EOH 2504, Principals of Environmental Exposure: Lecture 5, Strategies for and Design of Exposure Assessment Studies Design of an exposure study specifies the procedures that will be used to answer these three questions.Critical Elements of an Exposure Assessment StudyCritical Elements of an Exposure Assessment StudyCase Example—Determination of Well Water Ingestion Risk to Arsenic at Little Blue Coal Combustion Wastesite-Shippinsport PA.Case Example—Determination of Well Water Ingestion Risk to Arsenic at Little Blue Coal Combustion Wastesite-Shippinsport PA.Preliminary DEP Data Analysis to Determine Study Area and Population Potentially Affected (Redox Conditions )to Identify Sampling AreaSampling and GeneralizationComprehensive samplesProbability SamplesComprehensive samples: Advantages/DisadvantagesChoosing a Probability Sample for an Exposure Assessment StudyProbability Sampling ContinuedStratified SamplingOversamplingMultistage Sampling MethodologyNon-Probability Sampling-Anecdotal and Convenience SamplingAdvantages and Disadvantages on Non-Probability SamplesExposure Assessment ApproachesDirect-Personal Monitoring ApproachesSlide Number 21Slide Number 22EOH 2504, Principals of Environmental Exposure: Lecture 5, Strategies for and Design of Exposure Assessment Studies Conrad (Dan) Volz, DrPH, MPH Bridgeside Point100 Technology DriveSuite 564, BRIDGPittsburgh, PA 15219-3130Office phone; 412-648-8541Cell phone; 724-316-5408Fax; 412-624-3040University of Pittsburgh email address; [email protected] Professor, Environmental and Occupational Health, University of Pittsburgh, Graduate School of Public Health: http://www.pitt.edu/~cdv5/Director-Principal Investigator - Center for Healthy Environments and Communities (CHEC): http://www.chec.pitt.edu/Director, Environmental Health Risk Assessment Certificate Program http://www.publichealth.pitt.edu/interior.php?pageID=82Design of an exposure study specifies the procedures that will beused to answer these three questions.* Magnitude: What is the pollutant concentration?* Duration: How long does the exposure last?* Frequency: How often do exposures occur?Critical Elements of an Exposure Assessment StudyEpidemiology, RiskAssessment, Risk Management or Analyses of Exposure Status and Trends/ Preliminary or Detailed Study.Careful selection of study objectives1. Epidemiology- Develop estimates of population exposure for quartiles of exposure range.2. Risk Assessment- Develop probabilistic models of exposure so that sensitivity analysis of risk can be performed for the 95% Confidence Interval of the mean exposure or for deciles of exposure.3. Risk Management- Develop measures of exposure that inform development of institutional, engineering controls or behavioral changes.Is this study being done to test a hypothesis concerning exposure in any area over space and time—detail that hypothesis (null and alternate) and provide conditions of rejection of the null hypothesis.Critical Elements of an Exposure Assessment Study•Link objectives to measurementparameters in a cost-effective manner. •Two critical and oftenoverlooked elements of the study design are development of astatistical analysis plan and quality assurance (QA) objectives. •For general population studies, methods for measurement and analysis ofcontaminants in collected environmental or biological samples must besufficiently sensitive to determine their concentration at typicalambient levels. •For multimedia studies, method detection limits mustbe consistent across media.Always pilot the study before committing to the study methodology.Case Example—Determination of Well Water Ingestion Risk to Arsenic at Little Blue Coal Combustion Wastesite-Shippinsport PA.• Study objectives—1. Determine risk of cancer and non-cancerendpoints from ingestion of well water possibly contaminated with arsenic leaching from Little Blue CCW landfill//EPA document http://www.earthjustice.org/library/reports/epa-coal-combustion-waste-risk-assessment.pdfcalculates humans exposed via the groundwater-to drinking-water pathway, arsenic in CCW landfills poses a 90th percentile cancer risk of 5x10-4 for unlined units.Case Example—Determination of Well Water Ingestion Risk to Arsenic at Little Blue Coal Combustion Wastesite-Shippinsport PA.• Study objectives—2. Determine if exposure varies when well is simply purged of one volume of water (as DEP and Industry perform) or changes as a result of normal usage patterns—grass watering, washing, dish washing, showering etc.Preliminary DEP Data Analysis to Determine Study Area and Population Potentially Affected (Redox Conditions )to Identify Sampling AreaSampling and Generalization• The appropriateness of the generalization is determined by considering if the sample is randomly selected in such a way as to be representative of the larger population of interest (Whitmore, 1988). (Does it have internal validity and external validity?).• For continuous outcomes, the percentages of key attributes, such as demographic factors, should be similar between the sample and the population. • A descriptive study can provide credible data, although the extent to which these can be generalized a very limited.Comprehensive samples• Complete populations can be used to collect a full picture of the process being studied, especially when the total population is relatively small such as families in a neighborhood.• The main reasons for studies of this nature are either a small population size, a need for a complete evaluation of the problem, high potential risk, high variability among units or legal requirements.Probability Samples• Surveys consist of a random sampling of subjects from the population of interest. This approach aims to remove selection bias and is useful for generalizing results beyond the study sample.• A truly random sample is independent of human judgment. Every unit in the total population has a known above-zero likelihood of being included in the sample. Effective study design allows researchers to draw statistically valid inferences about the general population that the sample is designed to represent (Kish, 1965).Comprehensive samples: Advantages/Disadvantages• Advantages of this type of study are1. That a complete description of the exposure is given, and 2. There is no need for generalization because all potential subjects are covered.• Disadvantage of this approach, if the population is large, 1. lies in