PRINCIPLES OF TOXICOLOGY - Biology East Borneo

21.3 TYPES OF EPIDEMIOLOGIC STUDIES: ADVANTAGES AND DISADVANTAGES 513The association must be shown repeatedly in different studies of different populations. The associationshould be preferably strong, as determined by a measure of risk. Finally, ideally, if the exposure isremoved, the amount of disease (i.e., the incidence) should decrease and/or new disease should beprevented.As stated above, a disease–exposure association is considered established if there are repeatedsimilar findings in both toxicologic and in multiple epidemiologic studies. Further proof would betoxicologic and epidemiologic studies which show that when the exposure is removed, the amount ofthe particular disease decreases or disappears.Obviously disease–exposure connections are much easier to prove in the case of acute, as opposedto chronic, health effects in both humans and laboratory animals. An illustration is that although theacute effects of carbon monoxide, such as death by asphyxiation, have been easy to establish, thelong-term effects of carbon monoxide exposure associated with heart toxicity have been much moredifficult to prove. The reason for this is that animals or people must be followed for longer periods oftime and may be affected by many other concurrent exposures during that time. In addition, sincehumans have longer lifespans than many other animals, as well as subtle differences in enzymaticsystems and often different routes of exposure, the extrapolation between diseases found in laboratoryanimals to human disease in the general human population associated with a pollutant exposure isproblematic, especially for chronic diseases such as cancer.Ultimately, if the findings disagree between epidemiologic studies with regard to a possibleassociation between a particular exposure and a human health effect, the interpretation of theseepidemiologic studies must depend on the “weight of evidence.” In other words, issues such as thevalidity of the individual studies, the biological plausibility of the association, and the existence orabsence of supporting toxicologic and other scientific evidence must all be taken into account.21.3 TYPES OF EPIDEMIOLOGIC STUDIES: ADVANTAGES AND DISADVANTAGESDifferent types of epidemiologic studies have been conducted (Table 21.2). Although predominantlyan observational discipline, epidemiologic principles are used in experimental situations such asclinical trials. In observational epidemiologic studies, the study population is not manipulated. Inexperimental epidemiology, like toxicologic studies, population members are intentionally distributedto different groups to evaluate the effect of a particular intervention.TABLE 21.2 Types of Epidemiologic StudiesObservationalDescriptiveCase seriesSurveillanceEcol ogicAnalyticalPrevalence/cross-sectional studiesCase controlCohortRetrospectiveProspectiveNested/synthetic case controlExperimental/interventionClinical trials/randomized controlled trialsField and community trialsSource: Adapted from Beaglehole et al. (1993).