Chapter 2Chapter 2

Environmental Epidemiology

Learning ObjectivesLearning ObjectivesBy 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

What is Environmental What 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 smokeo A major concern in the field is causality

Contributions to Contributions to Environmental HealthEnvironmental Health

• Concern with populationso Environmental epidemiology studies a population in

relation to morbidity and mortality.

• Use of observational datao 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

Contributions to Contributions to Environmental HealthEnvironmental Health

• Methodology for study designso 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

• Ecologico unit of analysis is a population rather than an individual

• Case-Controlo compares subjects who have that condition/disease with

patients who do not have the condition/disease but are otherwise similar

• Cohorto largely about the life histories of segments of populations, and

the individual people who constitute these segments

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].”

Odds Ratio EquationOdds Ratio Equation

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

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.

RR EquationRR Equation AA + B

RR = CC + 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.

Two Classes of Two Classes of Epidemiologic StudiesEpidemiologic Studies• Descriptive

o Depiction of the occurrence of disease in populations according to classification by person, place, and time variables.

• Analytico Examines causal (etiologic) hypotheses regarding the association

between exposures and health conditions.

Measures of Disease Measures of Disease FrequencyFrequency

• Prevalenceo Refers to the number of existing cases of a disease, health condition, or

deaths in a population at some designated time

• Point prevalenceo 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 illPoint Prevalence = at a point in time

Total number in the group

Measures of Disease Measures of Disease FrequencyFrequency

• Incidenceo 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 Rateo 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)

Measures of Disease Measures of Disease FrequencyFrequency

• Case fatality rateo 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”

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.

Study Designs Used in Study Designs Used in Environmental Environmental EpidemiologyEpidemiology

Experimental •In epidemiology, most are Intervention Studies

o Involves intentional change in some status of the subjectso 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.

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

What is the What 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.

Corners of the Corners of the “Triangle”“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.”

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).

Bias in Environmental Bias 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 effecto Employed workers have lower mortality rates than general

populationso The healthy worker effect could introduce selection bias into

occupational mortality studies.

• Confoundingo The existence of other factors that contribute to the outcome of

the study

Limitations of Limitations of Epidemiologic StudiesEpidemiologic Studies

• Long latency periods• Low incidence and

prevalence• Difficulties in exposure

assessment• Nonspecific effects

ConclusionConclusion