Causes of Blisters
When the outer layer of skin (epidermis) is subjected to repeated contact friction or shear forces (shear is a tearing process in the skin caused by tissue layers moving in opposing directions), a blister typically results. The friction or shear forces cause tears in the second and third layers of the skin, and pressure forces fluid into the open spaces. Fluid fills up the open spaces in the deeper layers of the skin, while the top layer remains intact. In situations where blood vessels in the skin also break, blood drains along with the fluid, forming what are often called "blood blisters."
Blisters on the feet typically result from repetitive stress to exposed areas, particularly those that protrude or are raised, such as bunions and hammer toes, and on normal weight-bearing areas (pressure points).
The main culprit is an improperly fitted shoe or boot. Sometimes the design—for example, a too narrow toe box, a too wide heel area, raised seams in the shoe or boot lining--can lead to blisters even when shoes and boots are properly fitted.
Blistering is more frequent and more severe when the feet get wet, which softens the skin, making it much more vulnerable to friction and shear forces. Most friction blisters tend to form in the heel area, where friction from movement of the heel causes the skin to tear.
Blisters also are commonly found:
- on the sides of the big toe and the fifth (“pinky”) toe, when the toe box of the shoes or boots “squeeze” the forefoot;
- on the bottom of the toes, where friction from toe off (the final stage of the running gait, where the toes propel the runner forward as the foot is pushed off from the ground) occurs;
- on the tops and tips of the toes in situations such as hammer toes, where the feet or toes have changed shape due to other issues;
- on the tops of the toes, often from raised seams in socks;
- on the sides and back of the heels, when the heel area of the shoe or boot rubs and causes friction.