Environmental Epidemiology


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Environmental Epidemiology

  1. 1. Chapter 2Chapter 2 Environmental Epidemiology
  2. 2. Learning ObjectivesLearning Objectives By the end of this chapter the reader will be able to: • Define the term environmental epidemiology • Describe three major historical events in environmental epidemiology • Provide examples of epidemiologic tools used in environmental health • Identify types of associations found between environmental hazards and health outcomes
  3. 3. What is EnvironmentalWhat is Environmental Epidemiology?Epidemiology? • The study of diseases and health conditions (occurring in the population) that are linked to environmental factors. o These exposures usually are involuntary • Smoking versus secondhand smoke o A major concern in the field is causality
  4. 4. Contributions toContributions to Environmental HealthEnvironmental Health • Concern with populations o Environmental epidemiology studies a population in relation to morbidity and mortality. • Use of observational data o Epidemiology is primarily an observational science that takes advantage of naturally occurring situations in order to study the occurrence of disease. • Immoral to conduct experiments like these on humans
  5. 5. Contributions toContributions to Environmental HealthEnvironmental Health • Methodology for study designs o Characteristic study designs used frequently in environmental epidemiology: • Cross-sectional o involve observation of all of a population, or a representative subset, at one specific point in time • Ecologic o unit of analysis is a population rather than an individual • Case-Control o compares subjects who have that condition/disease with patients who do not have the condition/disease but are otherwise similar • Cohort o largely about the life histories of segments of populations, and the individual people who constitute these segments
  6. 6. Odds Ratio (OR)Odds Ratio (OR) • A measure of association for case- control studies. • Exposure-odds ratio: o Refers to “… the ratio of odds in favor of exposure among the cases [A/C] to the odds in favor of exposure among the non- cases [the controls, B/D].”
  7. 7. Odds Ratio EquationOdds Ratio Equation Note that an OR >1 (when statistically significant) suggests a positive association between exposure and disease or health outcome.
  8. 8. Relative Risk (RR)Relative Risk (RR) • The ratio of the incidence rate of a disease or health outcome in an exposed group to the incidence rate of the disease or condition in a non-exposed group.
  9. 9. RR EquationRR Equation A A + B RR = C C + D Notes: When an association is statistically significant: RR >1 indicates that the risk of disease is greater in the exposed group than in the nonexposed group. RR<1 indicates possible protective effect.
  10. 10. Two Classes ofTwo Classes of Epidemiologic StudiesEpidemiologic Studies • Descriptive o Depiction of the occurrence of disease in populations according to classification by person, place, and time variables. • Analytic o Examines causal (etiologic) hypotheses regarding the association between exposures and health conditions.
  11. 11. Measures of DiseaseMeasures of Disease FrequencyFrequency • Prevalence o Refers to the number of existing cases of a disease, health condition, or deaths in a population at some designated time • Point prevalence o Refers to all cases of a disease, health condition, or deaths that exist at a particular point in time relative to a specific population from which the cases are derived. Number of persons ill Point Prevalence = at a point in time Total number in the group
  12. 12. Measures of DiseaseMeasures of Disease FrequencyFrequency • Incidence o The measure of the risk of new disease or mortality within a defined period of observation in a specific population. • The population at risk • Incidence Rate o the number of new cases per population in a given time period Number of new cases Incidence Rate = over a time period Total population at risk x multiplier (e.g., 100,000)
  13. 13. Measures of DiseaseMeasures of Disease FrequencyFrequency • Case fatality rate o Provides a measure of the lethality of a disease. Number of deaths due to disease “X” CFR (%) = x 100 during a time period Number of cases of disease “X”
  14. 14. Major Historical Figure:Major Historical Figure: John SnowJohn Snow • An English anesthesiologist who linked a cholera outbreak in London to contaminated water from the Thames River in the mid-1800s. • Snow employed a “natural experiment,” a methodology used currently in studies of environmental health problems.
  15. 15. Study Designs Used inStudy Designs Used in Environmental EpidemiologyEnvironmental Epidemiology Experimental •In epidemiology, most are Intervention Studies o Involves intentional change in some status of the subjects o Some experimental designs are randomly assigned while others are not (quasi-experimental) •This type of study is not as practical as observational methods Case Series •Information is gathered about patients that have a particular disease over time.
  16. 16. Study EndpointsStudy Endpoints In evaluating health effects of occupational exposures to toxic agents, researchers may study various endpoints using measures derived from: •Self-reported symptom rates •Physiologic or clinical examinations •Mortality
  17. 17. What is theWhat is the Epidemiologic Triangle?Epidemiologic Triangle? • Used for describing the causality of infectious diseases • Provides a framework for organizing the causality of other types of environmental problems Source: Reprinted with permission from RH Friis, TA Sellers. Epidemiology for Public Health Practice. 4th ed. Sudbury, MA: Jones and Bartlett Publishers; 2009:439.
  18. 18. Corners of the “Triangle”Corners of the “Triangle” • The term environment is defined as the domain in which disease-causing agents may exist, survive, or originate; it consists of “All that which is external to the individual human host.” • A host is “a person or other living animal, including birds and arthropods, that affords subsistence or lodgment to an infectious agent under natural conditions.” • Agent refers to “A factor, such as a microorganism, chemical substance, or form of radiation, whose presence, excessive presence, or (in deficiency diseases) relative absence is essential for the occurrence of a disease.”
  19. 19. CausalityCausality • Certain criteria need to be taken into account in the assessment of a causal association between an agent factor (A) and a disease (B).
  20. 20. Bias in EnvironmentalBias in Environmental Epidemiologic StudiesEpidemiologic Studies • A skew in the availability of data, such that observations of a certain kind may be more likely to be reported and consequently used in research. • The healthy worker effect o Employed workers have lower mortality rates than general populations o The healthy worker effect could introduce selection bias into occupational mortality studies. • Confounding o The existence of other factors that contribute to the outcome of the study
  21. 21. Limitations of EpidemiologicLimitations of Epidemiologic StudiesStudies • Long latency periods • Low incidence and prevalence • Difficulties in exposure assessment • Nonspecific effects
  22. 22. ConclusionConclusion