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