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:

• burns or scalds 
• sunburn 
• electrical or chemical injuries

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.

Blisters from friction or heat usually have an obvious cause, such as sunburn, wearing shoes that rub or heavy manual work.

Blisters are easy to recognise as raised pockets of skin that contain fluid. Blood blisters are usually red or a darker colour.

Blisters that develop without apparent injury to the skin from heat or friction may be the result of a medical condition. If you develop blisters and you're not sure why, see your GP. They will be able to investigate what's causing them.

Most blisters heal naturally and don't require medical attention. As new skin grows beneath the blister, your body will slowly reabsorb the fluid in the blister, and the skin on top will dry and peel off.

Friction blisters

The unbroken skin over a blister provides a natural barrier to infection. The skin should remain intact to avoid infection. Never pierce a blister with a needle as it could lead to an infection or slow down the healing process (although a doctor may choose to do this under sterile conditions). Allow the skin to peel off on its own once the skin beneath has healed.

You may wish to cover small blisters with a plaster (adhesive dressing). Larger blisters can be covered with a gauze pad or a dressing that you can tape in place. If you have a blister that's painful or in a position that makes it likely to burst (such as on the sole of your foot), cover it with a soft dressing to cushion and protect it. Change the dressing daily and wash your hands before touching the blister to avoid infection.

Burst blisters

If a blister has burst, don't peel off the dead skin on top of the blister. Allow the fluid inside to drain, then cover the blister and the area around it with a dry, sterile dressing to protect it from infection until it heals. Hydrocolloid dressings have been proven to help stop discomfort and encourage healing.

If the top layer of dead skin from a burst blister has been rubbed off already, do not pick at the edges of the remaining skin but follow the same instructions outlined above to protect it from infection.

If the blister is on your foot, avoid wearing the shoes that caused it, at least until it heals.

Blood blisters

Leave blood blisters to heal naturally. If a blood blister bursts, keep the area clean and dry. Protect it with a sterile dressing to prevent infection.

Blood blisters are often painful. Applying an ice pack to the affected area immediately after the injury can help to relieve pain. Apply the ice pack for between 10 and 30 minutes. To make sure that the ice doesn't touch your skin directly, place a towel over the affected area before applying the ice.

Infection

If a blister becomes infected, it's important that you see a medical professional so that it can be treated. Your GP may prescribe antibiotics.

Medical conditions

If you have blisters that are caused by a separate medical condition, such as chickenpox, herpes or impetigo, you should visit your GP so that they can discuss how to treat the underlying condition.

There are several ways that you can prevent developing blisters from friction, sunburn or chemicals. Blisters that are caused by a medical condition often can't be prevented, so the cause will need to be treated by a GP.

Friction

Wearing comfortable, well-fitting shoes and clean socks will help you avoid getting blisters on your feet. Blisters are more likely to develop on moist skin. If you have sweaty feet, wearing moisture-absorbing socks or changing your socks twice a day can help to prevent blisters.

If you regularly exercise or play sport, wearing special sports socks, or thicker wool socks, can help keep your feet dry and reduce your risk of getting a blister.

If you're going for a long walk, wear shoes that fit properly, are comfortable and that you have worn before. Brand new shoes that aren't broken in may not be comfortable and could rub. If you feel a hot area on your foot while walking, exercising or playing sport, you should stop immediately. If possible, tape some padding over the area.

Wearing protective gloves when using tools, e.g. a shovel or pickaxe, and when doing manual work, such as gardening, will help prevent blisters developing on your hands.

Heat and sunburn

Be careful when dealing with heat such as steam, flames or boiling water. Make sure you use the right safety equipment in working environments that involve heat or chemicals.

Objects that get very hot, such as a stove or a kettle, should also be treated with care to avoid getting blisters that are caused by burn or scalds.

While you're out in the sun make sure that you use sunscreen. Cover your skin with clothing, including wearing a hat, in order to avoid getting blisters from sunburn. Moisturiser, aftersun or calamine lotion can ease the discomfort if you do get sunburnt.

Chemicals

Wear protective gloves when handling detergents, cleaning products, solvents and other chemicals. Avoid any unnecessary contact with chemicals, and be careful when dealing with them.