Blisters are often the result of an injury to the skin from friction. This can be caused by shoes that rub, for example.

They can also occur when skin is exposed to excessive heat, such as sunburn or a scald, as a reaction to a chemical substance, or as a symptom of an underlying medical condition.

If the skin is exposed to friction or heat, it can tear the upper layer of skin (epidermis) from the layers beneath. The surface of the skin remains intact but is pushed outwards because serum (blood without red cells or clotting agents) collects in the newly created space between the layers of skin.

Friction

Friction blisters are common in people who are very active, such as sports players and those in the military. They are usually caused by poor-fitting shoes.

A blister can develop if the skin is rubbed for a long period or if there is intense rubbing over shorter periods.

Friction blisters often occur on the feet and hands, which can rub against shoes and handheld equipment, such as tools or sports equipment. Blisters also form more easily on moist skin and are more likely to occur in warm conditions.

Medical conditions

A number of medical conditions cause blisters. The most common are:

• chickenpox – a childhood illness that causes itchy red spots
• cold sores
• herpes – a sexually transmitted infection (STI) that most commonly affects the groin 
• impetigo – a contagious bacterial skin infection
• pompholyx – a type of eczema

There are also several rarer conditions that can cause blisters. They are:

• bullous pemphigoid – a skin disease that causes large blisters and usually affects people over 60 years of age 
• pemphigus vulgaris  – a serious skin condition where blisters develop if pressure is applied to the skin; the blisters burst easily, leaving raw areas that can become infected
• dermatitis herpetiformis – a skin condition that causes intensely itchy blisters, usually on the elbows, knees, back and buttocks; blisters usually develop in patches of the same shape and size on both sides of the body
• epidermolysis bullosa – a group of rare inherited skin disorders that cause the skin to become very fragile; any trauma or friction to the skin can cause painful blisters
• chronic bullous dermatosis of childhood – a condition that causes clusters of blisters to develop on the face, mouth or genitals