EOH 2504, Principals of Environmental Exposure: Lecture 5, Strategies for and Design of Exposure Assessment Studies

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1 EOH 2504, Principals of Environmental Exposure: Lecture 5, Strategies for and Design of Exposure Assessment Studies Conrad (Dan) Volz, DrPH, MPH Bridgeside Point 100 Technology Drive Suite 564, BRIDG Pittsburgh, PA Office phone; Cell phone; Fax; University of Pittsburgh address; Assistant Professor, Environmental and Occupational Health, University of Pittsburgh, Graduate School of Public Health: Director-Principal Investigator - Center for Healthy Environments and Communities (CHEC): Director, Environmental Health Risk Assessment Certificate Program

2 Design of an exposure study specifies the procedures that will be used to answer these three questions. * Magnitude: What is the pollutant concentration? * Duration: How long does the exposure last? * Frequency: How often do exposures occur?

3 Careful selection of study objectives 1. 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 Epidemiology, Risk Assessment, Risk Management or Analyses of Exposure Status and Trends/ Preliminary or Detailed Study.

4 Link objectives to measurement parameters in a cost-effective manner. Two critical and often overlooked elements of the study design are development of a statistical analysis plan and quality assurance (QA) objectives. For general population studies, methods for measurement and analysis of contaminants in collected environmental or biological samples must be sufficiently sensitive to determine their concentration at typical ambient levels. For multimedia studies, method detection limits must be consistent across media. Always pilot the study before committing to the study methodology. Critical Elements of an Exposure Assessment Study

5 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-cancer endpoints from ingestion of well water possibly contaminated with arsenic leaching from Little Blue CCW landfill//epa document -coal-combustion-waste-risk-assessment.pdf calculates 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.

6 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.

7 Preliminary DEP Data Analysis to Determine Study Area and Population Potentially Affected (Redox Conditions )to Identify Sampling Area

8 Sampling 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.

9 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.

10 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).

11 Comprehensive samples: Advantages/Disadvantages Advantages of this type of study are 1. 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 the expense 2. all individuals in all locations must be monitored at all times.

12 Choosing a Probability Sample for an Exposure Assessment Study choose a population for investigation choose an appropriate unit for sampling and analysis (e.g., person, household, neighborhood, city, etc.) stratify as appropriate choose a sampling strategy (e.g., simple random sampling, multistage sampling).

13 Probability Sampling Continued Advantages; 1. make general statements about the population under investigation. The advantages 2. having results that represent the population, taking into account the possible error due to sampling. Disadvantages; 1. the complicated sample selection 2. difficulty in maintaining compliance from participants and 3. the potentially complex statistical analysis. 4. randomized surveys of insufficient sample size may miss rare hazardous events or small populations with high exposure or risk.

14 Stratified Sampling Stratified sampling may be used to obtain more precise survey results if exposures are more homogeneous within strata than between them. Possible strata include urban, suburban and rural populations, or occupationally exposed and non-occupationally exposed individuals.

15 Oversampling Oversampling of target populations or contaminants also may yield substantial increases in the precision of results. Because the individuals anticipated to have the highest exposures to a particular pollutant may be rare in the population being studied, oversampling can be considered to obtain more precise estimates of exposure. Before committing substantial resources to oversampling, special care must be taken to ensure that assumptions or data used to support a rationale for selecting the oversampled population are accurate; otherwise erroneous oversampling may decrease the precision of the study results (Callahan et al., 1995).

16 Multistage Sampling Methodology Multistage sampling designs utilize clusters of sampling units thereby limiting sampling locations to manageable areas. Depending on the scope of the study, the stages of probability sampling necessary may include: 1. selection of primary sampling unit (e.g., a city) 2. selection of sample area segments (e.g., blocks within the city) 3. selection of sample housing units within sample segments (e.g., residences within the blocks) 4. selection of sample individuals within sample housing units 5. selection of sample time points within the monitoring period (Callahan et al., 1995).

17 Non-Probability Sampling-Anecdotal and Convenience Sampling Consists of selecting a sample based on the selfreporting of conditions, such as complaint cases for "sick building" syndrome. Data collected in this manner are potentially subject to biased reporting. It is difficult to generalize results unless causal relationships are very strong or unless there is little reason to believe that a confounder or an unmeasured significant factor is relevant. In general, such studies are used for description or exploration of a given situation.

18 Advantages and Disadvantages on Non-Probability Samples Advantages of targeted anecdotal studies are: 1. The inexpensive and quick ways in which they aid in the design of future studies. For example, when exploring protocols, determining stratification variables, potential biases and confounders, and identifying the units of analysis, the use of cooperative volunteers can simplify field operations. Disadvantages 1. The uncertainty of the results of these studies is due to potential biases from the nonrandom and possibly non-representative sample (i.e., responder bias). 2. Since the population in such non-probability sample studies is often made up of volunteers, there is usually some factor present which distinguishes them from those who do not choose to participate. This factor could influence the results; in particular, those who participate may tend to consider themselves strongly affected or not affected by the pollutant being studied and may alter their responses or behaviors as a result. This phenomenon is a special case of responder bias, often termed self-selection bias. 3. Also, a poorly designed study can fail to control for temporal and spatial variability, as well as meteorological, site and source bias. This bias is a result of a single, "random-day", or grab sampling and single-location sampling, which decreases the potential or generalization.

19 Exposure Assessment Approaches Direct approaches include personal exposure monitoring and biological markers of exposure. Indirect approaches include environmental sampling- combined with exposure factor information, modeling and questionnaires.

20 Direct-Personal Monitoring Approaches Exposure route Media Environmental sample Biological sample Inhalation air personal monitor breath /urine Ingestion water tap water blood Ingestion food duplicate portion feces/breast milk Dermal soil/dust dermal patch blood/urine/fat skin wipe

21 Case Study-Modeling of Criteria Pollutant PM 2.5 in Pittsburgh Region-Mean of 10 year means Is this exposure? Figure PM 2.5 (ug/m 3 ) mean of annual means air concentrations by air monitoring stations throughout southwestern Pennsylvania. Inverse distance weighting was applied to formulate interpolation values. Coal fired power plants (CFPP_Events) are depicted as points. Basemap includes boundaries, places and transportation from ESRI s Online Services (ERSI, 2009; USEPA, 2009a). The EPA s 24 hour PM 2.5 regulatory standard is 15 µg/m 3 (USEPA, 2009a).

22 Case Study Modeling of Mercury Emissions from EPA TRI Sites (2006) Using Inverse Distance Weighting Deterministic Methodology Is this Exposure and if it isn't what is it? Figure total mercury air emissions in pounds from EPA listed Toxics Release Inventory (TRI) sites. Inverse distance weighting was applied to formulate interpolative values. EPA Hg emission point sources are depicted as blue circles. Basemap includes Pennsylvania, West Virginia, and Ohio boundaries, places, transportation and hydrography ESRI s Online Services (ERSI, 2009; USEPA, 2009b).