School Bus Scenario
AppendixEStationaryDiesel-Fueled EnginesHealth Risk Assessment Methodology
IntroductionThis appendix presentsthe methodology used to estimate the potential cancer risk from exposureto diesel particulate matter (PM) emitted from diesel-fuel stationaryengines. The methodology was developed to assist in developmentof the StationaryDiesel-Fueled Engine Airborne Toxic Control Measure (ATCM). The estimated risks andassumptions used to determine these risks are not based on a specificengine location or operating parameters. Instead, general assumptionsbracketing a fairly broad range of possible operating scenarios wereused. Exposures were estimatedat varying downwind distances, including the “point of maximum impact”(PMI) as determined using air dispersion modeling. The estimatedrisk ranges are used to provide a “qualitative” assessment of the potentialrisk levels near operating stationary diesel-fueled engines. Actualrisk levels will vary due to site specific parameters, including horsepowerrating and configuration of the engine, emission rates, operating schedules,site configuration, site meteorology, and distance to receptors.Source DescriptionThe following methodologywas developed to provide estimates of the potential cancer risk associatedwith exposures to diesel PM emissions from stationary diesel-fueledengines. Stationary diesel-fueledengines are generally categorized as either prime engines or emergencyback-up engines. Prime engines are used to power equipment suchas compressors, cranes, generators, pumps, and grinders. Emergencyback-up engines are used solely for emergency back-up electric powergeneration or water pumping. The main difference between primeand emergency back-up engines is that prime engines usually operateconsiderably more hours per year. The methodology used inthis risk assessment is consistent with the Tier-1 analysis presentedin the draft Office of Environmental Health Hazard Assessment (OEHHA),Air Toxics Hot Spots Program Risk Assessment Guidelines: The Air ToxicsHot Spots Program Guidance Manual for Preparation of Health Risk Assessments(OEHHA, 2002a). The OEHHA draft guidelines and this assessmentutilize health and exposure assessment information that is containedin the Air Toxics Hot Spot Program Risk Assessment Guidelines, PartII, Technical Support Document for Describing Available Cancer PotencyFactors (OEHHA 2002b); and the Air Toxics Hot Spot Program Risk AssessmentGuidelines, Part IV, Technical Support Document for Exposure Analysisand Stochastic Analysis (OEHHA 2000), respectively.Modeling AssumptionsFor this modeling exercisewe used a matrix of parameters. We modeled engines of 200, 550,and 1500 horsepower, and varied both the emissions rate and the hoursof operation for each horsepower rating. For each engine horsepower,we modeled five diesel PM emission factors: 0.01, 0.15, 0.40, 0.55, and1.0 grams/brake hp-hour. We also varied the hours of operationand evaluated the risks for the following hours of operation: 10, 20,30, 40, 50, 100, 200, 300, 400, 500, and 1000 hours/year. Foreach case we calculated the risk at varying downwind distances. Model UsedThe PM emissions are modeledin this scenario using the United States Environmental Protection Agency’sIndustrial Source Complex Short Term Model – Version 3 (ISCST3 Date:00101). The ISCST3 is an air dispersion model that allows an estimationof the annual average above ambient diesel PM concentrations.1 The potential cancer risk to nearby residential receptors is obtainedby multiplying annual average above ambient concentration of dieselPM by the unit risk factor (URF) for diesel PM (300 excess cancers/ug/m<sup>3over a 70-year exposure period). The results are expressed asan estimate of potential cancer risk in chances per million. Meteorological DataMeteorological data aresite-specific parameters that are used in air dispersion models to calculateconcentrations of emissions and subsequent risk. For this scenario,West Los Angeles, 1981, meteorological data were selected as the inputto the ISCST3 model. The West Los Angeles meteorological datatend to provide higher estimates of risk than most of the other meteorologicaldata sets compiled by ARB. This is because the West Los Angelessite tends to have the lowest average wind speed and more persistentwind directions, which result in less dispersion of pollutants.Model Parameters and Emission FactorsThe key modeling parametersand emission factors are presented in Table 1. We used the ruraldispersion coefficient to provide a more conservative (higher) estimateof the predicted concentration and the estimated potential cancer risk.
Table 1: Modeling and Health Risk Assessment ParametersModeling Paramenters Model ISCST3 (Version 00101)Engine Horsepower (at 100% load) 200 HP, 550 HP, 1500 HPEngine Operation Load 75%Emission Factor 0.01, 0.15, 0.40, 0.55, 1.00 g/bhp-hrOperation Hours (annual) 10, 20, 30, 40, 50, 100, 200, 300, 400, 500, 1000Source Type PointDispersion Setting RuralReceptor Height 1.5 mStack Information: Stack Diameter 4 in, 6 in, and 13 in Stack Height 3 m Stack Temperature 622 K Stack Exhaust Velocity 59.8 m/s, 73.1 m/s, and 42.5 m/sTime Emissions Emitted 3 p.m.Meteorological Data West L. A. (1981)Release Height Same as the stack heightHealth Risk Assessment Parameters Receptor’s Hypothetical Exposure Time 70 years, 50 weeks per yearAdult Daily Breathing Rate Range 271 - 393 l/kg body weight -day 1Adult Body Weight 70 kgDiesel PM Unit Risk Factor 300 excess cancers/μg/m<sup>31. The low end of the breathing rate rangeis the mean of the OEHHA breathing rate distribution and the high endis the 95<sup>th percentile of the distributionResultsWe have included threesets of tables, one set for each modeled horsepower (200, 500, and 1500). Each set of tables contains five sub-tables, one for each emission factor(0.01, 0.15, 0.40, 0.55 and 1.0 g/bhp-hr). Each emission factortable comprises a matrix of downwind distances and hours of operation,with the calculated risks for each combination. The low-end andhigh-end of the risks presented in the tables are corresponding to the65<sup>th (mean) and 95<sup>th percentile breathing rates,respectively. Additionally, the tables are coded using variedlevels of shading. The moderately shaded squares denote the low-endpotential cancer risks of between one and ten per one million people. The darkest squares show the low-end risk levels between 11 and 100potential cancer cases per million. The white squares show thehighest calculated risks, those exceeding 100 potential cases per millionpeople. As can be seen, the estimated cancer risk from stationarydiesel-fueled engines varies depending on the emission rate, horsepowerand annual hours of operation for a given engine.Estimated risk as a function of emission factor:For the range of enginehorsepowers modeled, all those engines that emitted0.01 g/bhp-hr or less could run at least 1000 hoursper year without exceeding the lowest range of estimated risks, thoseof 10 or less potential cancer cases per year. For the 0.15 g/ bhp-hrengines, most combinations of horsepower, hours of operation and downwinddistance did not exceed the lowest range of risks, with those combinationsresulting in the higher risk ranges occurring at 200-plus operatinghours and low to moderate downwind distances. For engines with emissionsof 0.4 g/bhp-hr or more the trend was to find higher risks at low tomoderate downwind distances and longer operating times continues, withthe proportion of moderate to high risk level results increasing asemission factors increase.Estimated risk as a function of hours of operation:Generally, as the hoursof operation increased, the number of engines that exceeded the lowestrisk range increased. However, most engines could operate for10 to 20 hours per year without exceeding the lowest range of risk. Estimated risk as a function of horsepower:For the engine configurationsevaluated in these scenarios, the smaller horsepower engine (200 hp),typically demonstrated higher near source risk for a given number ofhours of operation than the larger engines. In addition, the potentialcancer risk reached the point of maximum impact more rapidly for the200 hp engine than the larger engines. The larger engines hadthe point of maximum impact further from the engine due to the greaterplume dispersion that occurs with the large horsepower engines.
REFERENCES:Office of EnvironmentalHealth Hazard Assessment. Air Toxics Hot Spots Program Risk Assessment Guidelines: TheAir Toxics Hot Spots Program Guidance Manual for Preparation of HealthRisk Assessments; 2002. (OEHHA, 2002a)Office of EnvironmentalHealth Hazard Assessment. Air Toxics Hot Spots Program Risk Assessment Guidelines, Part II,Technical Support Document for Describing Available Cancer Potency Factors;2002. (OEHHA, 2002b)Office of EnvironmentalHealth Hazard Assessment. Air Toxics Hot Spots Program Risk Assessment Guidelines, Part IV,Technical Support Document for Exposure Analysis and Stochastic Analysis;2000. (OEHHA, 2000)
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