Oleum, which is a mixture of sulfuric acid and sulfur trioxide, is listed as a regulated toxic substance in 40 CFR §68.130. Sulfur trioxide is also listed individually as a regulated toxic substance. If a single process consists of one vessel containing oleum and one vessel containing sulfur trioxide, must the amount of sulfur trioxide in the oleum be aggregated with the amount of pure sulfur trioxide in the process when determining whether the threshold quantity for sulfur trioxide is exceeded?

No. If a process contains discrete amounts of both oleum and sulfur trioxide, the amount of sulfur trioxide that is a part of the oleum mixture should not be considered when determining whether the threshold quantity for sulfur trioxide is exceeded in that process. If the threshold is not met for either chemical, then the process is not subject to part 68, however, the process is subject to Section 112(r)(1), the general duty clause.