Cadmium

Like many heavy metals, cadmium has toxic effects in humans. Primary adverse effects are due to pulmonary and renal toxicity. There is evidence that cadmium may be a carcinogen, although this evidence is considerably weaker than that for other metals. Cadmium is present in the environment and may present a danger by occupational exposure. It is used in nickel-cadmium batteries, electroplating, smelting, and mining. Shellfish are a dietary source of cadmium and may contain from 100 to 1,000 mg per kilogram of this metal (65). Estimated daily intake in the United States from all sources is 10 to 40 mg per day. Cigarette smoke is another source of exposure, and it is estimated that smoking one or two packs per day doubles daily intake of cadmium (65).

Animal and human studies show that cadmium has a special affinity for the placenta (66) and suppresses mitochondrial succinic dehydrogenase and cytochrome oxidase activities. Its concentrations in the placenta far exceed those in cord blood, suggesting that the placenta serves as a barrier against fetal exposure (67).

Animal data reveal that when injected into rodents in early gestation, cadmium may be highly teratogenic. In a series of studies, facial malformations, undescended testes, hydrocephalus, and resorptions were noted in exposed fetuses (65). Other researchers have found evidence of severe growth retardation, fetal death, and neonatal anemia under similar experimental conditions. In a recent study (68), female rats exposed to 5 and 10 mg per kilogram of cadmium before fertilization demonstrated a dose-dependent increase in infertility. No effects on pregnancy were seen in the rats that had litters after such an exposure, however. It has been reported that multiple skeletal defects and abnormal bone formation may occur secondary to effects on calcium metabolism following cadmium exposure (67).

Single doses of cadmium administered to male rodents resulted in marked testicular atrophy (69). Similar exposures in humans do not appear to have the same result, possibly because of the protective effect of the testicular protein metallothionein, which preferentially binds cadmium.

Human data are limited, and much of it is anecdotal; but some studies suggest a decrease in the birth weight of exposed neonates (70). The most recent study on occupational exposure to cadmium was a retrospective epidemiologic study conducted by Berlin and co-workers (71) on female workers in a nickel-cadmium battery plant. When 157 children born to exposed mothers were compared with 109 children born to unexposed controls, no differences in mean birth weight were detected. Only smoking had a negative effect on birth weight in this study. Interestingly, it has been suggested that cadmium in cigarette smoke contributes significantly to the increased incidence of low birth weight in infants born to women who smoke. One of the main effects of cadmium toxicity is decreased absorption and metabolism of nutrients like iron in the gastrointestinal tract. Even though they may not significantly decrease nutrient absorption in the mother, low levels of cadmium may cause a significant decrease in the availability of nutrients to the fetus, resulting in IUGR.

Counseling patients with cadmium exposure during pregnancy may be difficult. Both OSHA and the American Conference of Governmental Industrial Hygienists (ACGIH) have set occupational limits for cadmium exposure. Whereas the data from animal experiments indicate that cadmium in high doses injected parenterally is highly teratogenic, insufficient data are available to suggest that cadmium presents a major hazard to pregnant women. Cadmium's poor absorption from the gastrointestinal tract or protection of the fetus by the placental barrier may account for this scarcity of data. Certainly, a targeted ultrasound examination should be offered to pregnant women who have had known exposure, and cadmium toxicity may be explored in infertile couples. Currently, insufficient evidence exists to offer termination of pregnancy based solely on a history of cadmium exposure.

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