Subchronic Sodium Chlorate Exposure in Drinking Water Results in a Concentration-Dependent Increase in Rat Thyroid Follicular Cell Hyperplasia

Abstract

Chlorine dioxide (ClO ₂) is an effective drinking water disinfectant, but sodium chlorate (NaClO₃) has been identified as a potentially harmful disinfection by-product. Studies were performed to describe the development of thyroid lesions in animals exposed to NaClO₃ in the drinking water. Male and female F344 rats and B6C3F₁ mice were exposed to 0, 0.125, 0.25, 0.5, 1.0, or 2.0 g/L NaClO₃ for 21 days. Additional male F344 rats were exposed to 0, 0.001, 0.01, 0.1, 1.0, or 2.0 g/L NaClO ₃ for 90 days. Female F344 rats were exposed to 0, 0.5, 1.0, 2.0, 4.0, or 6.0 g/L of NaClO₃ for 105 days. Thyroid tissues were processed by routine methods for light microscopic examination, and follicular cell hyperplasia was diagnosed using a novel method. Thyroid hormone levels were altered significantly after 4 and 21 days. NaClO₃ treatment induced a concentration-dependen t increase in the incidence and severity of thyroid follicular cell hyperplasia. Male rats are more sensitive to the effects of NaClO₃ treatment than females. Follicular cell hyperplasi a was not present in male or female B6C3F₁ mice. These data can be used to estimate the human health risk that would be associated with using ClO ₂, rather than chlorine, to disinfect drinking water.