Environmental Epidemiology (15) Flashcards
epidemiology:
the study of the distribution and determinants of health and diseases, morbidity, injuries, disability, and mortality in populations
**environmental epidemiology:
the study of diseases and health conditions (occurring in a population) that are linked to environmental factors
environmental epidemiology exposures are usually voluntary/involuntary
involuntary
issues surrounding this field include exposure-response gradients, discovery of how occupational exposures cause harmful effects, identification of vulnerable workers, and input into programs for the prevention of occupationally related diseases
occupational health
**wrote the chimney sweeps story: environmental (occupational) cause for cancer; population medicine… a population of chimney sweeps
Sir Percival Pott (1775)
**“father of field epidemiology,” cholera in London
John Snow (1854)
Environmental epidemiology studies a population in relation to…
morbidity and mortality.
Epidemiology: generally observational or clinical?
observational
4 study designs used frequently in environmental epidemiology:
cross-sectional, ecologic, case-control, cohort
Prevalence:
refers to the number of existing cases of a disease, health condition, or deaths in a population at some designated time
Point prevalence (definition):
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
Point prevalence (formula):
(number of persons ill) / (total number in the group) at a point in time
Case fatality rate (definition):
provides a measure of the lethality of a disease
Case fatality rate (formula):
CFR (%): [(number of deaths due to disease X) / (number of cases of disease X)] • 100 during a time period
The two types of study designs:
Experimental, Observational
Type of study design that uses randomization of human subjects into treatment and control groups (clinical trials) – VERY impractical and SELDOM used within epidemiology
Experimental
“Natural experiments” where the investigator acts as a disinterested observer; data is collected on past and present exposures and illness in a study population
Observational
Two classes of observational studies:
Descriptive, Analytical
Descriptive Study:
type of observational study in which disease/exposure trends and patterns are described and characterized, which generates a hypothesis for further study; depiction of the occurrence of disease in populations according to classification by person, place, and time variables
Analytical Study:
type of observational study in which illness and exposure variables are considered to understand whether there is a significant association between disease and exposures (explores causality and utilizes hypothesis testing)
Which type of observational study is hypothesis generating, and which is hypothesis testing?
Descriptive – hypothesis generating, Analytical – hypothesis testing
The two primary functions of descriptive studies:
- to assess variations in the occurrence of disease in populations
- to aid in the development of etiologic hypothesis
case series:
observational; hypothesis generating; collecting and analyzing information on a disease or condition that subjects have in common (ex. land applications of biosolids and health effects)
cross sectional:
observational; hypothesis generating; examines the distributions of disease prevalence and exposure in a defined population at a particular point in time; limited value for investigating rare disease or diseases of short duration, but can be useful for persistent diseases and is low-cost
ecologic studies:
observational; hypothesis generating; correlate group or population level measures of disease incidence with group or population measures of exposure (ex. # of WNV disease in each Ohio county and # WNV + mosquitoes… is there a correlation?)
Limitations of ecologic studies:
nothing is known an an individual or local level about the disease incidence, for instance, there may not be a high number of human cases because the local health department implemented a very strong control and prevention program
case control studies:
observational; hypothesis testing; subjects are chosen based upon disease status (cases have the disease being studied; controls do not)
case control studies are more efficient for rare/common diseases, good for diseases that develop quickly/take a long time to develop, can study only one potential exposure/multiple exposures
rare, long time to develop, more than one potential exposure
Odds ratio:
a measure of association for case-control studies; = AD/BC (OR > 1 suggests positive association between exposure and disease or health outcome)
Cohort studies:
observational; hypothesis testing; subjects are defined based upon exposure status and followed over time to determine the incidence of the health outcome
Timing of cohort study: prospective vs. retrospective
Prospective: follows exposure to outcomes forward in time (e.g. test blood lead levels over time, then assess intelligence) (gold standard, but expensive and seldom done in environmental epi)
Retrospective: exposure and outcome already occurred at time study begins (e.g. breast implant surgery patients having disorders)
Relative risk (definition and equation):
the ratio of the incidence 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 = [A/(A+B)] / [C/(C+D)]
(RR>1 - risk is greater in exposed vs nonexposed., = 1 no risk, <1 indicates possible protective effect)
Parts of epidemiological triangle:
host, agent, environment
Criteria for causality:
- strength of association
- consistency of association
- temporality** (ALWAYS needs to be satisfied)
- coherence
- specificity
- biological gradient
- biological plausibility
Bias:
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:
employed workers have lower mortality rates than general populations, which could introduce selection bias into occupational mortality studies
Confounding:
the existence of other factors that contribute to the outcome of the study; arises from complex correlations of exposures in real life
Effect of bias:
moves the measured relative risk away from the true value
Selection bias:
arises from methods used to obtain study participants from the base population
Information bias (also, misclassification/observation/measurement bias):
involves misclassification of either health outcome or exposure status; the major issue in environmental epidemiology studies
Classic example of recall bias:
mothers of infants with birth defects are more apt to remember information about potential exposures than mothers with healthy infants
Limitations of epidemiologic studies (4):
- long latency periods
- low incidence and prevalence
- difficulties in exposure assessment
- nonspecific effects
**What is the major issue in environmental epidemiology studies?
exposure misclassification! (there is an error in the classification of an exposure; i.e. an exposed subject is classified as non-exposed or vice versa)
Characteristics of environmental epidemiology (3):
- deals with nondisease effects
- involves numerous variables
- tends to be community-specific
Weaknesses of environmental epidemiology (3):
- sample size is insufficient
- important variables “uncontrolled”
- exposure estimation invalid
Strengths of environmental epidemiology (3):
- engages the real world
- unique perspective on disease/health
- basis for action despite ignorance of mechanism
Ecological fallacy/bias (limitation fo ecologic study):
researcher cannot assume that an association/correlation found at a population level will represent an association at the individual level
Benefits of ecologic studies (3):
- inexpensive
- can use existing data sets
- studies on individuals do not reveal complex relationships of the transmission of infectious diseases in the environment and how ecological factors effect that transmission
Case control studies are more/less costly and more/less efficient that cohort studies.
less, more
Limitations of case-control studies:
- can only test one (or two) diseases at a time
- not good for rare exposures
- exposure data may not be accurate (biased) because it is collected back in time (retrospectively) – recall bias, exposure misclassification, selection bias
- difficult to determine if exposure came before disease
- confidentiality must be protected
Selection of controls in case control studies:
- must be free of the disease at the time of selection
- should meet the “would criterion” (would be case if got disease
- up to 4:1 ratio of controls:cases
Cohort studies:
- are efficient for common/rare exposures
- are efficient for common/rare outcomes
- generally more/less accurate than case control studies
- can/cannot evaluate multiple effects of an exposure
- Efficient/inefficient for diseases with long latent periods
- Less/more vulnerable to prospective bias
- Less/more vulnerable to retrospective bias
- can/cannot measure disease incidence
- temporal relationships between exposure and disease are/are not clear
- T/F expensive and time-consuming
- rare
- common
- more accurate
- can
- efficient
- less
- more
- can
- are
- True
**Circa 400 B.C., Hippocrates wrote…
…“On Airs, Waters, and Places,” which was about how environmental and host factors such as behaviors might influence the development of disease
**In 1662, John Graunt wrote a…
landmark publication based on Bills on Mortality, quantify patterns of birth, death, and disease occurrence, noting disparities across population groups
**1800: William Farr…
modern vital statistics and surveillance
**19th and 20th centuries-
epidemiological methods applied in investigation of disease occurrence of communicable nature
**1930s and 1940s-
non-communicable diseases were also investigated for their risk-factors, i.e., Doll and Hill- linking lung cancer to smoking
**1960s and early 1970s-
health workers applied epidemiologic methods to eradicate naturally occurring smallpox worldwide
**1980s -
extended to the studies of injuries and violence
**Contemporary epidemiology-
HIV/AIDS, SARS, MDRTB, Avian Flu, also, biologic warfares and bioterrorism
