Section 203.1370 Significant

a) "Significant" means, in reference to a net emissions increase or the potential of a source to emit any of the following pollutants, a rate of emissions that would equal or exceed any of the following rates:

Pollutant	Emissions Rate

CO	100 tpy of CO, except under subsection (b)
NO₂	40 tpy of NO_X
SO₂	40 tpy of SO₂
PM₁₀	15 tpy of PM₁₀
PM_2.5	10 tpy of direct PM_2.5 emissions; 40 tpy of SO₂, 40 tpy of NO_X, 40 tpy of VOM, or 40 tpy of ammonia, to the extent that any such pollutant is defined as a precursor for PM_2.5 in Section 203.1340.
Ozone	40 tpy of VOM or NO_X, except under subsections (c) and (d).
Lead	0.6 tpy

b) For areas classified as serious nonattainment for CO where stationary sources significantly contribute to ambient CO levels, as determined under rules issued by the USEPA, pursuant to the CAA, notwithstanding the significant emissions rate for CO in subsection (a), significant means an increase in actual emissions of CO that would result from any physical change in or change in the method of operation of a major stationary source, if such increase equals or exceeds 50 tpy.

c) For areas classified as serious or severe nonattainment for ozone, notwithstanding the significant emissions rate for ozone in subsection (a), an increase in emissions of VOM or NO_X shall be considered significant if the net emissions increase of such air pollutant from a stationary source located within such area exceeds 25 tons when aggregated with all other net increases in emissions from the source over any period of 5 consecutive calendar years which includes the calendar year in which such increase occurred. This provision shall become effective beginning November 15, 1992, or such later date when an area is classified as a serious or severe nonattainment area for ozone.

d) For areas classified as extreme nonattainment for ozone, notwithstanding the significant emissions rate for ozone in subsection (a), any increase in emissions of VOM or NO_X from any emissions unit at a major stationary source of VOM or NO_X shall be considered significant.

e) For major stationary sources located outside designated nonattainment areas for purposes of Subpart R, an increase in emissions of a regulated NSR pollutant shall be considered significant if it would equal or exceed the rate listed in subsection (a), notwithstanding the attainment status in the area.

(Source: Added at 49 Ill. Reg. 6237, effective April 23, 2025)