With regard to thresholds in mixtures, how is reporting under Sections 311 and 312 handled if a facility has a number of different mixtures on-site and each is under 10,000 pounds but the mixtures contains an aggregated quantity of an extremely hazardous substance (EHS) that exceeds its reporting threshold?

If extremely hazardous substances are components of a mixture, the quantity of the extremely hazardous substance in each mixture shall be aggregated to determine if the threshold value has been reached for the facility.  You must include the quantity present in the mixture even if you are also counting the quantity of that particular mixture toward the threshold level for that mixture.  Reporting may be accomplished by reporting on the component or the mixture even if the amount of the mixture(s) is below the reporting threshold (55 FR 30632, July 26, 1990).