Disinfection by-products (DBPs) are contaminants found in drinking water. Chlorite is one of the DBPs that are formed as a by-product of the chlorination/ozonation process used to purify water to acceptable drinking water standards. The potential effects of drinking water contaminants to affect adversely the immune system is a concern of both the Environmental Protection Agency (EPA) and the National Institute of Environmental Health Sciences (NIEHS). Several drinking water DBPs have been identified and selected for evaluation of their potential effects on the immune system in a joint project between the EPA and the NIEHS.
The National Toxicology Program (NTP) requested that a dose range-finding study be performed in order to establish the potential effects of sodium chlorite (SCI) on the immune system and to determine doses that could be used in a full immunotoxicology study. These studies were conducted in female B6C3F1 mice. The animals were exposed to SCI based on the concentration of the test article in the drinking water. Five SCI concentrations of 0.1, 1, 5, 15 and 30 mg/L were utilized. SCI solutions were prepared fresh every two weeks in tap water and stored refrigerated. The in-life phase of these studies was carried out between 23 February 1999 and 12 November 1999.
The baseline toxicology studies are summarized in Table ES-1. SCI was administered in the drinking water from water bottles for 28 days at 0.1, 1, 5, 15 and 30 mg/L/day. There was no statistical difference in drinking water consumption from animals exposed to SCI as compared to the tap water controls. There were few notable changes among the standard toxicological parameters that were assessed. There was no significant difference in body weight gain between the exposed and control animals after experimental period; however, combined body weights measured on day 22 of the exposure were significantly higher (<5%) in the 1, 5, and 15 mg/L SCI exposure groups than the measured control weights. No gross pathological lesions were observed in the SCI-exposed animals; furthermore, there were no differences observed in either terminal body weight, thymus or lung weight. Absolute spleen, liver and kidney weights were increased (10-23%) in several of the SCI exposure groups. In most cases, this increase was not observed when the organ weights were expressed as relative (% body weight) weight; a 15% increase in the relative spleen weight was observed in the 5 mg/L SCI dose group. The erythrocyte count, hemoglobin, hematocrit, MCV, MCH, MCHC, platelets, leukocyte counts and leukocyte differential were unaffected by SCI. Reticulocyte counts were significantly increased (34%) in the 15 mg/L exposure group.
The immunological studies are summarized in Table ES-2. Few immunological parameters were affected by SCI. Spleen cell numbers were unaffected by SCI exposure. Additionally, phenotypic enumeration of the spleen cells revealed that the absolute number of B cells, total T cells, T helper cells, macrophages, and natural killer cells were unchanged after 28 days of SCI exposure. A 26% increase in T suppresser/cytotoxic cells was measured in the 30 mg/L exposure group. Exposure to SCI produced a stimulatory trend in the antibody-forming cell (AFC) response to sheep erythrocytes; however, the response at each SCI exposure group was not significantly different from the response measured in the control group. Peritoneal macrophage activation, serum IgM antibody titer to sheep erythrocytes, splenic mixed leukocyte response (MLR) and basal natural killer (NK) cell activity were all unaffected by SCI exposure. Augmented NK activity was inhibited at the lowest SCI dose, but not at higher exposure levels.
In conclusion, SCI, when administered in the drinking water at doses 0.1 to 30 mg/L, produced few toxicological and immunological effects. A slight increase in spleen weight and reticulocyte counts was noted; however, these changes were not dose dependent. Additionally, an increase in the absolute number of splenic CD8+ T cells was observed in the highest SCI exposure group, and a stimulatory trend was measured in the splenic antibody-forming cell (AFC) response to sheep erythrocytes. Lastly, augmented NK activity was inhibited at the lowest SCI dose, but not at higher treatment levels. Based on the results of this range-finding study, SCI is not likely to have immunotoxicological effects in humans.