An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

Calculating thresholds for toxic substances with concentration qualifiers

Several toxic substances are listed as regulated substances under 40 CFR §68.130 with concentration qualifiers (e.g., "conc 37% 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.

There are four regulated substances that have concentration qualifiers:
  • 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)