Question # 172

172. If a covered facility only knows the range of concentration of a Section 313 toxic chemical in a mixture, is it required to use the upper bound concentration to determine thresholds? Use of the average or midpoint of the range will avoid overestimating emissions. If a metal mixture contains a range of 1 to 10 percent of three metals together, how can this information be used to determine thresholds?
The upper bound should be used if the person knows only the upper bound concentration. For the combination of three toxic chemicals, the owner/operator of the facility should split the upper bound among the three toxic chemicals based on the knowledge that it has, so the total equals 10 percent. If a range is available, using the midpoint or average is reasonable. In this case, if there is a range of 1 to 10 percent of a mixture of three toxic chemicals, the facility would divide the midpoint (5 percent) by three. Therefore, the facility would assume 1.33 percent of each of the toxic chemicals in the mixture. The owner/operator of the facility does not have to assume 10 percent maximum for each toxic chemical.