Why Does Permanent Marker Stick To Human Skin, But Blood Doesn’t?

Permanent marker sticks to skin because its alcohol solvent (typically n-propanol or n-butanol) carries dye into the outer keratin layer, where it lingers until the dead cells flake off. Fresh blood has no such solvent, so it sits on the surface and rinses off, until it dries, when clotted proteins and oxidized hemoglobin glue it in place.

Here’s a throwback to my childhood days – back when I was in 6th grade, I got in a fight with a classmate. He had an ink pen, which he deliberately swung a few inches away from me, leaving a blue streak of ink across my face and neck.

I was mighty angry, but we’re not here to discuss all that, we’re only interested in the fact that ink stains (like that one) stick pretty well to human skin. When I got home that day, I had to spend quite a bit of time in the bathroom scrubbing my face and neck like crazy to get that stain to disappear.

On the other hand, blood, which, if you think about it, is just another pigment, doesn’t stick to the human skin as ‘adamantly’ as ink. However, if blood dries on the skin, then it also becomes very hard to remove. Why is that?

Much of this can be explained by the structure of skin and fabric, and by understanding how ink or blood ‘particles’ interact with other objects.

Ink And Skin

When you draw on your skin with a permanent marker, ink travels from the tip to your skin in a solvent, typically an alcohol like n-propanol or n-butanol (a Sharpie safety data sheet lists both, along with diacetone alcohol). The solvent evaporates quickly, but before it does, it carries dye into the keratinized outer layer of your skin (the stratum corneum). Most of the pigment stays on the surface, but a small amount tucks itself into the dead-cell brickwork of that layer. In other words, ink really can get under your skin… literally.

The dye and pigment particles in marker ink are tiny, far smaller than a red blood cell (which is about 7.5 micrometers, or roughly 0.0003 inches, across), and the alcohol carrying them is smaller still. That is why the alcohol-and-dye combo can slip between the corneocytes in your stratum corneum, where a chunky red blood cell simply cannot fit.

When you try and wash ink stains off your skin, they usually come off easily. If the ink is not water soluble, using soap can help to accelerate the ‘cleaning’. However, even when you use soapy water, there may still be a little ink left, which is very stubborn, that you can’t get completely rid of. That’s the small portion of the ink that got inside the top layer of your skin. So, when you really go to town while scrubbing your skin to remove that stain, you essentially scrub off some of the outer layer of your skin.

Is that brief contact with a marker dangerous? Probably not. Standard fine-point Sharpies carry the ACMI AP (Approved Product) seal, meaning a toxicologist has reviewed them and judged the ink non-toxic when used as intended. That said, Sharpie itself does not recommend using their markers on skin, and the heavy-duty Magnum, King Size and Touch-Up markers contain xylene, an industrial solvent that can irritate skin and shouldn’t go anywhere near it. If you want to draw on your kid for face-paint day, stick to cosmetic body markers or washable Crayola pens.

Blood stains, however, are a different story.

Blood Stains

The composition of blood is quite different from that of ink, which accounts for the difference in how blood and ink interact with either skin or fabric.

Red blood cells are about 7.5 micrometers across, much bigger than the dye in marker ink, so they cannot push past the keratin of the stratum corneum to begin with. And unlike marker ink, blood does not arrive with its own alcohol solvent (it is roughly 92% water), so there is nothing to carry pigment into the skin. Barring stray pores and hair shafts, fresh blood has little to grab onto and rinses off the skin easily (a lot easier than ink).

However, blood stains do not come off easily from fabrics, because clothes have many holes, pores and crevices where blood can enter and be absorbed by the fabric itself. This makes the blood stain ‘stick’ to the fabric, making it difficult to remove quickly.

However, if the blood stain is dry, then the dynamics change.

If blood is left on skin or cloth long enough to dry, two things happen at once. First, the clotting machinery that blood is built for kicks in: fibrinogen turns into long, sticky fibrin threads that weave a mesh around the red and white cells, locking them onto whatever surface they are sitting on. Second, as the water evaporates, the proteins in that mesh harden and the iron in hemoglobin oxidizes (which is also why old bloodstains turn brownish-rust rather than bright red).

The result is a hardened, glue-like film that bonds to the host surface. That is what makes dried blood stains so much harder to scrub off than fresh ones, especially without an enzyme detergent or some kind of cleaning agent.

Here is a fun fact about blood stains: most household stains come out more easily in hot water, but blood is the opposite. Hot water denatures the proteins in the stain (think of how an egg white turns from clear and runny to solid white when you cook it) and bonds them permanently to the fibers. That is why every laundry guide, from the University of Georgia Extension on down, tells you to use cold water on blood, and a protein-digesting (protease) enzyme cleaner if the stain has already set.