Exempt Compounds

The United States Environmental Protection Agency (U.S. EPA) excludes certain organic compounds from the regulatory definition of a 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.

Definitions

Rule 102 (PDF) - Definition of Terms contains a complete and updated list of exempt compounds. 

The following are not considered exempt solvents in the South Coast AQMD, except in the following limited applications:

tertiary-Butyl Acetate (tBAc)

Rule 1151 (PDF) - Motor Vehicle and Mobile Equipment Non-Assembly Line Coatings exempts tBAc.  Rule 1113 (PDF) - Architectural Coatings contains the following limited exemption for 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."

Dimethyl Carbonate (DMC)

In addition, Rule 1144 (PDF) - Metalworking Fluids and Direct-Contact Lubricants exempts DMC from the VOC limits and prohibition of sale as follows:

"...used as a cooling solvent in computed numerically controlled (CNC) machines where permeable media are used to maintain a vacuum that holds the part in place during cutting provided that the equipment existed at the time of rule adoption, is enclosed and an exhaust fan discharges the exhaust air from the equipment outside of the building."

Prohibitions

The South Coast 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 (PDF) - Architectural Coatings, Rule 1143 (PDF) - Consumer Paint Thinners and Multi-Purpose Solvents, and Rule 1168 (PDF) - Adhesive and Sealant Applications:

"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."