Unsourced material may be challenged and removed. (September 2007). Methemoglobinemia (or methaemoglobinaemia) is a disorder characterized by the presence of a higher than normal level of methemoglobin (metHb, i.e., ferric [Fe 3+ ] rather than ferrous [Fe 2+ ] haemoglobin) in the blood.igns and symptoms of methemoglobinemia (methemoglobin level above 1%) include shortness of breath, cyanosis, mental status changes (~50%), headache, fatigue, exercise intolerance, dizziness and loss of consciousness.