35 Ill. Adm. Code 302.590 establishes an acceptable additive risk level of one in 100,000 (10(-5)) for establishing Tier I criteria and Tier II values for combinations of substances exhibiting a carcinogenic or other nonthreshold toxic mechanism. For those discharges containing multiple nonthreshold substances, application of this additive standard shall be consistent with Sections 352.302 and 352.303.