Blisters are usually caused by injury to the skin from friction, such as shoes that rub, or heat, such as from sunburn or a scald. They may also occur as a reaction to a substance, or as a symptom of an underlying medical condition.
Friction or heat on the skin can create a tear between the upper layer of the skin (the epidermis) and the layers underneath. When this happens, the surface of the skin remains intact but is pushed outwards as serum (blood without red cells or clotting agents) collects in the newly created space between the layers of skin.
Friction
Friction blisters commonly occur among active people who regularly play sports and are in the military. They're 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.
Blisters often occur on the feet and hands, which can rub against shoes or 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.
Heat
Extreme heat can sometimes cause blisters to develop on the skin in cases such as:
Skin reaction
A blister can also sometimes form when a person’s skin comes into contact with a cosmetic, detergent, solvent or other chemical. They can also develop following an allergic reaction to an insect bite or sting.
Medical conditions
A number of medical conditions cause blisters. The most common are:
- chickenpox – a childhood illness that causes itchy red spots
- common 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 several rarer conditions that can cause blisters. They are described below.
- Bullous pemphigoid – a skin disease that causes large blisters and usually affects people over 60 years old.
- Pemphigus – 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.
- Chronic bullous dermatosis of childhood – a condition that causes clusters of blisters to develop on the face, mouth or genitals.
