Ill. Admin. Code tit. 35, § 203.2510
For this Subpart, the emissions from a new major stationary source or major modification will be considered to cause or contribute to a violation of a NAAQS if such source or modification would exceed the following significance levels at any locality that does not or would not meet the applicable NAAQS.
| Pollutant | Significant Level (μg/m3) | ||||
| Annual Average | 24-hour Average | 8-hour Average | 3-hour Average | 1-hour Average | |
| SO2 | 1.0 | 5 | 25 | ||
| PM10 | 1.0 | 5 | |||
| PM2.5 | 0.3 | 1.2 | |||
| NO2 | 1.0 | ||||
| CO | 500 | 2,000 |
(Source: Added at 49 Ill. Reg. 6237, effective April 23, 2025)