Intermezzo Example

In Iraq an outbreak of intoxications due to the consumption of fungicide-treated grain occurred in 1971-72. Of a total of 6530 hospital admissions, 459 patients died. The symptoms were typical for organic mercury intoxication. Analytical data showed a definite association between the consumption of bread baked from fungicide-treated grain and the mercury concentrations in hair and blood.

Examples of predominant risk factors are smoking (historical obduction data suggest that lung cancers were relatively rare before the "smoking era" started), asbestos (almost 80% of mesothelioma incidence can be traced back to occupational asbestos exposure in the past), and vinyl chloride (only two cases of angiosarcoma, a blood vessel tumor of the liver, in a cohort of workers alerted a medical company doctor to suspect the PVC-monomer). Toxic effects following the intake of food are often associated with accidental contamination. In the history of epidemiology there are many examples of this kind. In 1956 the accumulation of methylmercury in fish from Minamata Bay in Japan caused severe neurological disorders in many people as well as neurological birth defects (see also Section 20.2.4). In 1981 contaminated cooking oil caused an epidemic of acute lung disease in Madrid, Spain, with more than 20,000 cases over a period of three months. The contaminant caused severe immunotoxic effects. Toxic effects are often caused by components of vegetable origin. In India many people suffer from paralysis of the lower limbs, caused by the consumption of chick peas. In Africa a syndrome of severe neuropathological symptoms is caused by chronic hydrocyanic acid intoxication as a result of the poor man's diet consisting only of cassava. However, this kind of one-to-one associations between risk factors and diseases (disease specificity) is rare. In most cases, the risk factor under investigation is one of several, which reduces the sensitivity of epidemiological studies substantially.

If an association between the risk factor under investigation and another risk factor is found, one has to take into account the possibility of confounding. For example, comparison of an urbanized population with a rural population will show that the lung cancer incidence rate is higher in the first group. Thus, one might conclude that exposure to the urban ambient air induces lung cancer. However, the difference in lung cancer incidence may well be explained by differences in smoking habits and occupation, two major risk factors for lung cancer associated with urban life.

Inaccurate response parameters may also dilute the association between a risk factor and a disease and thus decrease the sensitivity of epidemiological methods. This can be best illustrated by studies on the association between radiation exposure and leukemia. A substantial risk increase could have escaped observation in epidemiological studies, if all types of leukemia had been combined. Only by studying the various types of leukemia individually, the association between exposure and acute nonlymphatic leukemia was revealed.

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