Does hydrogen peroxide remove blood stains?

Hydrogen peroxide does something different from every other treatment for blood. Cold water and enzymes take the stain out of the material. Peroxide leaves it there and destroys the part of it that has colour.

Why it fizzes

Blood contains catalase, an enzyme that breaks hydrogen peroxide down into water and oxygen. The foaming you see is that reaction. It is a reliable sign that there is blood present and it is not a measure of how much stain is being removed, so there is no reason to keep reapplying until it stops fizzing.

Where it is the right answer

On white cotton and white bedding, where a biological soak has stalled on an old mark, oxidising the remaining colour is the only route left and the material has no dye to lose. On pale marble it is what the stone trade specifies for organic stains, applied as a poultice rather than wiped on.

Where it is the wrong answer

On anything coloured. Restoration companies are direct about this: it discolours fabric, so it belongs only on white or very light material and only after a hidden test that has been allowed to dry. Carpet and upholstery are worse cases again, because neither can be rinsed, so the peroxide keeps working after you have stopped, and a lightened patch in a fitted carpet or a sofa cushion cannot be replaced. On finished leather it attacks the pigment layer that gives the hide its colour.

Strength and freshness

Household peroxide is 3 percent. The stone trade calls for 12 percent, which is hair bleaching strength, so a poultice made with the household version is the cautious version of that method and needs more repeats. Peroxide also decomposes on the shelf, so an old half used bottle is largely water and a disappointing result may be the bottle rather than the method.