I have written often about this topic and it is without a doubt the most questioned/challenged topic in my 5-Day Advanced Process Safety course. Several toxic substances are listed as regulated substances under 40 CFR §68.130 with CONCENTRATION QUALIFIERS (e.g., “conc 37% or greater”). The four(4) regulated substances that have concentration qualifiers are:
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Ammonia (conc 20% or greater)
- Hydrochloric acid (conc 37% or greater)
- Hydrogen fluoride/hydrofluoric acid (conc 50% or greater)
- Nitric acid (conc 80% or greater)
What does this concentration mean?
When determining whether a threshold amount of these substances exists in a process, should I consider the weight of the entire solution, or simply the amount of the regulated toxic substance in the solution?
If a regulated substance has a specific concentration listed, you need only consider solutions/mixtures with concentrations of the regulated substance that are at or above this concentration. You do not need to consider solutions/mixtures with concentrations of a regulated substance below the listed concentration when you determine threshold quantities.
Examples: Ammonia has a specified concentration of 20%. Hence, you do not need to consider a 15 percent ammonia solution, but do need to account for a 25 percent ammonia solution. For any regulated substance without a listed concentration cutoff, you must consider the quantity of the regulated substance if the concentration exceeds one percent.
Once you have determined that the solution/mixture must be accounted for, you need to determine whether you exceed the threshold quantity. For regulated toxic substances, you must consider only the weight of the regulated substance in the solution/mixture towards the threshold quantity.
Examples: The Threshold Quantity for ammonia solutions is 20,000 pounds. If you have more than 100,000 total pounds of a 20% ammonia solution, then you would have more than 20,000 pounds of ammonia in the solution and would have more than a threshold quantity. Similarly, if you have more than 50,000 total pounds of a 40% solution, you would also have more than a threshold quantity.
Source: https://www.epa.gov/rmp/calculating-thresholds-toxic-substances-concentration-qualifiers
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”).
Source: https://www.epa.gov/rmp/how-are-thresholds-regulated-toxic-substances-mixtures-determined