Assessment on Perchlorate Exposure Through Drinking Water. D. Ting, R. Howd, and A. Fan; Office of Environmental Health Hazard Assessment, Cal/EPA, and Department of Environmental Health Hazard Assessment
The Office of Environmental Health Hazard Assessment conducted a risk assessment to set a health protective level for perchlorate found in California drinking water. Perchlorate competitively blocks iodide from entering the thyroid and affects thyroid hormone synthesis and regulation. Most of the health effects of perchlorate observed in animals and humans are related to this effect. Pregnant women and their fetuses that are suffering from marginal iodine deficiency are identified as the sensitive populations. Pregnancy itself presents a stress on the thyroid. In areas of marginal iodine deficiency (intake level <100 µg/day), there is an increased risk of thyroid enlargement and goiter in pregnant women. Developing fetal brains are sensitive to iodine deficiency; even mild iodine deficiency has been linked to a reduction of IQ of the child. It was considered a paradox that in areas of iodine deficiency, some children with functioning thyroid glands had more severe brain damage than those without a thyroid gland. This is identified as a maternal effect. For prevention of brain damage in the offspring, iodide has to be supplied early in the first trimester before the fetal thyroid is functioning. A significant fraction of the United States’ population has been shown to have low iodine intake. Urinary iodine data from the National Health and Nutrition Examination Survey III (1988-1994) showed that among women of childbearing age, 14.9 % had urinary iodine concentration below 5 µg/dL (equivalent to an intake level of 50 µg/day).
As pregnant women that have low iodine intake are already at risk of developing goiter and a reduction of IQ in their offspring, any additional impairment of thyroid function is deemed undesirable. The proposed health protective level for perchlorate is set at a value that it is not likely to inhibit thyroid iodide uptake based on human studies.
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