Page 359 - Industrial Ventilation Design Guidebook

5.3 TOXiCITY AND RISKS INDUCED BY OCCUPATIONAL EXPOSURE TO CHEMICAL COMPOUNDS 3 I 5

rare (e.g., if it had induced cleft palate), it would have taken much longer to
identify the causal relationship between the use of thalidomide and the
malformations. In the case of thalidomide, the causal relationship was clear;
about 84% of mothers whose children had limb malformations had taken
thalidomide. It is estimated that thalidomide damaged about 7000-10 000
children, mainly in Western European countries.
Another well-known chemical teratogen is methyl mercury. Environmental
health disasters in Japan, in Minamata Bay and Nigeta in the 1950s, and in Iraq in
1971, have provided detailed information of the effects of methyl mercury on fe-
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tuses. Exposure to methyl mercury during pregnancy affected mainly the central
nervous system of the children, and these changes were permanent. The most im-
portant sign was progressive retardation of psychomotor development of a child
that seemed to be normal at birth. In addition, the exposure may also have caused
blindness, deafness, and convulsions which appeared as the child grew (see Fig.
5.51). Methyl mercury seriously disturbs the normal organization of various brain
structures during organogenesis. It binds to SH groups of proteins and also disturbs
DNA and RNA synthesis. It used to be thought that the mother could somehow
protect the growing fetus from chemical insults. Methyl mercury is a compound
which reveals the fallacy of the claim that doses of methyl mercury which do not
damage the mother cannot be teratogenic to the fetus. Furthermore, male fetuses
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seem to be more sensitive than female fetuses to the effects of this compound.

FIGURE 5.51 Dose-response relationships for methyl mercury. 176 (Used with permission.)