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) - Definitions of Terms contains a complete list of exempt compounds and classifies VOC exempt compounds as Group I and Group II Exempt Compounds. Many VOC rules include prohibitions from using Group II Exempt Compounds as they have potential toxic health risks, as well as being potential contributors to upper-atmosphere ozone depletion and other potential environmental impacts. In addition, some VOC rules include limited exemptions for certain compounds due to suspected toxicity concerns at the time of rule adoption. Please refer to the most recent version of each rule in the South Coast AQMD Rule Book to determine the current status of VOC exempt compounds.
Background on para-Chlorobenzotrifluoride (pCBtF CAS: 98-56-6) and tert-Butyl Acetate (t-BAc CAS: 540-88-5)
In 1994, the U.S. EPA exempted pCBtF from the definition of a VOC. In 1995, South Coast AQMD added pCBtF as a Rule 102 Group I exempt VOC compound; exempting it from the definition of a VOC for any application in the South Coast AQMD.
In 2004, the U.S. EPA exempted t-BAc from the definition of a VOC. Due to preliminary toxicity concerns, the South Coast AQMD did not allow an unlimited Rule 102 exemption but allowed limited exemptions in Rule 1113 - Architectural Coatings and Rule 1151 - Motor Vehicle and Mobile Equipment Non-Assembly Line Coating Operations.
In 2018, the Office of Environmental Health Hazard Assessment (OEHHA) finalized their t-BAc health risk assessment, concluding that it had a higher cancer potency than previously estimated. In 2020, OEHHA finalized their assessment of pCBtF determining it was a more potent carcinogen than t-BAc.
The South Coast AQMD's Stationary Source Committee recommended staff take a precautionary approach for exempt compounds with toxic endpoints, recommending prioritizing reducing toxicity over reducing VOC emissions. Based on that recommendation, staff has been amending each VOC rule, category by category, to determine the best approach to reduce the health risks associated with pCBtF and t-BAc use in coatings and solvents used within the South Coast AQMD.
Status of pCBtF and t-BAc Prohibition Efforts
The following VOC rules that have been amended to include future effective prohibitions from using pCBtF and t-BAc:
- Rule 1168 - Adhesive and Sealant Applications (Amended Nov. 2022);
- Rule 1151 - Motor Vehicle and Mobile Equipment Non-Assembly Line Coating Operations (Amended Nov. 2024);
- Rule 1171 - Solvent Cleaning Operations (Amended Jun. 2025);
- Rule 1107 - Coating of Metal Parts and Products (Amended Dec. 2025);
- Rule 1144 - Metal Working Fluids and Direct-Contact Lubricants (Amended Apr. 2026);
- Rule 1124 - Aerospace Assembly and Component Manufacturing Operations (Amended Apr. 2026); and
- Rule 1136 - Wood Coating Operations (Amended Jun. 2026).
South Coast AQMD continues to amend VOC rules to address the health risks from pCBtF and t-BAc. Click HERE to access a pdf summary of the pCBtF and t-BAc phase out schedule for each rule and category of material regulated.
Status of Dimethyl Carbonate (DMC)
Due to toxicity concerns, DMC was also not added to Rule 102 as an exempt compound.