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.
branched, or linear, completely methylated siloxanes (VMS) are not subject to
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.