How are thresholds for regulated toxic substances in mixtures determined?

Formaldehyde is listed as a regulated substance under 40 CFR 68.130 with the qualifier "(solution)." When determining whether a threshold amount of this substance exists in a process, should a person consider the weight of the entire solution, or simply the amount of formaldehyde in the solution? How are thresholds for other regulated toxic substances in mixtures determined?

The owner or operator of a stationary source need only consider the weight of the formaldehyde in the solution; not the entire weight of the mixture. This is the case for all listed toxic substances, including those with concentration qualifiers (i.e., "conc 80% or greater").