Sulfhemoglobinemia is a rare condition in which there is excess sulfhemoglobin (SulfHb) in the blood.
It causes cyanosis even at low blood levels.
It is a rare blood condition in which the β-pyrrole ring of the hemoglobin molecule has the ability to bind irreversibly to any substance containing a sulfur atom.
[1][2] When hydrogen sulfide (H2S) (or sulfide ions) and ferrous ions combine in the heme of hemoglobin, the blood is thus incapable of transporting oxygen to the tissues.
Another possible cause is occupational exposure to sulfur compounds.