Fact Sheet 13: Mixture Rule


The Illinois Pollution Control Board finalized a "mixture rule" by adopting Dockets B and C, which amended the TACO regulations (35 IAC Part 742). This amendment provides rules for developing remediation objectives when two or more contaminants of concern at a particular site have the same target organ or mode of action (i.e., they are similarly acting chemicals).

The mixture rule has been developed to meet the mandate set forth in Section 8 of the Illinois Groundwater Protection Act and in 35 IAC Part 620.601 and 620.615. For the Site Remediation Program, Section 58.5 of the Illinois Environmental Protection Act also requires the mixture rule.


In general, the remediation objectives developed by TACO for a single contaminant are calculated to pose no more than either a non-carcinogenic hazard quotient of 1 or a carcinogenic risk of 1 in 1,000,000 (See Fact Sheet 2). If multiple chemicals present at a site affect the same target, the potential exists for an unacceptable exposure. To safeguard against this, an analysis of the contaminants' mode of action is warranted during the development of risk-based remediation objectives for both carcinogens and non-carcinogens. It is important to understand not only the pathway into the body but also the toxic or carcinogenic endpoint.

For carcinogens, some protection against unacceptable exposure is built in. Even if ten carcinogens at a site have the same target (an unlikely event), the cumulative cancer risk for that target would be 1 in 100,000. This would be well within the acceptable risk range of 1 in 10,000 to 1 in 1,000,000. For noncarcinogens, there is no "acceptable range," and therefore, no safeguard for cumulative effects. Two noncarcinogenic contaminants, each at the acceptable hazard quotient of 1, would result in an unacceptable hazard quotient of 2.

The Tables

Three tables in TACO are pertinent to the mixture rule:

  • Appendix A, Table E, Similar-Acting Noncarcinogenic Chemicals
  • Appendix A, Table F, Similar-Acting Carcinogenic Chemicals
  • Appendix A, Table H, Chemicals Whose Tier 1 Class I Groundwater Remediation Objective Exceeds the 1 in 1,000,000 Cancer Risk Concentration

Tables E and F contain similar acting chemicals listed by endpoint of concern. If a site has two or more contaminants affecting the same target, (i.e., listed in Table E or F) the mixture may require analysis (see Application Table below).

Table H lists those Tier 1 groundwater objectives that are not based on a 1 in 1,000,000 risk factor. The majority of these groundwater objectives are adjusted upward due to laboratory detection limitations. Therefore, these contaminants' Tier 1 objectives cannot be used when determining if the cumulative cancer risk lies within the acceptable risk range. The actual 1 in 1,000,000 risk concentration (as provided in the third column of Table H) must be used in mixture calculations.

Tier 1

The mixture rule does not apply to carcinogens or noncarcinogens in soil because of the inherent conservative nature of the Tier 1 objectives provided for noncarcinogens and the acceptable risk range for carcinogens. In addition, human exposure to soil contaminants for inhalation or ingestion is not direct; that is, the vapors or particulates must first become airborne for inhalation and soil must be disturbed and transported for ingestion. These factors offer another layer of safety.

Groundwater for carcinogens must be evaluated in Tier 1 whenever one of the COCs is listed in Appendix A, Table H, regardless of whether or not the Tier 1 objective is met. If none of the COCs are listed in Table H, the mixture rule does not apply. Groundwater for noncarcinogens must be evaluated when one contaminant exceeds its Tier 1 objective. At this point, all contaminants attacking the same target that are present at the site are brought into the mixture evaluation.

Tier 2

Soil is exempt from the mixture rule for carcinogens because of the allowable risk range. This range does not exist for noncarcinogens, consequently, the mixture rule must be applied to noncarcinogens in soil under Tier 2. Carcinogens in groundwater must be evaluated in the mixture rule if one of the contaminants is listed in Table H. Noncarcinogens in soil or groundwater must always be addressed in Tier 2 if two or more COC's have the same target or mode of action.

Tier 3

All scenarios in Tier 3 must use the mixture rule since so many possibilities for remediation and contaminant management exist. Therefore, with any Tier 3 proposal, a demonstration must be made that cumulative effects have been adequately addressed.

Adjusting the Remediation Objectives

For noncarcinogens two methods are available:

  1. A weighted average may be calculated using the following equation:
    Wave =X1
      +  X2
       +    X3
     +...  Xn


  2. In this equation the original remediation objective for each contaminant is represented by CUO. The maximum level of each contaminant detected at the site is represented by the corresponding X.

    Because the original noncarcinogenic objectives are based on a hazard quotient of 1, the weighted average must also be 1 or less to be equally protective for a particular target. If the weighted average is greater than 1, additional corrective action must be conducted for those contaminants which have exceedances.

  3. You may divide each contaminant's original objective by the number of contaminants detected at the site that attack the same target. For example, if there are three contaminants that affect the kidney (e.g. acetone, toluene, and pyrene), the objective for acetone would be adjusted as follows:
    Adjusted CUO for Acetone

    The same exercise would then be conducted for toluene and pyrene. Each of the contaminant's on-site levels is then compared to its respective adjusted objective. If any of the contaminants exceed their adjusted remediation objective, further corrective action will be required for those COCs which have exceedences.

    For carcinogens:

    The cumulative risk of carcinogenic contaminants attacking the same target must not exceed 1 in 10,000. Therefore, the risk from all on-site similar acting carcinogens must be added together. If this cumulative risk level is greater than 1 in 10,000, corrective action must be taken to reach an acceptable risk level.


The table below outlines the scenarios in which the mixture rule must be used.

 Tier 1Tier 2Tier 3

Mixture rule applies if a carcinogenic COC listed on Appendix A, Table H is detected.