Background: On October 20, 1992, > 40 children from one elementary school visited the school nurse due to the acute onset of blue lips and hands, vomiting, and headache during and after the school lunch periods. Forty-nine children were seen by physicians that day and 14 were hospitalized. Laboratory analysis revealed methemoglobinemia in many of the children. All recovered in 36 hours.
Objective: A case-control study was supplemented by environmental and laboratory investigations to determine the outbreak source.
Methods: Cases were selected based on the laboratory diagnosis of methemoglobinemia (methemoglobin level > 2%). Children whose methemoglobin levels were missing or < 2% were excluded from analysis. Controls were obtained by selecting every third name from a school roster. The parents of 29 students who met the case definition and 52 controls were interviewed.
Results: All 29 cases and 33% (17/52) of the controls ate soup during the school lunch (odds ratio undefined, lower 95% confidence limit 16.1). Two pots of soup were prepared from ready-to-serve cans, which were diluted with water and enriched with a commercially prepared flavor enhancer. The school's boiler, dormant during the previous 5 months, was restarted on the morning of the outbreak. The boiler also served as a tankless hot water heater. Laboratory analysis of the soup identified abnormally high quantities of nitrite (459 ppm) and sodium metaborate, major components of the boiler water treatment solution. Undiluted soup from the same lot had 2.0 ppm nitrites; the flavor enhancer had 2.2 ppm nitrites. Nitrites were present in the hot potable water system (4 to 10 ppm) and absent in the cold potable water system.
Conclusions: This outbreak of methemoglobinemia due to nitrite poisoning was traced to soup contaminated by nitrites in a boiler additive. Nitrites are ubiquitous and potentially hazardous inorganic ions. Extreme caution should be used when the possibility for toxic human exposure to nitrites exists.