"Subject to regulation" means, for any air pollutant, that the pollutant is subject to either a provision in the CAA, or a nationally-applicable regulation codified by USEPA in 40 CFR 50 through 99, that requires actual control of the quantity of emissions of that pollutant, and that such a control requirement has taken effect and is operative to control, limit or restrict the quantity of emissions of that pollutant released from the regulated activity. Pollutants subject to regulation include, but are not limited to, GHGs as defined in Section 204.430.
Ill. Admin. Code tit. 35, § 204.700