Chapter 2
experimental methodology
difficult to achieve in environmental epidemiology; assignment of groups of individuals with different environmental exposures is generally impractical if not unethical; consequently, in order to study the effects of environmental exposures researchers must use observational methods
goals of epidemiologic research in occupational health
encompass the description of exposure-response gradients, discovery of how occupational hazards may cause harmful effects, characterization of vulnerable workers, input into programs for prevention of occupationally related disease
strengths of environmental epidemiology
engages the real world, unique perspective on disease/health, basis for action despite ignorance of mechanism
odds ratio (OR)
measure of association between exposure and outcome used in case control studies 2x2 table OR= AD / BC odds ratio of more than 1 suggests a positive association (provided results are statistically significant)
Examples of Agent factors
microbial agents responsible for zoonotic or foodborne disease toxic chemicals toxic metals airborne particulates and gases radiation: ionizing and nonionizing
clinical trials
design in which there is manipulation of an exposure variable and random assignment of subjects to either a treatment group or a control
physiologic or clinical examinations
"harder" evidence than self-reports, but may be expensive or impractical to conduct
epidemiology is important to the study of environmental health problems because
1) many exposures and health effects associated with the environment occur at the population level 2) epidemiologic methods of natural experiments and observational techniques are appropriate 3) study designs used in epidemiologic research can be applied directly to the study of environmental health issues 4) epidemiology aids in the development of hypotheses and the study of causal relationships
limitations in measuring exposure
1- dependence on indirect, surrogate estimates of exposure and dose 2- uncertainty regarding pathways of exposure 3- probably low dose levels in most settings 4- frequent inability to develop useful dose-response data
Buffler: 3 major requirements for successful epidemiologic investigation
1- direct and accurate estimates of the exposures experienced by individual members of the study population 2- direct and accurate determination of the disease status of individual members of the study population 3- appropriate statistical summarization and analysis of the individual data pertaining to disease and exposure
limitations in detecting disease
1- long and variable latency periods between exposure and diagnosis 2- etiologic nonspecificity of disease clinical features 3- small population size coupled with low disease frequency 4- observer bias in reporting illness occurence
Disease clustering
A closely grouped series of events or cases of a disease or other health-related phenomena with well-defined distribution patterns in relation to time or place or both
descriptive epidemiology
Depiction of the occurrence of disease in populations according to classification by person, place, and time variables studies aim to delineate the patterns and manner in which disease occurs in populations
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.
case series study
Information about patients who share a disease in common is gathered over time weakest for making causal assumptions, but useful for developing hypotheses for further study
incidence rate
The rate at which new events occur in a population. The numerator is the number of new events that occur in a defined period; the denominator is the average population at risk of experiencing the event during this period by some multiplier
ecologic studies
a study in which the units of analysis are populations or groups of people rather than individuals different from most other types of epidemiologic research in regard to the unit of analysis assumption is made that outcome rates would be comparable in exposed and nonexposed groups if the exposure did not take place major problem stems from uncontrolled factors, but may open the next generation of investigation
point prevalence
all cases of or deaths from a disease or health condition that exist at a particular point in time relative to a specific population from which the cases are derived; can be calculated by dividing the number of persons ill by the total number in the group to; the resulting number is measured by "at a point in time"
Snow's cholera experiment
attempted to extrapolate from the health effects of exposures to high doses of environmental substances what the effects of low doses would be
selection bias
bias in the estimated association or effect of an exposure on the outcome that arises from the procedures used to select individuals into the study or the analysis
observational study designs
case series, cross sectional, ecologic, case control, cohort
recall bias
cases may remember an exposure more clearly than controls
cohort study
classifies subjects according to their exposure to a factor of interest and then observes them over time to document the occurrence of new cases (incidence) type of longitudinal design able to evaluate many different outcomes but few exposures either prospective or retrospective; prospective participants must be certified as being free of the outcome of interest
observational designs used for hypothesis testing
cohort, case control studies
Environmental epidemiology
complex field that in some cases provides insights into environmentally caused diseases and in others provides unclear results that must be followed up by other studies special concern is causality
Epidemiology aids the environmental health field through
concern with populations use of observational data methodology for study designs descriptive and analytic studies
observational designs used for hypothesis generation
cross sectional, case series, some ecologic correlations, and proportionate mortality comparisons
Characteristics of environmental epidemiology
deals with nondisease effects, involves numerous variables, tends to be community specific
Classes of Epidemiologic Studies
descriptive and analytic
analytic epidemiology
examines causal (etiologic) hypotheses regarding the association between exposures and health conditions proposes and evaluates causal models that employ both outcome variables and exposure variables etiologic studies are planned examinations of causality; importance of low level exposures is explored and greater refinement in exposure-effect relationships is sought
cross sectional study
examines the relationship between diseases and other variables of interest as they exist in a defined population at one particular time type of prevalence study in which the distribution of disease and exposure are determined, although it is not imperative for the study to include both exposure and disease may be used to formulate hypotheses that can be followed up in analytic studies
non-observational and partly observational study designs
experimental and quasi-experimental
for an environmentally associated health outcome by be considered a topic of environmental epidemiology
exposure factors must lie outside the individual's immediate control
Sir Percival Pott
first to describe an environmental cause of cancer (chimney sweepers) established an occupational hygiene control measure--recommendation that chimney sweeps bathe once a week
epidemiology contribution to public health
focus on entire populations and by its use of descriptive and analytic methodologies findings can aid in controlling environmental exposures and developing health policies for protecting the public
mortality research
frequently uses a retrospective cohort study design; mortality experience in cohort can be compared with the expected mortality in the general population by using the SMR advantages: may be relevant to agents that have a subtle effect over a long time period
self report questionnaires
inexpensive but not always reliable
intervention study
investigation involving intentional change in some aspect of the status of the subjects study designs are: clinical trials (randomized controlled trials) and quasi-experiments (community trials)
for an environmental epidemiologic research study to be valid
level of exposure in a population must be assessed validly
low incidence and prevalence
low level environmental exposures challenge epidemiologic researchers who have difficulty applying standard lab methods to determine exposure levels; consequently unable to establish definitively whether exposure to a particular agent has occurred
approach of environmental epidemiology
mainly the relationship between environmental exposures and their unknown hazards; identification of previously unrecognized exposures to known hazardous agents and the quantification of such risks, estimation of the amount of exposures that individuals have to have to hazards, assessment of risks associated with exposures, and evaluation of procedures to prevent exposures
quasi experimental study
manipulation of an exposure variable occurs, but individual subjects are not randomly allocated to the study conditions; ex: community trial, which tests an intervention at the community level; in some quasi experimental designs study units may be assigned randomly to study conditions, however in other research assignment of study units may be arbitrary
bias
many different types of bias; particularly important for environmental epidemiology are those that impact study procedures such as recall bias and selection bias
prevalence
number of existing cases of or deaths from a disease or health condition in a population at a designated time prevalence measures are used to describe the scope and distribution of health outcomes in the population prevalence data contribute to the accomplishment of two of the primary functions of descriptive epidemiology: to assess variations in the occurrence of disease in populations and to aid in the development of etiologic hypotheses
healthy worker effect
observation that employed populations tend to have a lower mortality experience than the general population
incidence
occurrence of new disease or mortality within a defined period of observation in a specified population incidence measures are central to the study of causal mechanisms with regard to how exposures affect health outcomes used to describe the risks associated with certain exposures
epidemiologic triangle
one of the fundamental models of causality; provides a framework for organizing the causality three major factors: agent, host and environment environment: domain in which disease causing agents may exist, survive or originate host: person or other living animal that affords subsistence or lodgment to an infectious agent under natural conditions agent: a factor whose presence, excessive presence, or relative absence is essential for the occurrence of disease
case fatality rate (CFF)
provides a measure of the lethality of a disease CFR(%)= (number of deaths due to a disease during period/number of cases of the disease during period)x 100
standardized mortality ratio (SMR)
ratio of the number of deaths observed in the study group or population to the number that would be expected if the study population had the same specific rates as the standard population ratio, multiplied by 100 when percentage is greater than 100% the SMR in the study population is elevated above that found in the comparison population
methodology for study designs
research generally aims to portray the frequency of disease occurrence in the population or to link disease outcomes to specific exposures characteristic study designs: cross-sectional, ecologic, case-control and cohort
weaknesses of environmental epidemiology
sample size is insufficient, important variables "uncontrolled", exposure estimation invalid
Hill's Criteria of Causality
strength, consistency, specificity, temporality, biological gradient, plausibility, coherence strength: strong associations give support to a causal relationship consistency: observed repeatedly by different persons, in different places, circumstances and times specificity: one constrained to a particular disease-exposure relationship; a given disease results from a given exposure and not from other types of exposures Temporality: we must observe the cause before the effect biological gradient: dose-response curve; shows a linear trend in the association between exposure and disease plausibility: association must be biologically plausible from standpoint of contemporary biological knowledge coherence: cause and effect interpretation should not seriously conflict with the generally known facts of the natural history and biology of the disease
environmental epidemiology
study of diseases and health conditions that are linked to environmental factors
Epidemiology concerned with
study of distribution and determinants of health and diseases, morbidity, injuries, disability, and mortality in populations
case control study
subjects who participate in the study are defined on the basis of the presence or absence of an outcome of interest cases and controls generally are matched according to criteria such as age, sex, race or other variables exposure to the factor is determined retrospectively, meaning exposure has already occurred in the past advantage is that they can examine many potential exposures disadvantage is that in most circumstances they can examine only one or a few outcomes may provide more complete exposure data than cross sectional studies, but unmeasured exposure variables as well as methodological biases may remain
natural experiments (analytic epidemiology)
take advantage of naturally occurring situations in order to test causal hypotheses
confounding bias
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
relative risk (RR)
the measure of association used in cohort studies; is defined as 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= A/(A+B) / C/(C+D) relative risk greater than 1 (and statistically significant) indicates that the risk is greater in the exposed group (positive association between exposure and outcome) if relative risk is less than 1 (and statistically significant) the risk is lower among the exposed group (protective effect)
latency period
the time interval between initial exposure to a disease-causing agent, and the appearance of the disease or its manifestations in the host
central concern of environmental epidemiology
to be able to assert that a causal association exists between an agent factor and a disease in the host
concern with populations
unique characteristic of epidemiology vs clinical focus on the individual
outcome variable in epidemiologic studies
usually a specific disease, cause of mortality, or health condition
nonspecific effects
when an outcome is nonspecific it can be associated with several or many different environmental exposures
Causality
whether research findings represent cause and effect associations
distinguishing characteristics of study designs
whether they involve the individual or group as the unit of analysis