When human feces turn black after exposure to air, it is often due to a chemical reaction called "oxidation." The color change is primarily caused by the breakdown of hemoglobin, a protein found in red blood cells that carries oxygen.
Here's the scientific explanation for the process:
Iron content: Hemoglobin contains iron, which gives blood its red color. When blood is digested in the gastrointestinal tract and reaches the large intestine, some of the iron is converted to a compound called "ferric iron" (Fe3+).
Oxidation: When feces are exposed to air, oxygen from the atmosphere interacts with the ferric iron in the hemoglobin, leading to its further oxidation. As a result, the ferric iron is transformed into a dark-colored compound called "ferric oxide" or "ferrihemate."
Black color: The presence of ferrihemate in the feces imparts a black or dark brown color to them.
It's important to note that while black feces can occur due to oxidation, it can also be a sign of certain medical conditions or the intake of certain medications or supplements. If someone experiences persistent black stools or has concerns about their stool color, it is essential to seek medical advice to determine the underlying cause. In some cases, black stools may indicate bleeding in the upper gastrointestinal tract, which can be a serious medical issue.