Acquired methemoglobinemia results when the rate of formation of methemoglobin exceeds the rate of reduction after exposure to certain compounds. Many compounds have been implicated and the exact mechanism is not well understood. Complex reactions involving metabolites with free radical intermediates have been hypothesized. Nitrites and aniline derivatives which most frequently produce methemoglobinemia. Because of its use as a meat preservative and its similarity in appearance to table salt, sodium nitrite is frequently the cause of methemoglobinemia.
Nitrites are also present in many medicinal agents.
Mosts reports of nitrate- induced methemoglobinemia, however, are due to infants fed well water contaminated with high levels of nitrates. Nitrate salts are converted by enteric bacteria to nitrites which are then rapidly absorbed from the gastrointestinal tract. Silver nitrate used on burns is also a common source as bacteria infecting burns may similarly cause nitrite production. Aniline and its derivitives are found in many household products such as inks, shoe polishes, paints and varnishes, and in some red and orange wax crayons. Other drugs commonly associated with methemoglobinemia include phenacetin, sulfonamides, and pyridium.