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the appearance of methemoglobin in the blood as a result of the toxic effect of various chemical substances (nitrates and nitrites, aniline, pyridine) that are capable of converting hemoglobin to methemoglobin (MtHb) upon entering the body through the skin, digestive tract, or lungs. With considerable methemoglobinemia, oxygen starvation, or hypoxia, may result. Methemoglobin may be excreted with the urine (methemoglobinuria); this is sometimes damaging to the kidneys. Methemoglobinemia is treated with antidotes (methylene blue, ascorbic acid) and oxygen therapy.