A blister is a small pocket of fluid within the upper layers of the skin. Blisters can be filled with blood (known as blood blisters) or with pus (if they become infected). However, most blisters are filled with a clear fluid called serum. Serum is the part of the blood that remains after red blood cells and clotting agents have been removed.
A blister usually forms because the outer layer of the skin has become damaged. Fluid collects under the damaged layer of skin, cushioning the tissue underneath, protecting it from further damage and allowing it to heal.
A blood-blister usually forms when a small blood vessel close to the surface of the skin ruptures (breaks) and blood leaks into a tear between the layers of skin. This can happen if the skin is crushed, pinched or squeezed very tightly.
Blisters can also form as the result of certain medical conditions.
Blisters are usually caused by injury to the skin from heat or from friction, which creates a tear between the epidermis—the upper layer of the skin—and the layers beneath. When this happens, the surface of the skin remains intact, but is pushed outwards as serum seeps into the newly created space between the layers.
Short periods of intense rubbing can cause a blister, as can any rubbing of the skin continued long enough. Blisters are most common on the hands and feet, as these extremities are susceptible while walking, running, or performing repetitive motions. Blisters form more easily on moist skin than on dry or soaked skin, and are more common in warm conditions.
Sometimes, the skin can blister when it comes into contact with a cosmetic, detergent, solvent or other chemical; this is known as contact dermatitis. Blisters can also develop as a result of an allergic reaction to an insect bite or sting.
There are also a number of medical conditions that cause blisters. The most common are chickenpox, herpes, impetigo, and a form of eczema called dyshidrosis. Other, much rarer conditions that cause blisters include:
Before going for a long walk, it is important to ensure that shoes have been broken in. If a hot area on the foot is felt, taping padding over the affected area can prevent the formation of a blister.
To avoid blisters on the hands, gloves should be worn when using tools such as a shovel or pickaxe, doing manual work such as gardening, and handling detergents, cleaning products, solvents and other chemicals.
Sunscreen and sun protection should also be used during the hottest part of the day to avoid blisters from sunburn, and moisturizing, after-sun or calamine lotions can help to ease discomfort in the case of burns.
The unbroken skin over a blister provides a natural barrier to infection, and patients should try to keep blisters intact and unbroken in order to avoid infection, ideally only allowing it to break on its own once the skin underneath has healed.
A common treatment used by medics in the U.S. Army is to drain the fluid from a blister and to inject the same amount of compound tincture of benzoin, to help seal the space created by the blister, to serve as a local antiseptic, and to prevent further abrasion or loss of skin.