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Exempt Compounds


The United States Environmental Protection Agency (U.S. EPA) excludes certain organic compounds from the regulatory definition of a Volatile Organic Compound (VOC) based on the compound's negligible contribution to the formation of ground-level ozone, better known as smog. Smog is formed when VOCs photochemically react with oxides of nitrogen (NOx) in the atmosphere. The potential for a VOC to react with NOx to form smog, i.e., the maximum incremental reactivity (MIR) of a VOC, varies from compound to compound.

As a result, air quality regulations focus on VOCs that increase ground-level ozone concentrations, and exclude compounds with negligible reactivity. The U.S. EPA exempts compounds by comparing the reactivity of a given organic compound to that of ethane. Compounds with reactivity levels lower than, or equal to, ethane under the assumed conditions, may be deemed negligibly reactive.  Compounds that are more reactive than ethane continue to be considered reactive VOCs, and therefore are subject to air quality regulation. 

Rule 102 - Definition of Terms contains a complete and updated list of exempt compounds.  In addition to the exempt compounds listed in the Rule 102, some rules include limited exemption.  Rule 1113 (b)(71) contains the following limited exemption for tertiary-Butyl Acetate (tBAc):

"VOLATILE ORGANIC COMPOUND (VOC) is as defined in Rule 102 – Definition of Terms. For the purpose of this rule, tertiary butyl acetate (tBAc) shall be considered exempt as a VOC only for purposes of VOC emissions limitations or VOC content requirements and will continue to be a VOC for purposes of all recordkeeping, emissions reporting, photochemical dispersion modeling, and inventory requirements which apply to VOCs, when used in industrial maintenance coatings, including zinc-rich industrial maintenance coatings and non-sacrificial anti-graffiti coatings."

The AQMD classifies exempt compounds as Group I and Group II Exempt Compounds.  Some rules include prohibitions from using certain Group II Exempt Compounds as they have potential toxic health risks, as well as being contributors to upper-atmosphere ozone depletion and other potential environmental impacts.  The following general prohibition is included in Rule 1113 and Rule 1143 -Consumer Paint Thinners and Multi-Purpose Solvents:

"No person shall supply, sell, market, offer for sale, manufacture, blend, or repackage any architectural coating, consumer paint thinner or multi-purpose solvent in the District subject to the provisions of this rule with any materials that contain in excess of 0.1% by weight any Group II exempt compounds listed in Rule 102.  Cyclic, branched, or linear, completely methylated siloxanes (VMS) are not subject to this prohibition."

This provision is already in effect in Rule 1143.  For Rule 1113, this provision is effective January 1, 2012, except that products manufactured prior to the effective date may be sold until January 1, 2013 and used until January 1, 2015.

This page updated: August 23, 2011