Effect vs. Association Section
Epidemiologists often compare the frequency of disease among an exposed group and a non-exposed group in order to assess the association of that exposure with the occurance of the disease. In the most general sense, an exposure is any characteristic that potentially affects the health outcome, including environmental factors, lifestyle practices, genetic factors, belonging to a particular sociodemographic group, family medical history or an administered treatment. Some epidemiologists reserve the use of the term 'association' for situations when the exposure or risk factor of interest is one which cannot be changed, such as a genetic factor, while using the term 'effect' to more generally describe relationships. Under this framework, a genetic factor may be 'associated' with a health outcome while a lifestyle factor could have an 'effect'. Consider the definitions below....there is little difference between an observed effect and an observed association, other than the type of exposure defining the two groups.
Effect – A quantitative measure of the increased or decreased prevalence, rate or risk for an exposed population compared to an unexposed population. Epidemiologists use this term when the risk factor or exposure can be changed (e.g. smoking cigarettes).
Example: The effect of smoking (compared to never smoking) in the study population was to increase the rate of hospitalization 1.9 times.” 1.9 is a measure of the effect (the value may be an odds or rate ratio, depending upon the study design).
Association – A quantitative measure of the increased or decreased prevalence, rate or risk for one population compared to another population. Epidemiologists often use this term when the risk factor or exposure cannot be changed (e.g. male).
Basic Concepts of Exposure: Section
In a general sense, exposure can be defined is any of a subject's attributes (association) or any agent (effect) with which the subject may come into contact . These attributes or agents may be relevant to his or her health (Armstrong et al., 1998).
This definition would include smoking, drinking, exposure through an occupation (farmers, pesticide applicators, etc.), age (e.g., menopause >> endogenous estrogen levels) as exposures.
For an environmental factor, exposure can be more precisely defined as contact of some agent at the boundary between humans and the environment, at a specific concentration, over an interval of time (Wallace, 1995). Exposures can be harmful or beneficial.
Harmful - Environmental Tobacco Smoke, (ETS), Asbestos, ...
Beneficial Factors - Vitamin D intake, Colonoscopy as a preventive measure for colon cancer, ...
Continuum from Emission of a Contaminant to a Health Effect Section
Figure 1 below depicts multiple steps leading to a particular environmental exposure.
In this example, the source emissions are not associated with a direct health effect. Following the flowchart, it appears the emissions must be transported and transformed, then accumulate in the environment. Next, a human must come in contact with or be exposed to these transformed emissions, at a sufficient internal dose (some may be eliminated). If the dose is high enough to have a biological effect, the disease may be expressed, resulting in a health effect. This pollutant travels a long path before it causes a health effect. Different individuals process and eliminate toxins at different rates, which also affects the expression of disease. It is obvious proving an emission is related to a health impact can be difficult because of a lengthy chain of events as well as individual variation. If biomarkers are present in such a sequence and detectable during early expression of the disease, we may be able to detect disease a little earlier.