174. A covered facility is told by its supplier that the mixture the facility receives contains as much as 80 percent of 4-aminobiphenyl, a listed toxic chemical, and as little as 20 percent. How should the facility estimate the concentration of 4-aminobiphenyl in this mixture?
If the facility knows the upper and lower bound concentrations in a mixture (i.e., 80 and 20 percent), it should use the midpoint of these concentrations for threshold determinations. In this instance, 50 percent should be used because it is the midpoint between 80 and 20 percent.