Environmental Epidemiology

1. Chapter 2
Environmental Epidemiology

2. Learning 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. What is Environmental
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 Smoking is not a typical focus but the effects of
smoking on others that choose not to smoke would
be.
o A major concern in the field is causality

4. Contributions to
Environmental Health
• Concern with populations
o Environmental epidemiology studies a population in
relation to morbidity and mortality.
o Sometimes called population medicine
• Example: Is lung cancer mortality higher in areas with higher
concentrations of “smokestack” industries?
• 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. Contributions to
Environmental Health
• Methodology for Study Designs
o Generally aims to portray the frequency of disease occurrence in
the population or to link disease outcomes to specific exposures.
Characteristic study designs used frequently in environmental
epidemiology
• Cross-sectional
• Ecologic
• Case-control
• Cohort

6. Two Classes of
Epidemiologic Studies
• Descriptive
o Depiction of the occurrence of disease in populations according to
classification by person, place, and time variables.
o An example of a pattern derived from descriptive studies is disease
clustering
• Grouped series of events or cases of a disease with well defined
distribution patterns in a certain time or place
• Analytic
o Examines causal (etiologic) hypotheses regarding the association
between exposures and health conditions.
• Proposes and evaluates causal modals that employ both
outcome variables and exposure variables.
• One approach to this is to take advantage of natural occurring
events (natural experiments)

7. Measures of Disease
Frequency
• 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

8. Measures of Disease
Frequency
• Incidence
o The measure of the risk of new disease or mortality within a defined period
of observation in a specific population. (e.g., a week, month, year, or
other time period)
• 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)

9. Measures of Disease
Frequency
• 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”

10. Major Historical Figures:
• John Snow
o An English anesthesiologist who linked a cholera outbreak in
London to contaminated water from the Thames River in the mid-
1800s.
o Snow employed a “natural experiment,” a methodology used
currently in studies of environmental health problems.
• Sir Percival Pott (1714-1788)
o London surgeon
o Thought to be the first individual to describe environmental cause
of cancer
• Chimney sweeps had high incidence of scrotal cancer due to
contact with soot.

11. Study Designs Used in
Environmental 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.

12. Study Designs Used in
Environmental Epidemiology
Cross-sectional
• involve observation of all of a population, or a
representative subset, at one specific point in
time
Ecologic
• unit of analysis is a population rather than an
individual

13. Study Designs Used in
Environmental Epidemiology
Case-Control
• compares subjects who have that
condition/disease with patients who do not
have the condition/disease but are otherwise
similar
Cohort
• largely about the life histories of segments of
populations, and the individual people who
constitute these segments

14. Odds Ratio (OR)
• A measure of association between exposure
and outcome 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].”

15. Odds Ratio Equation
Note that an OR >1 (when statistically significant)
suggests a positive association between exposure and
disease or health outcome.

16. 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.
o Used in Cohort Studies

17. RR 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.

18. Study 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

19. What is the
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.

20. 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.”

21. Causality
• Certain criteria need to be taken into account in
the assessment of a causal association between
an agent factor (A) and a disease (B).
• See more detailed definitions in the text

22. Bias in Environmental
Epidemiologic 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 (Results in a distortion of a measure)

23. Limitations of
Epidemiologic Studies
• Long latency periods
• Low incidence and
prevalence
• Difficulties in exposure
assessment
• Nonspecific effects

24. Conclusion
Environmental Epidemiology is one of the fields that research
fundamental questions regarding the role of environmental
exposures in human health.
Table 2-6 lists the general characteristics, weaknesses, and
strength of Environmental Epidemiology