A Preliminary Risk-Based Screening Approach For Air Toxics .

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A PreliminaryRisk-Based ScreeningApproach for Air ToxicsMonitoring Data Sets

U.S. Environmental Protection AgencyAir, Pesticides, and Toxics Management DivisionAtlanta, Georgia xic/October 2010(Version 2)Cover Photo: Greg Noah/EPA

DisclaimerThe information and procedures set forth here are intended as a technical resource to thoseconducting risk-based evaluations of air toxics monitoring data. This document does notconstitute rulemaking by the Agency, and cannot be relied on to create a substantive orprocedural right enforceable by any party in litigation with the United States. As indicated by theuse of non-mandatory language such as “may” and “should,” it provides recommendations anddoes not impose any legally binding requirements. In the event of a conflict between thediscussion in this document and any Federal statute or regulation, this document would not becontrolling.The general description provided here may not apply to a particular situation based upon thecircumstances. Interested parties are free to raise questions and objections about the substance ofthis methodology and the appropriateness of its application to a particular situation. EPA Region4 and other decision makers retain the discretion to adopt approaches on a case-by-case basis thatdiffer from those described in this document where appropriate. EPA Region 4 may take actionthat is at variance with the recommendations and procedures in this document and may changethem at any time without public notice. This is a living document and may be revisedperiodically. EPA Region 4 welcomes public input on this document at any time. Commentsshould be sent to Dr. Kenneth Mitchell (mitchell.ken@epa.gov).version 2

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PART I:BACKGROUNDWhat This Preliminary Screening-LevelMethodology Is NotThe purpose of this document is to provide arisk-based methodology for performing aninitial screen of air toxics monitoring datasets in outdoor air. This methodology isnecessary because:1.Many Region 4 State, local, andtribal (R4 SLT) air agencies havebeen collecting air toxics data for anumber of years;2.These Agencies want to evaluate thedata sets to determine what theresults indicate with regard to thepotential for exposures of potentialpublic health concern;3.4.This preliminary screening-level methodology is nota substitute for a thorough risk assessment. Instead,the application of this process will commonly resultin a “short list” of chemicals and geographiclocations that should be the focus of more rigorousrisk evaluation. This short list of chemicals arecharacterized in this document as posing exposuresof potential public health concern and is only meantto imply that the chemicals failed the screeninganalysis. To clarify the actual level of concern posedby any given chemical that fails the screen willnecessarily require a more in-depth risk analysis andmay even require the collection of additional data.(Analysts may decide to carry all detected chemicalsthrough a subsequent risk assessment, whether theyfail the screen or not. While this is somewhat morework, the availability of computer tools such asspreadsheets and databases make this a relativelytrivial exercise. Carrying all chemicals through therisk assessment process also has the benefit of furtherclarifying for stakeholders which chemicals are thelikely risk drivers and which are likely not.)The risk-based approaches forevaluating air toxics have madesignificant strides in recent years;however, many R4 SLTs are still inthe process of developing theirexpertise in this area. This maturingexpertise, as well as resource issues,have had the effect of hinderingmany R4 SLTs in their efforts todevelop a detailed risk evaluation oftheir monitoring data sets;Ultimately, this methodology is not an end in itself.Instead, it should be viewed as a tool that can helpnarrow the focus of SLTs to important chemicals andlocations as they work to strengthen their riskassessment skills.5.As they work to develop their riskassessment expertise [e.g., bybecoming more familiar with the fulldetails of the EPA's Air Toxics RiskAssessment (ATRA) ReferenceLibrary1], R4 SLTs need a concisemethodology that they can use toefficiently screen existing monitoringdata sets to identify whether anychemicals are potentially posingexposures of public health concern inspecific geographic areas;There is a need to standardize theprocedures used by R4 SLTs toproduce uniform risk-based screeningresults. This document presents astep in that direction.It is expected that the application of thisscreening-level methodology by R4 SLTswill allow them to better address air toxicsissues by focusing their limited resources forfurther analysis only on those geographicareas and chemicals for which the availabledata indicate a potential for exposures ofpublic health concern. The method may alsoprovide a risk basis for a decision to1version 2

would come to the same conclusion. Anychemicals that do not pass the screeningcriteria would become the primary focus forany number of follow-up activities.continue (or not continue) a givenmonitoring effort. For example, monitoringsites that consistently indicate a lowpotential for exposures of public healthconcern, by application of this screeningmethodology, might reasonably bediscontinued and the monitoring resourcesshifted to other locations. This methodologywill also help R4 SLTs better understand thedata quality objectives (DQOs) thatmonitoring studies should meet for theresults to be used in a risk-based decisionmaking framework.For example, decision makers might choose,based on the screening level results, toperform a more extensive analysis of thesefailing chemicals to help confirm or deny theoutcome of the screening level assessment.Specifically, a likely next step an analystwill generally recommend for chemicalsfailing the screen is to develop more rigorousestimates of potential exposure, such as 95%upper confidence limits (95% UCL) of thearithmetic mean using the full set ofmonitoring data, as described in the ATRAReference Library, Volume 1, Appendix I.The analyst may also recommend theapplication of an exposure model (seewww.epa.gov/ttn/fera), and may alsoindicate a need for additional air qualitymonitoring or air dispersion modeling tohelp clarify potential exposures and risks.It should be noted that performing thisscreening-level methodology in an adequatefashion necessarily requires the analyst tohave already learned some of thefundamentals of risk assessment (e.g.,understanding data quality requirements forair toxics monitoring data sets used in a riskbased decision making framework). To thatend, this document attempts to point analyststo key references that they should be familiarwith as they apply the methodology.A.In some circumstances, decision makers maychoose “action oriented” alternatives torespond to the screening results. Forexample, consider a screening levelassessment that identifies a chemical ofpotential public health concern that canreadily be linked to a specific source. Ifthere are inexpensive and available riskreduction options for the emission source,the decision makers may simply choose totake actions to reduce potential exposures tothat chemical rather than perform furtheranalysis.Overview of the Screening-LevelMethodologyThe basic concept behind this risk-basedinitial screening level methodology is toevaluate air monitoring data sets using aframework that is, by design, relativelysimple to perform yet conservative (i.e.,health protective) in nature. This initialscreening methodology is designed, throughthe use of conservative decisions, to identifypollutants for which risks are unlikely to beof concern. Accordingly, if all of themonitoring data "pass the screen" using thisapproach, the analyst may be able toconclude that the monitoring results areindicative of acceptably low risk and that amore robust analysis (were one to be done)The basic steps of the screening process areoutlined below. The details of each of thesesteps are discussed in detail in the sectionsthat follow.2version 2

1.Identify the monitoring data sets tobe screened and the geographic areasand time frames that the monitoringdata in question represent.2.Assess the data to determine if theyare of sufficient quantity and qualityto perform the screen.3.For each chemical detected at leastonce in the data set, create astatistical summary of the monitoringresults for that chemical. Thestatistical summary will commonlyinclude the following: Number ofvalid samples collected andfrequency of detection, the methoddetection limits (MDLs), and rangeof detected values.4.For each detected chemical in thedata set, compare the maximummonitored value to the suggestedchronic screening level valueprovided in Appendix A and theacute values provided in Appendix B(the basis for using the maximumvalue found as a surrogate forexposure is provided in Part I,Section D below). Summarize theresults of the comparison process in atable. Highlight chemicals whosemaximum monitored values exceedtheir respective screening values(chronic and acute). For eachchemical whose maximum monitoredvalue exceeds a screening value,review the full data set and determinethe percentage of detections that areat or above the screening value.5.Augment the results described inStep 4 with ancillary informationabout chemicals that fail the screen(e.g., possible sources, applicableregulations, estimated backgroundconcentrations, NATA national scaleassessment results for the geographicarea, etc.).6.Describe areas of uncertainty in theanalysis.7.Based on the screening resultsprovided in Step 4, the ancillary datadeveloped in Step 5, and theuncertainty analysis developed inStep 6, develop a written descriptionof the analysis, including a discussionabout the possibility that a publichealth threat exists that requiresfurther analysis. Include in thisdiscussion an overall statement of theconfidence in the results.Systematic PlanningSystematic Planning is necessary to define the type,quantity, and quality of data a decision maker needsto make a decision and is performed before collectingor generating environmental data. The Data QualityObjectives (DQO) Process is an example of asystematic planning process that assessors would useto translate a decision maker’s aversion to decisionerror into a quantitative statement of data qualityneeded to support a decision. EPA requires that asystematic planning process such as the DQO processbe used for all EPA environmental data collectionactivities.For more information on EPA’s quality program, seewww.epa.gov/quality.3version 2

These steps are shown pictorially in Exhibit1. An example is provided in Appendix D toillustrate how to apply this methodology toan air toxics monitoring data set.EXHIBIT 1Flow Diagram for Preliminary Risk-BasedScreening of Air Toxics Monitoring DataSTEP 1Identify Monitoring Data Sets to be ScreenedAt the end of the screening process, theanalyst will generally have sorted thedetected chemicals at each monitor into twogroups. The first group consists ofchemicals that “pass the screen.” Thesechemicals are below screening levelconcentrations for both chronic and acuteexposures. Decision makers may decide topursue evaluation of these chemicals nofurther.STEP 2Evaluate the Quantity and Quality of Data forScreening-level AnalysisSTEP 3Develop Monitor-Specific Statistical Summaries forEach Chemical Detected at Each MonitorThe second group consists of chemicals that“fail the screen.” These chemicals are at orabove screening level concentrations forchronic and/or acute exposures. Thesechemicals, at a minimum, will commonlyrequire a more in-depth analysis (e.g., a moredetailed risk assessment) to clarify thepotential risks associated with the monitoredconcentrations.STEP 4Screen Chemicals Against Chronic and Acute ScreeningValues - Highlight Chemicals that Fail the ScreenSTEP 5Identify Ancillary Data for Failing ChemicalsSTEP 6Evaluate Uncertainties in the AnalysisAs noted previously, all detected chemicalscan easily be carried forward to the full riskassessment given the available computertools to automate the process and theanalysts may choose to do so. The benefit ofcarrying all detected chemicals forward is tofurther clarify which chemicals are the likelyrisk drivers and which are likely not. Thiswill also help avoid a potentialmisperception by some stakeholders thatanalysts are trying to “hide important data.”STEP 7Write-up Analysis, including Statement of OverallConfidence in the Resultschemical in the air to which a person couldbe continually exposed for a lifetime(assumed to be 70 years) and which wouldbe unlikely to result in a deleterious effect(either cancer or noncancer health effects).B. Derivation of Chronic ScreeningValuesThe suggested chronic screening values usedin this methodology are presented inAppendix A. The starting point for theIn this methodology, a chronic screeningvalue is used to indicate a concentration of a4version 2

derivation of these screening values is theOffice of Air Quality Planning andStandards’ (OAQPS) list of recommendedchronic inhalation toxicity values for theHazardous Air Pollutants (HAPs).2Specifically, the methodology uses theOAQPS recommended inhalation unit risk(IUR) value for cancer causing agents andinhalation reference concentration (RfC) fornoncancer health effectsa as a starting pointand performs the following manipulations toderive a final chronic screening value:i.Chronic vs. AcuteWhat’s the Difference?Chronic exposure is continuous or multipleexposures that occur over an extended period oftime or a significant fraction of an animal’s orperson’s lifetime.Chronic health effects are effects that occur as aresult of repeated or long term (chronic) exposures(IRIS definition).Acute exposure is one or multiple exposuresoccurring within a short time frame relative to thelifetime of an animal or person (e.g., approximately24 hours or less for humans).Chronic screening value for“noncancer” (and in some cases,cancer) health endpoints. For the“noncancer” screening value (whichin some cases, is also a cancerscreening value), the chronic RfCswere used as a starting point sincechronic RfCs are, by definition, anestimate of the concentration of achemical in the air to whichcontinuous exposure over a lifetimeis expected to result in littleappreciable deleterious effects to thehuman population, includingsensitive subgroups. However, mostambient air contains a mixture ofchemicals which may result in acumulative hazard that is notaccounted for by assessing chemicalson an individual basis. To accountfor possible exposure to multiplecontaminants, the noncancer chronicscreening value for each chemicalwas selected to be one tenth of itschronic RfC [i.e., (0.1) x (RfC) x(1000)]. Noncancer screening valuesare presented in Appendix A as an airAn acute health effect may occur within a shortperiod of time following an acute exposure, forexample, minutes to a few days. (Some acuteexposures may also lead to chronic health effects.)The ATRA Library, Volume 1, Chapter 12 providesdetails on chronic vs acute toxicity data.concentration in ug/m3. (Since RfCsare reported as mg/m3 in the OAQPStable, multiplication by 1000 isnecessary to convert mg to ug).Calculating the noncancer screeningvalues in this fashion is conservativesince it is unlikely that a personwould be continuously exposed overa lifetime to 10 chemicals that behavein a toxicologically similar manner.bii.Chronic screening value for cancerhealth endpoints. For cancer, theIUR for a chemical is used as astarting point to derive an airbThis rationale has been previouslyemployed by Region III Superfund programin their table of risk based concentrations -aNote that some RfCs are developedto be protective of both cancer andnoncancer health ndex.htm.5version 2

Chapter 12 (Section 12.7) of Volume1 of the ATRA Reference Librarydiscusses various approaches todealing with chemicals that have notoxicity information.concenration corresponding to aspecific individual cancer risk level.In this methodology, the cancerscreening risk level was selected asone in one million (written 1E-06 or1x10-6) which is the lower end of thecancer risk range cited in the 1989Benzene NESHAP (approximately1E-04 to approximately 1E-06) as therange of risk used for regulatorydecision making for the air toxicsprogram.3 The 1E-06 level of riskwas also selected to take into accountthe potential for simultaneousexposure to multiple carcinogens.Specifically, one would have toexperience the unlikely scenario ofcontinuous lifetime exposure to 100cancer causing agents (all at aconcentration corresponding to a risklevel of 1E-06) to approach the upperend of the above noted risk range(approximately 1E-04). The chronicscreening value for cancer iscalculated by simply dividing theIUR into a risk of one in a million[(1E-6)/(IUR)]. Cancer screeningvalues are presented in Appendix Aas air concentrations in ug/m3.iii.The suggested screening levels in thismethodology were selected for the reasonsstated above and because this approach hasprecedent in other risk-based environmentalprograms (see footnote b). If a SLT decidesto use different screening levels, it isencouraged to document why it chose analternate value and why the alternate value isin line with the screening level concept (i.e.,a simple approach counterbalanced withconservative inputs and decision criteria).[NOTE: The OAQPS Toxicity Values tablesare not static and changes are made fromtime to time which may not be reflected inthe current version of this screening levelmethodology. Analysts are encouraged toroutinely review the OAQPS ToxicityValues tables for changes and to adjust thescreening levels presented here, asnecessary. This applies to both chronic andacute values presented in Appendices A andB.]C. Derivation of Acute Screening ValuesFinal chronic screening value forboth cancer and noncancer effects.The final chronic screening value fora chemical is simply the lower of theconcentration values calculated inSteps i and ii above. The finalchronic screening values arepresented as an air concentration inug/m3. A quick review of AppendixA shows that a number of chemicalshave no final chronic screeningvalue, indicating no data in thetoxicological references upon whichOAQPS relies for toxicity values.Many air pollutants can cause adverse healtheffects after short-term (acute) exposure torelatively high concentrations that last froma few minutes to days. Depending on theexposure circumstances and the chemicalsinvolved, acute exposures may be of greaterconcern than chronic exposures. AppendixB provides a discussion of how to performan acute risk-based screening levelevaluation along with a selection of availableacute toxicity values.6version 2

ATRA reference library. The text box on thenext page describes several commonapproaches for evaluating exposures.D. Issues Regarding Risk-based AnalysisUsing Monitoring DataIn this preliminary risk-based screeningapproach, monitoring data are used torepresent exposure. The screening valuespresented in Appendix A apply tocontinuous lifetime exposures to thegeneral population, including sensitivesubpopulations (even though these values arecommonly derived from studies involvingdiscontinuous exposures). As such, it wouldbe most useful to have monitoring data thatare also representative of the same timeframe (i.e., continuous lifetime exposure).This follows from the general riskassessment principle that the time framesassociated with exposure data and toxicitydata should match in order for the two typesof data to be computationally combined in arisk-based analysis.To avoid having to perform such calculationsfor each chemical detected at a monitor in apreliminary risk-based screen of the typedescribed here, a less onerous, y

air toxics monitoring data sets used in a risk-based decision making framework). To that end, this document attempts to point analysts to key references that they should be familiar with as they apply the methodology. A. Overview of the Screening-Level Methodology The basic concept behind this risk-based initial screening level methodology is to

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