Ill. Admin. Code tit. 35, § 291.201 – Study Area and Background Concentrations

a) The study area shall include all territory surrounding the subject facility which encompasses a common aggregation of sources, usually an urbanized/industrial area bounded by areas which are now undeveloped. The analysis must consider the following:
1. 1) All point and area source emissions originating within the urbanized/industrial area must be considered in the analysis.
2. 2) Any emission source located beyond the undeveloped boundary of the urbanized/industrial area if such source contributes one microgram per cubic meter or more to the sulfur dioxide and/or particulate annual average; 10 micrograms per cubic meter or more to the maximum sulfur dioxide and/or particulate 24-hour concentration; or, 50 micrograms per cubic meter or more to the maximum 3 hour sulfur dioxide concentration within the study area.
3. 3) The effect of the subject facility outside of the territory defined above when such facility contributes the same annual and/or short-term concentrations in other urbanized/industrial areas located outside of the study area.
4. 4) The influence of topography and geography on the dispersion of air pollutants when performing the analyses to determine the study area or impacted areas outside the study area.
b) Background concentrations contained in the "clean" air entering the study area may be considered to be 40 micrograms per cubic meter for particulates and 10 micrograms per cubic meter for sulfur dioxide when calculating annual average concentrations. When calculating the second highest short-term concentration for purposes of comparison to the short-term primary standards, the critical meteorological conditions associated with such second highest concentration must be identified. The background concentration associated with these meteorological conditions shall be estimated using available air quality data and information pertaining to emission sources located outside of the study area.