Chapter 2Chapter 2
Environmental Epidemiology
Learning ObjectivesLearning Objectives
By the end of this chapter the reader will be able to:
• Define the term environmen...
What is EnvironmentalWhat is Environmental
Epidemiology?Epidemiology?
• The study of diseases and health conditions
(occur...
Contributions toContributions to
Environmental HealthEnvironmental Health
• Concern with populations
o Environmental epide...
Contributions toContributions to
Environmental HealthEnvironmental Health
• Methodology for study designs
o Characteristic...
Odds Ratio (OR)Odds Ratio (OR)
• A measure of association for case- control
studies.
• Exposure-odds ratio:
o Refers to “…...
Odds Ratio EquationOdds Ratio Equation
Note that an OR >1 (when statistically significant)
suggests a positive association...
Relative Risk (RR)Relative Risk (RR)
• The ratio of the incidence rate of a disease or
health outcome in an exposed group ...
RR EquationRR Equation
A
A + B
RR =
C
C + D
Notes:
When an association is statistically significant:
RR >1 indicates that ...
Two Classes ofTwo Classes of
Epidemiologic StudiesEpidemiologic Studies
• Descriptive
o Depiction of the occurrence of dis...
Measures of DiseaseMeasures of Disease
FrequencyFrequency
• Prevalence
o Refers to the number of existing cases of a disea...
Measures of DiseaseMeasures of Disease
FrequencyFrequency
• Incidence
o The measure of the risk of new disease or mortalit...
Measures of DiseaseMeasures of Disease
FrequencyFrequency
• Case fatality rate
o Provides a measure of the lethality of a ...
Major Historical Figure:Major Historical Figure:
John SnowJohn Snow
• An English anesthesiologist who linked a cholera
out...
Study Designs Used inStudy Designs Used in
Environmental EpidemiologyEnvironmental Epidemiology
Experimental
•In epidemiol...
Study EndpointsStudy Endpoints
In evaluating health effects of occupational
exposures to toxic agents, researchers may
stu...
What is theWhat is the
Epidemiologic Triangle?Epidemiologic Triangle?
• Used for describing
the causality of
infectious di...
Corners of the “Triangle”Corners of the “Triangle”
• The term environment is defined as the domain in
which disease-causin...
CausalityCausality
• Certain criteria need to be taken into
account in the assessment of a causal
association between an a...
Bias in EnvironmentalBias in Environmental
Epidemiologic StudiesEpidemiologic Studies
• A skew in the availability of data...
Limitations of EpidemiologicLimitations of Epidemiologic
StudiesStudies
• Long latency periods
• Low incidence and
prevale...
ConclusionConclusion
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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
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