Nitrite and nitrate ions are naturally occurring forms of nitrogen that can be present in ground and surface water and can be used as a food preservative because they inhibit the growth of Clostridium botulinum (1). Exposure to excessive levels of nitrite or nitrate may result in the acute syndrome of methemoglobinemia (MetHb), in which nitrite binds to hemoglobin. This report summarizes the findings of investigations of two incidents in which unintentional exposure to high doses of nitrite occurred through drinking potable water contaminated with additives to boiler conditioning fluids.
Incident 1
On October 20, 1992, a school nurse contacted the New Jersey Poison Information and Education System regarding the acute onset of illnesses in 49 children in first through fourth grades in one school (2). All of the children had visited the school nurse within 45 minutes after lunch because of blueness of the lips and fingers. The poison center, after ruling out a possible local stain, suggested that the children be examined at a hospital. When the children were examined, additional complaints included nausea, vomiting, and headache. An emergency department physician, in consultation with the poison center, made the presumptive diagnosis of MetHb on the basis of cyanosis with normal pulse oximetry readings of oxygen saturation >88%. Initial questioning by the poison center did not identify possible sources.
MetHb was diagnosed in 29 (59%) of the 49 students, and in 14, levels were >20% (range: 3%-47%; normal: <2% {3}). Manifestations among the 49 children included cyanosis (79%), nausea (69%), abdominal pain (68%), vomiting (66%), and dizziness (52%). All 14 of the children who were hospitalized were treated with supplemental oxygen and intravenous methylene blue. All patients recovered fully within 36 hours with no complications.
The field investigation indicated that the children with MetHb had eaten soup served during the second lunch period. The soup had been prepared from a commercially canned product that was taken directly from the can and heated before being served. To provide second servings, the soup was diluted with a 1:1 ratio of water obtained from hot and cold water taps in the school kitchen. Analysis of the leftover diluted soup detected nitrite levels of 459 parts per million (ppm). Samples of the original undiluted soup contained a nitrite level of 2 ppm.
Analysis of water from the hot water taps in the kitchen detected a nitrite level of 4 ppm to 10 ppm; samples from the cold water tap were negative for detectable nitrite. The hot water boiler had been serviced in May 1992 with commercial conditioner fluid containing nitrite and sodium metaborate, and had not been started until the morning of the incident in October. Boiler treatment solution had been added to the boiler during routine boiler conditioning approximately 2 weeks before the incident. Sodium metaborate levels were measured in the soup, and traces were found in the leftover diluted soup but not in the undiluted soup. During the investigation of the outbreak, the backflow check valve (which prevents backflow of water from the boiler to the potable water system) was tested and determined to be faulty and stuck in the open position. A section of the boiler also was used as a tankless water heater. In addition, the hot water coil tap and the tap for boiler treatment solution were in the same location, and neither tap was labeled. The school's water system was flushed; water from all taps was retested and was negative for nitrite and sodium metaborate. As a result of this incident, the school discontinued heating of water through the boiler coils and removed the hot water coil tap.
Incident 2
On March 23, 1996, the poison center was contacted by an office worker regarding the acute onset of blueness of skin in six of her office coworkers who had been meeting in a conference room. The poison center suggested that the workers be examined at a hospital. The presumptive diagnosis by the emergency department clinician in consultation with the poison center was MetHb; initial questioning by the poison center did not identify any methemoglobin inducers.
Four of the six workers were evaluated by physicians; MetHb was diagnosed based on analysis of blood samples (range of methemoglobin levels: 6%-16%). Two patients were treated with supplemental oxygen and intravenous methylene blue, and all recovered without complications within 24 hours.
All six workers had onset of illness after drinking coffee prepared with water from a nearby hot water faucet. Analysis of the leftover coffee detected a nitrite level of 300 ppm. Nitrate levels were >50 ppm in samples of hot water obtained from several sites in the building, including the tap where the water was obtained to prepare the coffee. All samples of cold water contained negligible amounts of nitrate (<0.1 ppm). During the subsequent field investigation of this outbreak, the backflow prevention valve was removed from the boiler in the building, determined to be defective, and replaced. Conditioning fluid in the boiler contained both nitrites and sodium borate. Traces of sodium borate were found in the coffee.
Reported by: RD Shih, MD, Dept of Emergency Medicine, Morristown Memorial Hospital, Morristown; SM Marcus, MD, New Jersey Poison Information and Education System, Dept of Emergency Medicine and Dept of Pediatrics, Newark Beth Israel Medical Center; CA Genese, BN Manley, KS Kolano, KC Spitalny, MD, F Sorhage, VMD, Acting State Epidemiologist, Div of Epidemiology, Environmental, and Occupational Health Svcs, New Jersey Dept of Health; B Waterson, F Clayton, Div of Environmental Health, Camden County Dept of Health and Human Svcs, Camden. Div of Field Svcs, Epidemiology Program Office; Health Studies Br, Div of Environmental Hazards and Health Effects, National Center for Environmental Health, CDC.
Editorial Note: Methemoglobinemia may result when hemoglobin is exposed to oxidizing agents such as nitrite or nitrate. These compounds cause the iron in the hemoglobin to be oxidized (Fe (superscript 2+) -->Fe (superscript 3+)), producing methemoglobin and a reduction in oxygen-carrying capacity (1). Manifestations of MetHb may include cyanosis, headache, nausea, vomiting, and dizziness; the syndrome usually is not fatal. Nitrates and nitrites are strong oxidizing agents and are established causes of this syndrome (4); amino- and nitro-aromatic compounds are 10 times more potent than sodium nitrite in oxidizing hemoglobin (5). An analysis of data from CDC's National Hospital Discharge Survey indicated that, during 1985-1990, only 18 cases of MetHb were recorded (CDC, unpublished data, 1997); in addition, data from CDC's Compressed Mortality File confirm the low case-fatality rate (cumulative incidence rate was 0.01 MetHb deaths per million population during 1979-1994). Based on data from the American Association of Poison Control Centers Toxic Exposure Surveillance System (6), during 1995 there were 970 cases of MetHb but no associated deaths; however, cases were not verified by laboratory analysis.
The two episodes described in this report resulted from ingestion of water originating from potable municipal sources but that had become cross-contaminated with boiler fluid because of defective backflow valves. This potential mechanism for nitrite exposure has not been widely recognized. The boiler fluids probably refluxed when the boilers were started, thereby generating high pressure and fluid reflux into the buildings' water systems. Although most municipalities have regulations requiring backflow valves on boilers to prevent such incidents in large buildings (7), there are no provisions for routine inspection and replacement of these valves. Building managers and personnel who service boilers should be informed about the potential problem and the need to turn off boilers during servicing to ensure a reverse pressure gradient is not produced. In addition, backflow valves should be inspected routinely to ensure proper operation, and conditioner fluid containers should include warning labels with specific instructions for replacing fluid and for proper operation of the safety valve backflow mechanism.
These two incidents underscore the need for health-care workers to consider this potential source of exposure in the differential diagnosis of MetHb. Other compounds with potential for inducing MetHb include organic nitrates (e.g., room deodorizer propellents and certain pharmaceutical agents), laundry ink, industrial solvents, some local anesthetics (benzocaine and lidocaine), sulfonamides, mothballs, and fungicides.
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