"Nearby", with respect to a specific structure or terrain feature:

a) For purposes of applying the formula provided in Section 203.1200(a)(2), means that distance up to five times the lesser of the height or the width dimension of a structure, but not greater than 0.8 km (½ mile); and

b) For conducting demonstrations under Section 203.1200(a)(3), means not greater than 0.8 km (½ mile), except that a portion of a terrain feature may be considered to be nearby if it falls within a distance of up to 10 times the maximum height of the feature, not to exceed 2 miles if such feature achieves a height, 0.8 km from the stack, that is at least 40 percent of the good engineering practice stack height determined by the formula provided in Section 203.1200(a)(2)(B) or 26 meters, whichever is greater, as measured from the ground-level elevation at the base of the stack. The height of the structure or terrain feature is measured from the ground-level elevation at the base of the stack.

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