Methodology to calculate carcinogenic metal emission limits from multiple sources using air dispersion modeling results

The U.S. Environmental Protection Agency (U.S. EPA) has set emission limits for heavy metals by comparing the results of computer air dispersion modeling to the potential most exposed individual risk. The potential most exposed individual risk is based on appropriate unit risk values (carcinogenic metals) or reference air concentrations (non-carcinogenenic metals). For the non-carcinogenic metals, the maximum ground level concentration of a compound is compared to the compound's reference air concentration. The aggregate risk to the most exposed individual for carcinogenic metals (i.e., arsenic, beryllium, cadmium, and chromium) is calculated by predicting the maximum annual average ground level concentration for each compound, calculating the estimated risk for each compound, calculating the estimated risk from that ambient concentration using the unit risk factor, and summing the risk for all four compounds. Therefore, for carcinogenic metals, if the emission of one of the four metals increases, the other three metal emissions must be reduced to keep the summation of the ratios of actual emissions to emission limits less than or equal to one. The objective of this study was to develop a methodology and the procedures whereby one can determine a cost-effective strategy for the control of carcinogenic metals from multiple sources within a particular facility in order to minimize the potential adverse health impact due to each carcinogenic metal emissions from one facility. To accurately predict the carcinogenic metal emissions from each source and to decide which emission control strategy for the facility was a cost effective, a computer program was developed that uses the receptor concentration results as predicted by air dispersion modeling. As an case study, a facility with five sources emitting carcinogenic metals was used to demonstrate the procedures followed in determining a cost-effective control configuration in order to meet emission requirements. Four of the sources were existing liquid incinerators, and the fifth source was a proposed solids incinerator. With the five sources emitting carcinogenic metals at the design rate, the facility would cause an unacceptable cancer risk to a most exposed individual. One or more of the existing sources, therefore, would have to be retrofitted with a high efficiency submicron particulate control system to remove carcinogenic metals. The results of the example study determined that either the emissions from source 3 or 4 should be controlled with at least a 85 percent removal efficiency for heavy metals. Eighty five percent control of either one of these two sources would enable the facility to meet the cancer risk to the most exposed individual with a 13 percent "safety margin". A 95 percent removal efficiency at either of these sources would give the facility a 22 percent "safety margin". Which of the two sources to control depended on the annual cost of control because emission control at source 3 or 4 would afford equal mass reduction of total carcinogenic metals at the most exposed individual location. Even a removal efficiency of 99.9 percent at source 1 or 2 would not reduce the cancer risk to the most exposed individual to an acceptable level.