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248 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

Case-controf Studies
In case-control studies, only one disease can be investigated. The cases in-
clude all patients with a certain disease observed in a hospital, city, or a larger
area in a given period of time. Their exposure histories are compared with
those of the controls. Thus, several exposures can be investigated. The expo-
sure data are not very accurate because they are obtained by interview. Espe-
cially in cases of serious diseases, patients are often desperate to seek some
reason for their disease. Therefore, patients of some other disease are usually
employed as controls to avoid this information bias. The selection of controls
is a crucial but extremely difficult task. Since factors such as age, sex, smok-
ing, living habits, and place of abode are known to be risk factors for several
diseases, the effects of these confounding factors are eliminated by matching.
However, overmatching should also be avoided. Odds ratio (see Table 5.86,c)
is used to express how often the cases have been exposed to various exposures
compared to controls. Case-control studies are common because they are in-
expensive and relatively easy to perform. If the disease studied is rare, this ap-
J
proach is also the only practical alternative. -
5.3.1.3 Classifications of Toxicology
The word toxicology originates from the Greek word toxicon, which
means arrow. In ancient times, arrows were dipped into plant poisons to in-
crease their lethality in hunting. Today, toxicology refers to that scientific dis-
cipline that explores the deleterious effects of chemicals or of physical or
biological factors on living organisms. Toxicology also explores the mecha-
nisms whereby chemicals, or physical or biological factors induce their harm-
ful effects in the organism.
There are several definitions and classifications of toxicology. One classi-
fication is based on the target organs which are harmfully affected by chemi-
cals. Hence, there are terms such as neurotoxicology, liver or hepatic
toxicology, kidney or renal toxicology, and toxicology of the eye (ocular toxi-
cology). Inhalational toxicology emphasizes the importance of the lungs as the
target organ of chemicals. In addition to these descriptive classifications, toxi-
cology can be divided into mechanistic toxicology, conducted mainly in uni-
versity and governmental research institutions, and descriptive or regulatory
toxicology, which is required for classification and labelling of chemicals for
registration purposes. Even if mechanistic toxicology is essential for under-
standing how chemicals and other factors induce their toxic effects, descrip-
tive toxicology is also important in characterizing the properties of a chemical
compound. Descriptive toxicology is a prerequisite for regulatory purposes
and risk assessment. When the qualitative requirements for risk assessment in-
crease, the importance of mechanistic information on chemicals in risk assess-
ment will also increase.
Toxicology can also be divided into different classes based on the goals it
serves. Clinical toxicology explores ways of treating poisoned patients, and
also aims to develop quick methods to diagnose poisonings. Forensic toxicol-
ogy is the science involved in detecting the role of poisons in fatalities. Environ-
mental toxicology assesses the importance of environmental pollution and the
effects of exposure through various environmental compartments on human
health. Ecotoxicology is interested in the effects of environmental chemicals on