A blister is a small pocket of fluid in the upper layers of the skin.
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 when the outer layer of the skin has been damaged. Fluid collects under the damaged layer of skin, cushioning the tissue underneath. This protects the tissue from further damage and allows it to heal.
Blisters are sometimes filled with blood (blood blisters) or yellow or green pus if they become inflamed or infected. An infected blister may be red, painful and hot.
A blood blister usually forms when a small blood vessel close to the surface of the skin breaks and blood leaks between the layers of skin. This can happen if the skin is crushed, pinched or tightly squeezed.
What causes blisters?
Most blisters are common and 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 (a highly contagious bacterial infection of the surface layers of the skin)
Read more about what causes blisters.
When to seek medical attention
You should visit your GP if you have blisters that you think are infected, blisters that are very painful, or blisters that keep coming back. Also visit your GP if you have blisters that develop in unusual places, such as on your eyelids or the inside of your mouth, or if they appear after contact with chemicals or other substances.
Most blisters heal naturally, usually between three and seven days, and don't require medical attention. However, speak to a medical professional if your blister starts weeping pus or becomes inflamed and swollen. Also seek medical advice if the blister was caused by:
Read more about treating blisters.
Even though it may be tempting, try not to burst a blister as it could lead to an infection or slow down the healing process. However, if it becomes very large or painful your doctor or nurse may decide to decompress the blister under sterile conditions.
If a blister does 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.
Preventing blisters
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
- use sunscreen
Read more about preventing blisters.
