Risk Assessment: Refinery MACT 1 and 2

Background

Section 112(f)(2) of the Clean Air Act (CAA) requires the U.S. EPA to determine for source categories subject to Maximum Achievable Control Technology (MACT) standards whether the emission standards provide an ample margin of safety to protect public health.  This assessment is carried out by the U.S. EPA in a two-step process that includes the primary objective of developing regulations to address any residual risk after implementation of the original Refinery MACT 1 and MACT 2 emission standards and work practices.  Following the risk assessment, the agency performs the prescribed second step of determining whether further requirements are necessary to provide an “ample margin of safety” under its evaluation of the original MACT rules.

The first step in the process of evaluating residual risk is to make a determination of what is an acceptable risk.  If the risks are not acceptable, then the U.S. EPA cannot consider cost in identifying the emissions standards necessary to bring the risks down to an acceptable level.  The second step is the determination of whether regulatory standards must be further strengthened in order to provide the required “ample margin of safety” necessary to protect public health.  The ample margin of safety is the level at which the standards must be set, unless an even more stringent standard is necessary to prevent an adverse environmental impact, taking into consideration costs, energy, safety and other relevant factors.

Risk Assessment

The determination of acceptable risk begins with a presumptive limit on the maximum individual lifetime risk (MIR) of approximately 1-in-10 thousand.  The MIR risk assessment considers the risk from both cancer and non-cancer exposures and considers acute exposures as well as chronic effects from life-time exposures.  Both long-term and short-term inhalation exposure concentrations and health risks from the petroleum refinery sector were addressed by the U.S. EPA in their MACT review by using the Human Exposure Model (Community and Sector HEM-3 version 1.1.0). 

The U.S. EPA evaluated risk using screening estimates of acute exposures and risks for each of the hazardous air pollutants (HAPs) of concern at the point of highest off-site exposure for each facility by assuming that a person is located at this spot at a time when both the peak (hourly) emissions rate and worst case dispersion conditions would occur.  The U.S EPA also conducted the screening analysis examining the potential for significant human health risks due to exposures via routes other than inhalation, such as by consumption of locally grown food.  They first determined whether any sources in the source category emitted any hazardous air pollutants known to be persistent and bio-accumulative in the environment (Lead(PB)-HAP).  For the petroleum refinery source categories, the U.S. EPA identified emissions of cadmium compounds, chlorinated dibenzodioxins and furans, lead compounds, mercury compounds, polychlorinated biphenyls, and polycylic organic matter.

In addition to assessing baseline inhalation risks and screening for potential multipathway risks, the U.S. EPA estimated risks considering the potential emission reductions that would be achieved by the different control options that they were considering for the MACT reassessment.  The U.S. EPA used the same emissions inventory that it used for their risk screening modeling and then applied emission reduction estimates for the different control options under consideration in order to calculate the post-control risk values.

The results of the chronic baseline inhalation cancer risk assessment indicated that, based on estimates of current actual emissions, the maximum individual lifetime cancer risk (MIR) posed by the refinery source category is 60-in-1 million, with benzene and naphthalene emissions from equipment leaks and storage tanks accounting for 98 percent of the MIR risk.  The total estimated cancer incidence from refinery emission sources, based on actual emission, levels is 0.3 excess cancer cases per year or one case in every 3.3 years.  When considering the MACT-allowable emissions, the maximum individual lifetime cancer risk was estimated to be up to 100-in-1 million, driven by emissions of benzene and naphthalene from refinery fugitives (e.g., storage tanks, equipment leaks and wastewater) and the estimated cancer incidence is estimated to be 0.6 excess cancer cases per year or one excess case in every 1.5 years.

The maximum modeled chronic non-cancer target organ-specific hazard index (TOSHI) value for the source sector, based on actual emissions, was estimated to be less than 1.  When considering MACT-allowable emissions, the maximum chronic non-cancer TOSHI value was estimated to be about 1.  All other HAP from the facilities modeled had worst-case acute hazard quotient values of less than 1, indicating that the HAP emissions are believed to be without appreciable risk of acute health effects.

To better understand the uncertainty of its multipathway screening analysis to either under predict or over predict risk, the U.S. EPA conducted a refined multipathway case study using the Total Risk Integrated Methodology (TRIM) TRIM.FaTE risk model for one of the petroleum refineries for which actual emissions data had been obtained.  The refinery was selected because of its nearby lakes and farms which provided a real lifetime exposure scenario, as well as the fact that the refinery was found to have one of the highest potential multipathway risks for PAH based on its Tier II risk analysis.  The refined analysis for this facility showed that the Tier II screening that had been performed for each pollutant at the site actually had over predicted the potential risk when compared to the refined analysis results.

Overall, the refined risk analysis predicted a potential lifetime cancer risk of 4-in-1 million for the MIR.  The non-cancer hazard quotient (HQ) was predicted to be well below 1 for all of the target organs.  The chronic inhalation cancer risk assessment estimated inhalation cancer risk around this same facility was found to be approximately 10-in-1 million, due in large part to emissions of naphthalene and 2-methylnaphthalene.  The U.S. EPA decided to look at these two results as additive, although they acknowledged it was highly unlikely, such that the person in the risk assessment with the highest chronic inhalation cancer risk was also assumed to be the same person with the highest individual multipathway cancer risk.  Therefore, the U.S. EPA combined these results to form the worst-case MIR for that facility which theoretically could be a 10-in-1 million risk.  While this refined assessment was performed on only a single facility, the results of this single refined analysis indicate that if refined analyses were performed for other sites, the risk estimates would consistently be lower than those estimated by their Tier II analysis.  In addition, the risks predicted by the multipathway analyses at most facilities are considerably lower than the risk estimates predicted by the inhalation assessment, indicating that the inhalation risk results are in all likelihood the primary factor that is causing any residual risk for petroleum refineries.

Summary – Environmental Risk Assessment Screening

In the Tier II screening analysis for PB-HAP, none of the individual modeled concentrations for any facility in the source category exceeded any of the ecological benchmarks.  For lead compounds, the U.S. EPA did not model any exceedances of the secondary lead National Ambient Air Quality Standards (NAAQS).  Therefore, the U.S. EPA did not conduct further risk assessment for lead compounds.

For acid gases, the average modeled concentration around each facility did not exceed any ecological benchmark.  In addition, for both hydrochloric acid (HCL) and hydrogen fluoride (HF), each individual concentration was below the ecological benchmarks for all facilities.  Therefore, the U.S. EPA did not conduct further risk assessment for acid gases.

The risk assessment results indicate that both the actual and allowable inhalation cancer risks to the individual most exposed are no greater than approximately 100–in-1 million, which is the presumptive limit of acceptability.  The MIR based on actual emissions is 60-in-1 million, approximately 60 percent of the presumptive limit.  Based on the results of the refined site specific multipathway analysis the U.S. EPA conclude that the ingestion cancer risk to the individual most exposed is significantly less than 100-in-1 million (1-in-10 thousand). In addition, the maximum chronic non-cancer TOSHI due to inhalation exposures is less than 1, and the U.S. EPA’s refined multipathway analysis indicates that non-cancer ingestion risks are estimated to be less than non-cancer risk from inhalation.  Finally, while the evaluation of acute non-cancer risks was very conservative, it also showed acute risks below a level of concern.  The U.S. EPA estimated that their proposed new standards would not affect the MIR, and do not by themselves justify additional emission reductions, but would if implemented would reduce the source category estimated cancer incidence by 15 percent. 

Conclusion

In conclusion, the U.S. EPA determined that the risks remaining after implementation of the existing Refinery MACT 1 and 2 rules were acceptable and did already provide “ample margin of safety” necessary to protect public health.

For more information about the proposed Refinery MACT revisions, refer to ALL4’s May 2014 and June 2014 4 The Record articles.