(Chapter 2)  Environmental Epidemiology
environmental epidemiology
study of diseases and health conditions (occurring in the population) that are linked to environmental factors
epidemiology’s contribution to environmental health
–concern with populations
–use of observational methods
–methodolgy for study designs
–descriptive and analytic studies
concern with populations
environmental epidemiology studies a population in relation to morbidity and mortality
use of obervational data
epidemiolgy is primarily an observational science that takes advantage of naturally occurring situations in order to study the occurrence of disease
methodology for study designs
1. cross-sectional
2. ecologic
3. case-control
4. cohort
2 classes of epidemiologic studies
1. descriptive
2. analytic
descriptive studies
depiction of the occurrence of disease in populations according to classification by person, place, and time variables
analytic studies
examine causal (etiologic) hypotheses regarding the association between exposures and health conditions
prevalence
measure of disease frequency and refers to existing cases of disease or deaths
point prevalence
–refers to all cases of a disease that exist at a particular point in time relative to a specific population from which the cases are derived

— Point prevalence = (number of persons ill) / (total number in the group) at a point in time

incidence
the occurrence of a new disease or mortality within a defined period of observation (week, month, year, etc) in a specific population

— incidence rate = (number of new cases / total population at risk) over a time period x multiplier (e.g. 100,000)

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Case fatality rate
–provides a measure of the lethality of a disease

— CFR (%) = (number of deaths due to disease “X” / number of cases of disease “X”) x 100 during a time period

John Snow
–english anesthesiologist who linked cholera outbreak in London to contaminated water from the Thames River in the mid-1800s

–used a natural experiment, commonly used today by environmental epidemiology

–miasma theory vs. fecal/oral transmission

–spatial analysis

Sir Percival Pott (1714-1788)
–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

Odds Ratio (OR)
–a measure of association for case-control studies

–exposure-odds ratio – ratio of odds in favor of exposure among the disease group (the case) to the ratio in favor of exposure among the no-disease group (the control)

relative risk
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
study endpoints
–self-reported symptom rates
–physiologic or clinical examinations
–mortality
epidemiologic triange
–host, environment, agent

–one of the fundamental models of causality used in epidemiolgic studies

–provides a framework for organizing the causality of other types of environmental problems

host
a person or other living animal, including birds or arthropods, that affords subsistence or lodgement to an infectious agent under natural conditions
agent
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. A disease may have a single agent, a number of independent alternative agents (at least one of which must be present) or a complex of two or more factors whose combined presence is essential for the development of the disease
environment
domain in which the disease-causing agent may exist, survive, or originate. consists of “all that is external to the individual human host”
causality
–a causal association must exist between an agent factor and disease in the host for there to be causality

–certain criteria need to be taken into account in the assessment of causal association between factor A and disease B

Hill’s Criteria of Causality
1. strength
2. consistency
3. specificity
4. temporality
5. biological gradient
6. plausibility
7. coherence
bias in epidemiologic studies
–bias: deviation of results or inferences from the truth, or processes leading to such deviation. any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth

–healthy worker effect: source of bias that results from the fact that employed populations tend to be healthier than the general population

–confounding: a situation in which a measure of the effect of an exposure with other factors that influence the outcome under study

limits of epidemiologic studies
1. long latency periods
2. infrequent occurrence of certain diseases
3. difficulties in exposure assessment
4. nonspecific effects
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