PRINCIPLES OF TOXICOLOGY

21.3 TYPES OF EPIDEMIOLOGIC STUDIES: ADVANTAGES AND DISADVANTAGES 513
The association must be shown repeatedly in different studies of different populations. The association
should be preferably strong, as determined by a measure of risk. Finally, ideally, if the exposure is
removed, the amount of disease (i.e., the incidence) should decrease and/or new disease should be
prevented.
As stated above, a disease–exposure association is considered established if there are repeated
similar findings in both toxicologic and in multiple epidemiologic studies. Further proof would be
toxicologic and epidemiologic studies which show that when the exposure is removed, the amount of
the particular disease decreases or disappears.
Obviously disease–exposure connections are much easier to prove in the case of acute, as opposed
to chronic, health effects in both humans and laboratory animals. An illustration is that although the
acute effects of carbon monoxide, such as death by asphyxiation, have been easy to establish, the
long-term effects of carbon monoxide exposure associated with heart toxicity have been much more
difficult to prove. The reason for this is that animals or people must be followed for longer periods of
time and may be affected by many other concurrent exposures during that time. In addition, since
humans have longer lifespans than many other animals, as well as subtle differences in enzymatic
systems and often different routes of exposure, the extrapolation between diseases found in laboratory
animals to human disease in the general human population associated with a pollutant exposure is
problematic, especially for chronic diseases such as cancer.
Ultimately, if the findings disagree between epidemiologic studies with regard to a possible
association between a particular exposure and a human health effect, the interpretation of these
epidemiologic studies must depend on the “weight of evidence.” In other words, issues such as the
validity of the individual studies, the biological plausibility of the association, and the existence or
absence of supporting toxicologic and other scientific evidence must all be taken into account.
21.3 TYPES OF EPIDEMIOLOGIC STUDIES: ADVANTAGES AND DISADVANTAGES
Different types of epidemiologic studies have been conducted (Table 21.2). Although predominantly
an observational discipline, epidemiologic principles are used in experimental situations such as
clinical trials. In observational epidemiologic studies, the study population is not manipulated. In
experimental epidemiology, like toxicologic studies, population members are intentionally distributed
to different groups to evaluate the effect of a particular intervention.
TABLE 21.2 Types of Epidemiologic Studies
Observational
Descriptive
Case series
Surveillance
Ecol ogic
Analytical
Prevalence/cross-sectional studies
Case control
Cohort
Retrospective
Prospective
Nested/synthetic case control
Experimental/intervention
Clinical trials/randomized controlled trials
Field and community trials
Source: Adapted from Beaglehole et al. (1993).