Hardly noticeable and barely bleeding, paper cuts are the mother of all library injuries. Anyone who deals with paper on a daily basis will have at some point suffered such an affliction. Paper cuts cause a seemingly out of proportion amount of pain due to the anatomy of our skin and the structure of paper. When very thin and held in place, a sheet of paper becomes inflexible and can exert very high levels of pressure – enough to slice through flesh! Yikes! Let’s go under the microscope to see what's happening...

Figure 1: A single sheet of paper at x30 magnification.

Figure 2: Paper cuts - small but mighty!

Most paper cuts result from new sheets of paper held strongly in place. A rogue sheet may come loose from the pack but remain held in position by the rest of the tightly-knit sheets. In paper, more resistance is felt when a force is applied parallel to a sheet of paper. This has to do with the paper’s tensile strength. Tensile strength measures the ability of a material to resist rupture when force is applied to one of its sides under certain conditions. Held in place, the sheet of paper becomes extremely resistant to buckling, stiffens, and acts as a razor.

Figure 3: A sheet of paper that strays from the pack can cause serious paper cuts!

A paper’s edge may appear to be smooth and flat, but on a microscopic scale paper edges are jagged. Paper cuts leave a wound more like one from a saw than a knife (a miniature papery saw).

Figure 4: Pages from a copy book at x30 magnification. Fibres at the surface give paper a serrated edge. The black lines are page lines.

Paper cuts are remarkably painful. They usually occur in the fingertips, which have a greater concentration of nerve cells (neurons) than the rest of the body – an evolutionary trait to protect us during the exploration of our environment. Neurons send chemical and electrical signals to our brain, and some of them, called nociceptors, detect potential harm. Paper cuts stimulate a large number of nociceptors in a very small area of the skin. Shallow paper cuts don’t bleed very much so pain receptors are left open to the air resulting in continuous pain as the wound cannot clot and seal. As we continue to use our hands, the wound flexes open, continually distressing these neurons.

Not only do paper cuts part the flesh with a micro-serrated paper edge, but they also damage skin either side of the wound due to the composition of the paper. Pain receptors are continuously irritated by the combination of cellulosic wood pulp, rags, grasses, chemically-coated fibres, and bacteria that make up paper. Paper may also include other additives such as chalk or china clay to make the paper easier to write on. Sizing gives us a great variety of papers to suit the specific type of ink we wish to apply, but involves mixing many additives into the pulp to determine the correct surface absorbency.

Paper cuts from envelopes can be particularly stingy due to the layer of glue along the sealing tab. The glue is made from gum arabic, which although edible to humans, can pack a punch if embedded inside a wound. Gum arabic is the product of hardened sap taken from two species of acacia trees, and is also used as a binder for watercolour painting, and in traditional lithography.

Figure 5: Gum arabic glue at x30 magnification coats the paper tab on an envelope.

Figure 6: Gum arabic glue at x200 magnification coats the paper tab on an envelope. When the gum is moistened it forms a seal with the adjacent paper.

When skin closes around the paper cut these foreign particles become trapped inside causing a great deal of pain. This is why a cut from a razor blade is usually less painful than that from a paper cut: razor blades make clean incisions without leaving behind any foreign particles. It hurts initially, but the pain soon ebbs away. Bleeding caused by a razor cut helps to wash away any infection-causing particles, while paper cuts bleed very little (this also reduces your chances of getting any sympathy!)

Figure 7: A razor blade at x50 magnification.

Figure 8: A razor blade at x200 magnification. The razor’s edge is smooth allowing a clean incision without introducing foreign bodies.

It might seem strange that sometimes needles for a flu jab require quite a bit of force to pierce the skin, yet paper (PAPER!) can slice through. This is due to the random orientation of collagen fibres in our skin allowing us to withstand pinpoint forces.

Figure 9: Human skin feeling the pressure under a sharp pin (x20 magnification).

Our skin does not have a comparable strength against shearing forces such as those exerted by paper, and so, we are susceptible to the small but mighty paper cut. Libraries can be dangerous places. Be careful out there!

Christina Duffy (@DuffyChristina)

 

Further learning:

Paper May Be the Unkindest Cut, Scientific American, Volume 306, Issue 3 , Mar 1, 2012 |By Steve Mirsky 

Why Do Paper Cuts Hurt So Much? Scientific American - Instant Egghead #25