 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).

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What is photochemical reactivity?

The potential contribution that each VOC makes to ozone formation depends on its photochemical reactivity, that is, its tendency to participate in photochemical reactions in the atmosphere in ways that contribute to ozone formation. One significant new development in the regulation of VOCs 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 form ozone. The higher the reactivity, the greater the potential contribution to ozone formation.

» Learn more about ozone control.

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What are hazardous air pollutants (HAPs)?

The 1990 amendments to the federal Clean Air Act created a new regulatory program designed to reduce emissions of 189 chemicals and chemical categories that are listed as hazardous air pollutants, or HAPs. The HAP list can be found at http://www.epa.gov/ttn/atw/188polls.html. The HAP list includes compounds with a wide range of toxicity. Some of the listed compounds are widely recognized to be very hazardous while others, including many of the listed solvents, are generally considered to have relatively low toxicity.

EPA has the authority to add or remove chemicals from the Clean Air Act HAP list, and any person may petition EPA to remove a substance from the HAP list. Such petitions currently are under review for ethylene glycol butyl ether (EGBE), methyl ethyl ketone (MEK), and methyl isobutyl ketone (MIBK).

In addition to the federal HAP program, most states have their own air toxics programs.

Learn more about the Federal regulation of hazardous air pollutants (HAPs).

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What is a MACT standard?

Under the Clean Air Act, EPA is required to develop regulations to reduce emissions of HAPs from "major sources," defined to include any facility that has the potential to emit, on an annual basis, 10 tons of any single HAP or 25 tons of any combination of HAPs. These regulations are based on "maximum achievable control technology," and so are often called MACT standards.

Learn more about the Federal regulation of hazardous air pollutants (HAPs).

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What does “residual risk” refer to?

In order to ensure that the technology-based MACT standards protect human health with an "ample margin of safety," the Clean Air Act requires EPA to assess residual risks to human health and the environment remaining after imposition of MACT. When considering whether additional controls should be required, EPA must consider not only health risks, but also costs, energy requirements, safety issues, and "other relevant factors." EPA's residual risk assessment is required by no later than eight years after a MACT standard has been established for a source category. EPA has been delayed in its implementation of this program, however, and has not yet completed its residual risk assessments for the MACT standards promulgated in 1993 and 1994.

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Solvents help boost the cleaning power of laundry detergents. They attack grease, which helps keep our clothing looking good even after a spill.


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