关键词：硝酸 硝酸盐
Methemoglobinemia is a blood disorder caused when nitrite interacts with the hemoglobin in red blood cells. Unlike hemoglobin, the methemoglobin formed in this interaction cannot carry sufficient oxygen to the body's cells and tissues. Although methemoglobinemia is rare among adults, cases have been reported among infants, where nitrate-contaminated well water was used to prepare formula and other baby foods. Nitrate in well water may result from point sources such as sewage disposal systems and livestock facilities, from nonpoint sources such as fertilized cropland, parks, golf courses, lawns and gardens or from naturally occurring sources of nitrogen.
Methemoglobinemia
Nitrate, a relatively non-toxic substance, occurs naturally as part of the nitrogen cycle. However, bacteria can convert nitrate to nitrite in the environment, in foods and in the human body.
Until infants reach about six months of age, their digestive system secretes lower amounts of gastric acid and the pH level in their digestive system is higher than most adults. Adults with a diminished capability to secrete gastric acid also can experience a rise in pH in their digestive system. In both situations, bacteria can proliferate, increasing the transformation of nitrate to nitrite.
Once in the blood, nitrite oxidizes iron in the hemoglobin of red blood cells to form methemoglobin, which lacks hemoglobin's oxygen-carrying ability. The nitrite can come from nitrate in drinking water or from food, some drugs or other sources. For additional information on hemoglobin oxidizing agents, read Extension Circular EC90-2502, Perspectives on Nitrates and consult your physician.
Although methemoglobin is continually produced in humans, an enzyme in the human body reduces methemoglobin to hemoglobin. In most individuals, methemoglobin is rapidly converted back to hemoglobin. Typically, less than 1 percent of the total circulating hemoglobin in a healthy adult is present in the form of methemoglobin. Infants, however, have a low concentration (about 60 percent of the adult concentration) of the reducing enzyme, as do some older individuals with an enzyme deficiency. In these people, methemoglobin is not converted to hemoglobin as readily.
When methemoglobin levels are elevated, the condition known as methemoglobinemia, often referred to as "blue baby syndrome", can result as the blood lacks the ability to carry sufficient oxygen to individual body cells.
Methemoglobinemia Signs and Symptoms
Infants suffering from methemoglobinemia may seem healthy but show intermittent signs of blueness around the mouth, hands and feet. They may have episodes of breathing trouble, some diarrhea and vomiting. In some cases, an infant with methemoglobinemia has a peculiar lavender color but shows little distress. Blood samples appear chocolate brown and don't turn pink when exposed to air. When the methemoglobin level is high, infants express a marked lethargy, excessive salivation and loss of consciousness. Convulsions and death can occur at extreme methemoglobin levels.
Consult your physician for additional information on methemoglobinemia signs and symptoms or for diagnosis if methemoglobinemia is suspected. Do not attempt to self-diagnose this condition.
Methemoglobinemia Treatment
If the condition is identified early and is not life-threatening, a change of drinking water to water with less than 10 milligrams per liter of nitrate-nitrogen is usually the only needed treatment. This will reduce methemoglobin to hemoglobin in two to three days. Severely affected infants
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