Methemoglobin
A form of hemoglobin in which iron has been oxidized, rendering it unable to carry oxygen effectively—a problem that can occur when hemoglobin exists outside red blood cells.
A variant of hemoglobin in which the iron atom in the heme group has been oxidized from the ferrous (Fe²⁺) to the ferric (Fe³⁺) state. In this oxidized form, hemoglobin cannot bind or transport oxygen to tissues. Small amounts naturally occur in the body and are continuously reduced back to functional hemoglobin by cellular enzymes. Elevated levels—whether from genetic conditions, certain medications, or chemical exposures—cause a condition called methemoglobinemia, which starves tissues of oxygen despite normal breathing.
What this means in real life
A toddler who eats nitrate-contaminated well water may develop bluish skin because their hemoglobin has been oxidized into methemoglobin, which can't deliver oxygen even though their lungs are working fine.
What it isn’t
It is not simply 'old' or 'used' hemoglobin that has already delivered its oxygen. Methemoglobin is chemically altered—its iron is in the wrong oxidation state—so it is structurally incapable of picking up oxygen in the first place.
Commonly misused online
Health forums sometimes conflate methemoglobin with carbon monoxide poisoning or anemia, but those involve different mechanisms: carbon monoxide blocks oxygen binding sites, anemia means too few red cells, while methemoglobin means the iron itself is chemically stuck.