A blister is a small pocket of fluid that forms in the upper layers of the skin.
It usually forms when the outer layer of the skin has been damaged. Fluid collects under the damaged skin, cushioning the tissue underneath. This protects the tissue from further damage and allows it to heal.
Most blisters are filled with a clear fluid called serum, which is the part of the blood that remains after red blood cells and clotting agents have been removed. However, blisters are sometimes filled with blood (blood blisters) or pus if they become inflamed or infected.
Blisters can develop anywhere on the body, but are most common on the hands and feet.
What causes blisters?
Blisters can be caused by:
friction to the skin
contact with chemicals, such as detergent
heat – for example, from sunburn or a scald
medical conditions, such as chickenpox and impetigo
When to see your GP
Most blisters heal naturally between three and seven days and don't require medical attention.
See your GP if you have blisters that:
you think are infected
are very painful
keep coming back
An infected blister will be filled with yellow or green pus and may be painful, red and hot.
It's important not to ignore an infected blister because it could potentially lead to secondary impetigo (a contagious bacterial infection of the skin) and further complications, such as cellulitis or sepsis.
You should also talk to your GP if you have blisters in unusual places, such as on your eyelids or inside your mouth, or if they appear after severe sunburn, burns or scalds or an allergic reaction, or after coming into contact with chemicals or other substances.
There are a number of things you can do to avoid getting blisters caused by friction, sunburn or chemicals. For example, you can:
wear comfortable, well-fitting shoes
wear gloves when handling chemicals
Causes of blisters
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, asa 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 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.
A number of medical conditions cause blisters. The most common are:
chickenpox – a childhood illness that causes itchy red spots
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
Blisters can sometimes form when your skin comes into contact with substances such as cosmetics, detergents and solvents.
Most blisters heal naturally and do not require medical attention.
As new skin grows underneath the blister, your body will slowly reabsorb the fluid in the blister and the skin on top will dry and peel off.
The unbroken skin over a blister provides a natural barrier to infection. It's important that the skin remains intact to avoid infection.
As tempting as it may be, try not to pierce a blister with a needle because it could lead to an infection or slow down the healing process. Allow the skin to peel off on its own after the skin beneath has healed.
You may choose to cover small blisters with a plaster. Larger blisters can be covered with a gauze pad or dressing that can be taped in place.
Painful blisters, or those in positions where they are likely to burst, such as on the sole of your foot, can be covered with a soft dressing to cushion and protect them. Change the dressing daily and wash your hands before touching the blister to avoid infection.
If a blister has burst, do not peel off the dead skin on top of the blister. Allow the fluid inside to drain and then cover the blister and the area around it with a dry, sterile dressing to protect it from infection until it heals.
Hydrocolloid dressings, available over the counter from pharmacies, have been shown to help prevent discomfort and encourage healing.
If the top layer of dead skin from a burst blister has already rubbed off, do not pick at the edges of the remaining skin. Follow the advice 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 should be left 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 relieve the pain (a bag of frozen vegetables works just as well). Between 10 and 30 minutes should help.
To stop the ice touching your skin directly, place a towel over the affected area before applying the ice pack.
If your blisters are caused by a medical condition, such as chickenpox, herpes or impetigo, your GP will be able to advise you about how to treat the underlying condition.
There are several ways to prevent getting blisters from friction, sunburn or chemicals.
Blisters caused by a medical condition often cannot be prevented, so the cause will need to be treated by a GP.
Wearing comfortable, well-fitting shoes and clean socks will help prevent blisters.
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 prevent them.
If you play sport or exercise regularly, wearing 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 you have worn before. Brand new shoes that aren't broken in may not be comfortable and may rub.
Stop immediately if you feel a hot area on your foot while walking, exercising or playing sport. If possible, tape some padding over the area.
Wear protective gloves when using tools such as shovels or pickaxes, and when doing manual work such as gardening. This 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 caused by burns or scalds.
Use sunscreen when in the sun. Keep your skin covered with clothing to avoid getting blisters from sunburn. You should also wear a sun hat.
Moisturiser, aftersun or calamine lotion can help ease discomfort if you do get sunburnt.
Always 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.