Response: The unknown acute toxicity statement is only required on the label and the SDS where the chemical mixture is already classified as acutely toxic for a particular route of exposure, and there are one or more other "relevant ingredients" (as defined under 29 CFR 1910.1200, Appendix A, A.1.3.3) of unknown acute toxicity for that particular route.

The approach in HCS 2012 for classifying mixtures for acute toxicity is tiered, and is dependent on the amount of information available for the mixture itself and for its ingredients. See A.1.3.1. Where the mixture has been tested to determine its acute toxicity, it is classified according to the criteria provided in Table A.1.1. See A.1.3.4. If there is no acute toxicity data for the mixture, A.1.3.5 requires that classification be based on bridging principles. If bridging principles cannot be applied, classification is based on the ingredients of the mixture. See A.1.3.6. Where there is data for all ingredients, an acute toxicity estimate (ATE) is followed, per A.1.3.6.1.

HCS 2012 specifies that classification of mixtures for acute toxicity may be carried out for each route of exposure, but is only required for one route of exposure as long as this route is followed (estimated or tested) for all ingredients and there is no relevant evidence to suggest acute toxicity by multiple routes. Where there is evidence of acute toxicity by multiple routes of exposure, classification is to be conducted for all appropriate routes of exposure. See A.1.3.2.

Where data are not available for one or more ingredients, the classification procedures in A.1.3.6.2 are followed. In this situation, A.1.3.6.2.3 of HCS 2012 provides:

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In the event that an ingredient with unknown acute toxicity is used in a mixture at a concentration ≥1%, and the mixture is not classified based on testing of the mixture as a whole, the mixture cannot be attributed a definitive acute toxicity estimate. In this situation the mixture is classified based on the known ingredients only. (Note: A statement that X percent of the mixture consists of ingredient(s) of unknown toxicity is required on the label and safety data sheet in such cases; see Appendix C to this section, Allocation of Label Elements and Appendix D to this section, Safety Data Sheets.)

Classifiers can present the unknown acute toxicity information on ingredients either as a single statement or as multiple statements, where routes are differentiated. If there is acute toxicity by more than one route of exposure and the classifier chooses to provide one statement in order to save space on the label or SDS, then the route with the highest total percentage unknown toxicity from one or more relevant ingredients will be used in the statement. The single statement on the label or SDS would state that "X% of the mixture consists of ingredient(s) of unknown acute toxicity."

Because it is possible to have ingredients with unknown toxicity for more than one route (e.g., oral, dermal, inhalation), differentiating the unknown toxicity statement by route is recommended.¹ As such, classifiers may also communicate the information as:

X% of the mixture consist of ingredient(s) of unknown acute oral toxicity

X% of the mixture consists of ingredient(s) of unknown acute dermal toxicity

X% of the mixture consists of ingredient(s) of unknown acute inhalation toxicity

In the example you provided (see question above), we will assume there is no relevant evidence for the ingredient suggesting acute toxicity by the inhalation route. If so, and your mixture is not classified for the inhalation route, then an unknown acute toxicity statement would not be needed for inhalation.