Three different facility prioritization procedures, including California Air Resource Board (CARB), South Coast Air Quality Monitoring District (SCAQMD), and Texas Commission on Environmental Quality (TCEQ), had been applied in this study to demonstrate the potential variation for performing a health risk assessment on air toxics. A case study of refinery had been done for this study.
These procedures had been used for five specific facilities that emit volatile organic air toxics in the refinery. These five specific facilities were fluid catalytic cracking (FCCU), vacuum distillation unit, delayed coking unit, hydrogen plant, and flare. There are sixteen toxic air pollutants categorized as volatile organic compounds that emitted by the five facilities, which were acetaldehyde, acrolein, aniline, benzene, 1,3-butadiene, isopropyl benzene, ethylbenzene, formaldehyde, n-hexane, methyl isobutyl ketone, styrene, toluene, 2,2,4-trimethylpentane, o-xylene, m-xylene, p-xylene, but only 13 toxic compounds put into calculation because the lack of health effects data.
The emission estimation results indicated that formaldehyde, benzene, n-hexane, and toluene were the critical constituents that contributed the dominant emissions from the selected facilities. Flare was the biggest emission source of formaldehyde (27.3 TPY), benzene (25.9 TPY) and n-hexane (25.0 TPY).
The results from CARB method, emission potency procedure and dispersion adjustment procedure, the five selected facilities were categorized as low priority for the carcinogenic effect and high priority for the non-carcinogenic effect. By categorized as high priority, the rest of four facilities should conduct the health risk assessment as the next step.
The results by SCAQMD Tier 1 indicate that all facilities had hazard indices score greater than 1 for carcinogenic and non-carcinogenic effects for the three categories. Only vacuum distillation unit cancer/chronic effect didn’t exceed the threshold by score 7.17E-01 (for 100 meters). The result showed all the facilities exceed the threshold of 1 for all the screening emission levels categories. The results by Tier 2 indicate that emissions from FCCU, hydrogen plant, and flare exceeded the threshold of 1 x 10-6 (one in one million) for MICR with score 1.29E-05, 6.97E-06 and 2.87E-04. All of the facilities exceeded the hazard index 1 only for the acute health effects for all the target organs. By exceeding the threshold for MICR and HIA, it means each selected facility should continue to Tier 3 and Tier 4 for detailed risk assessment process.
The results of TCEQ Tier 2 indicate that all selected facilities had GLCni score exceeded the ESLs more than two folds for downwash and non-downwash effects. The facilities had GLCni scores exceeded the ESLs more than two folds for the cancer, acute, and chronic health effects.
This study used sensitivity ratio (SR) as one of sensitivity analysis method. The results indicate the emission factor is the most sensitive input for the result of emission estimation. For the facility prioritization procedure, the sensitivity ratio only worked out for SCAQMD Tier 2 method. Since the results from Tier 2 gave the SR ratio almost equal to 1, it can be concluded that X/Q (dispersion factor) was the most sensitive parameters and would influence the Tier 2 results. The dispersion factor was determined by data of the height of the stack and the distance between the facility (source emitter) and the receptor.

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