Today the DHS Infrastructure Security Compliance Division (ISCD) published a new frequently asked question (FAQ) on the CFATS Knowledge Center web site. FAQ #1773 addresses the confusion about the way solid ammonium nitrate is described in the DHS chemical of interest (COI) list.
The COI lists solid ammonium nitrate as “Ammonium nitrate, solid [nitrogen concentration of 23% nitrogen or greater]”. The phrase ‘nitrogen concentration of 23% nitrogen or greater’ has caused some confusion since nitrogen is 22% of the number of atoms in an ammonium nitrate molecule or 32.6 wt% of the ammonium nitrate molecule.
The new FAQ #1773 asks: “In calculating whether a facility has a Screening Threshold Quantity (STQ) of solid ammonium nitrate in a mixture, does the facility look at the percentage of the nitrogen in the mixture or the percentage of the ammonium nitrate in the mixture?”
The response explains that the ‘33%’ listed in the concentration column of the COI list shows the minimum concentration of ammonium nitrate in a mixture that would make the total weight of that mixture count towards determining if the screening threshold quantity (STQ) for ammonium nitrate (2,000 lbs) has been met. It specifically states: “Facilities should NOT use the nitrogen concentration in a mixture to determine whether the mixture meets the minimum concentration requirements to be counted towards the STQ.”