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Chapter 2
Environmental Epidemiology
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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
• List study designs used in environmental epidemiology
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What is Environmental
Epidemiology?
• The study of diseases and health conditions
(occurring in the population) that are linked to
environmental factors.
• These exposures usually are involuntary.
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Epidemiology’s Contributions to
Environmental Health
• Concern with populations
• Use of observational data
• Methodology for study designs
• Descriptive and analytic studies
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Concern with Populations
• Environmental epidemiology studies a
population in relation to morbidity and
mortality.
– Example: Is lung cancer mortality higher in areas
with higher concentrations of “smokestack”
industries?
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Use of Observational Data
• Epidemiology is primarily an observational
science that takes advantage of naturally
occurring situations in order to study the
occurrence of disease.
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Methodology for
Study Designs
• Characteristic study designs used frequently in
environmental epidemiology:
– Cross-sectional
– Ecologic
– Case-Control
– Cohort
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Two Classes of
Epidemiologic Studies
• Descriptive
– Depiction of the occurrence of disease in
populations according to classification by
person, place, and time variables.
• Analytic
– Examines causal (etiologic) hypotheses
regarding the association between exposures
and health conditions.
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Measures of Disease Frequency
• Prevalence
• Point prevalence
• Incidence
• Incidence rate
• Case fatality rate
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Prevalence
• Refers to the number of existing cases of a
disease, health condition, or deaths in a
population at some designated time
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Point Prevalence
• 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.
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Formula for Point Prevalence
Number of persons ill
Point Prevalence = at a point in
Total number in the group time
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Incidence
• The occurrence of new disease or mortality
within a defined period of observation (e.g.,
week, month, year, or other time period) in a
specific population.
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Formula for Incidence Rate
Incidence Number of new cases over a time
Rate = period x multiplier
Total population at risk (e.g., 100,000)
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Case Fatality Rate (CFR)
• Provides a measure of the lethality of a
disease.
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Case Fatality Rate (CFR)
(continued)
Number of deaths due to disease “X”
CFR (%) = x 100 during a
Number of cases of disease “X” time period
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Major Historical Figure:
Sir Percival Pott (1714-1788)
• A London surgeon thought to be the first
individual to describe an environmental cause
of cancer.
• Chimney sweeps had high incidence of scrotal
cancer due to contact with soot.
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Major Historical Figure:
John 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.
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Study Designs Used in
Environmental Epidemiology
• Experimental
• Case Series
• Cross-Sectional
• Ecologic
• Case-Control
• Cohort
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Odds Ratio (OR)
• A measure of association for case- control
studies.
• Exposure-odds ratio:
– 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].”
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Odds Ratio Table
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Odds Ratio Equation
Note that an OR >1 (when statistically significant)
suggests a positive association between exposure and
disease or health outcome.
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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.
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Relative Risk (RR) Table
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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.
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Study Endpoints
• Self-reported symptom rates
• Physiologic or clinical examinations
• Mortality
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Figure 2-5 The epidemiologic triangle.
Source: Reprinted with
permission from RH Friis, TA
Sellers. Epidemiology for Public
Health Practice. 4th ed.
Sudbury, MA: Jones and
Bartlett Publishers; 2009:439.
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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
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Environment in 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.”
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Host in the “Triangle”
• 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.”
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Agent in the “Triangle”
• 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.”
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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).
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Hill’s Criteria of Causality
• Strength
• Consistency
• Specificity
• Temporality
• Biological gradient
• Plausibility
• Coherence
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Bias in Environmental
Epidemiologic Studies
• Definition of bias
• The healthy worker effect
• Confounding
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Definition of Bias
• “Systematic deviation of results or inferences from
the truth. Processes leading to such deviation. An
error in the conception and design of a study—or in
the collection, analysis, interpretation, reporting,
publication, or review of data—leading to results or
conclusions that are systematically (as opposed to
randomly) different from the truth.” – Porta M. A Dictionary of Epidemiology. 5th ed. New York, NY: Oxford
University Press; 2008.
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Healthy Worker Effect
• Refers to the observation that employed
populations tend to have a lower mortality
experience than the general population.
• The healthy worker effect could introduce
selection bias into occupational mortality
studies.
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Confounding
• Denotes “… the distortion of a measure of the
effect of an exposure on an outcome due to the
association of the exposure with other factors
that influence the occurrence of the outcome.”
– Porta M. A Dictionary of Epidemiology. 5th ed. New
York, NY: Oxford University Press; 2008.
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Limitations of Epidemiologic
Studies
• Long latency periods
• Low incidence and prevalence
• Difficulties in exposure assessment
• Nonspecific effects