Environmental Health Factors

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Chapter 2

Environmental Epidemiology

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

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.

Epidemiology’s Contributions to Environmental Health

  • Concern with populations
  • Use of observational data
  • Methodology for study designs
  • Descriptive and analytic studies

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?

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.

Methodology for
Study Designs

  • Characteristic study designs used frequently in environmental epidemiology:
  • Cross-sectional
  • Ecologic
  • Case-Control
  • Cohort

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.

Measures of Disease Frequency

  • Prevalence
  • Point prevalence
  • Incidence
  • Incidence rate
  • Case fatality rate

Prevalence

  • Refers to the number of existing cases of a disease, health condition, or deaths in a population at some designated time

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.

Formula for Point Prevalence

Number of persons ill

Point Prevalence = at a point in Total number in the group time

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.

Formula for Incidence Rate

Incidence Number of new cases over a time

Rate = period x multiplier

Total population at risk (e.g., 100,000)

Case Fatality Rate (CFR)

  • Provides a measure of the lethality of a disease.

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

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.

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.

Study Designs Used in Environmental Epidemiology

  • Experimental
  • Case Series
  • Cross-Sectional
  • Ecologic
  • Case-Control
  • Cohort

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].”

Odds Ratio Table

Odds Ratio Equation

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

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.

Relative Risk (RR) Table

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.

Study Endpoints

  • Self-reported symptom rates
  • Physiologic or clinical examinations
  • Mortality

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.

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

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.”

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.”

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.”

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).

Hill’s Criteria of Causality

  • Strength
  • Consistency
  • Specificity
  • Temporality
  • Biological gradient
  • Plausibility
  • Coherence

Bias in Environmental Epidemiologic Studies

  • Definition of bias
  • The healthy worker effect
  • Confounding

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.

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.

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.

Limitations of Epidemiologic Studies

  • Long latency periods
  • Low incidence and prevalence
  • Difficulties in exposure assessment
  • Nonspecific effects