Ill. Admin. Code tit. 35 § 352.530
A minimum of five representative toxicity tests is necessary to calculate a PEQ. If less than five test results are available and there is evidence that effluent toxicity may exist, additional toxicity testing shall be required consistent with Section 352.520(c). Whenever sufficient data exists, the PEQ is estimated to be the maximum representative value determined from Section 352.520(a) and (b), expressed in terms of acute and chronic toxicity units (TU[a] & TU[c]) increased by a multiplying factor from the table in Section 352.421. If more than 10 facility specific data values are available, and the PEQ is more than either 1.0 TU[a] or 1.0 TU[c], the Agency will proceed to consideration of dilution and mixing under Section 352.540 for the relevant effect (acute, chronic, or both). If less than 10 facility specific data values are available, and the PEQ is more than either 1.0 TU[a] or 1.0 TU[c], the Agency will follow the process set forth in Section 352.421(b) to determine whether to proceed to Section 352.540. If the PEQ is less than or equal to 1.0 TU[a] or less than or equal to 1.0 TU[c], no WET limit will be established in the permit for the relevant standard.
Ill. Admin. Code tit. 35, § 352.530