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Blisters Basics Blisters Basics

Blisters Basics

Blister Basics

By John Vonhof, EMT-P

A basic understanding of how blisters are formed is necessary to successfully treat them. Heat, friction, and moisture contribute to the formation of blisters. Studies have shown that the foot inside the shoe or boot is exposed to friction at many sites as it experiences motion from side to side, front to back, and up and down. These friction sites also change during the activity as the exercise intensity, movement of the sock, and flexibility of the shoe or boot changes.

The outer epidermis layer of skin receives friction that causes it to rub against the inner dermis layer of skin. This friction between the layers of skin causes a blister to develop. As the outer layer of the epidermis is loosened from the deeper layers, the sac in between becomes filled with lymph fluid. A blister has then developed. If the blister is deep or traumatically stressed by continued running or hiking, the lymph fluid may contain blood. When the lymph fluid lifts the outer layer of epidermis, oxygen and nutrition to this layer is cut off and it becomes dead skin. This outer layer is easily burst. The fluid then drains and the skin loses its natural protective barrier. The underlying skin is raw and sensitive. At this point, the blister is most susceptible to infection.

The friction against whatever is touching the skin causes the friction between the layers of skin. As identified earlier, the majority of foot problems can be traced back to socks, powders, and lubricants—or the lack of them. In order to prevent blisters, friction must be reduced. Friction can be reduced in three main ways: wearing double-layer socks or one inner and one outer sock, keeping the feet dry by using powders, or using a lubricant to reduce chafing. It has been found that rubbing moist skin tends to produce higher friction than does rubbing skin that is either very dry or very wet. Since the skin blisters more easily when soft and moist, it is important to understand the value of moisture-wicking socks coupled with knowing whether powders and/or lubricants are best and how to use them. Eliminating pressure points caused by poor-fitting insoles and/or ill-fitting shoes can also reduce friction.

Proactive Prevention

Athletes need to recognize that they need to find what will work on their feet. That goes for what they put on your feet and how they treat blisters. You can give suggestions, but what works for another may not work for them. Experienced athletes report their feet swell one to two sizes over an expedition length event. They should wear shoes that are one-half to one size larger than normal. A second insole or second pair of socks will help the shoes fit at the start.

Reduce calluses with a callus file, being careful not to go too deep. Many athletes like calluses, feeling they protect their skin from blisters. However, treating blisters deep under calluses is difficult and sometimes impossible. Use a silicone-based lubricant, like Hydropel® or Sportslick®, which helps drive moisture away from their skin and reduces friction between feet and shoes.

Many athletes have problems caused by poorly maintained toenails. Untrimmed nails catch on socks putting pressure on the nail, causing blisters and black toenails, and cut into other toes. Toenails should be trimmed straight across the nail. Leave an extra bit of nail on the outside corner of the big toe to avoid an ingrown toenail. After trimming, use a nail file to smooth the top of the nail down toward the front of the toe and remove any rough edges. If you draw your finger from the skin in front of the toe up across the nail and can feel a rough edge, the nail can be filed smoother or trimmed a bit shorter.

Skin ‘tougheners’ work three ways. They coat the feet for protection, toughen the skin, and if using tape, help the tape or blister patches adhere better to the skin. Tincture of benzoin is commonly used, however if it gets into a cut or open blister, it will momentarily hurt. New Skin® can also be used to toughen the skin. Remember though that it is possible to get blisters under toughened skin and under calluses.

Taping provides a barrier between the skin and socks to reduce friction. There are specific methods of taping toes, the balls of the feet, heals, the bottoms of the feet, between toes, and even for the whole foot. Duct tape is commonly used. Making the tape stick involves cleaning the feet well, using a tape adherent, and rounding corners of the tape. Taping can be done as a pre-event prevention measure or over a hot spot or blister during a race.

Frequent sock changes will help keep the feet in good condition. Wet or moist shoes and socks, from water or constant sweating, can cause problems as the skin softens, maceration occurs, and skin layers separate. Changing the socks also gives opportunity to reapply either powder or lubricant and deal with any hot spots before they become blisters.
Maintaining proper hydration helps reduce swelling of the feet, often common after hours of running, so the occurrence of hot spots and blisters is reduced. When you become fluid-deficient, the skin loses its normal levels of water in the skin and easily rubs or folds over on itself, leading to blisters. Adjusting shoelaces can relieve friction and pressure over the instep and make footwear more comfortable.

Athletes should wear gaiters to provide protection against the sand, rocks, and grit. These irritants cause friction and blisters as shoes and socks become dirty. Additional tips include elevating feet above the level of the heart when resting in order to reduce swelling, taking off shoes and socks to air feet whenever possible and soaking feet in cool water to reduce swelling and soreness.

Your Event Foot Care Kit

If you participate in events on a medical staff or on a crew providing foot care, a large kit is necessary. The kit should contain material based on the length and type of event. The key is to have the materials to patch blisters so athletes can run the next day. The following materials form the core of the kit. Add or eliminate materials based on your personal preferences and experience. Look in the tool section of your local hardware stores for a plastic toolbox with trays in which to store the materials.

Lubricant of choice
Tweezers for pulling blister
Sharp-pointed scissors
Powder of choice
Blister patches of choice in various sizes
Spenco® 2nd Skin in various sizes
Tapes of choice in a variety of sizes
(1/2 and 1 inch Micropore®, Leukotape®, and duct tape)
Tincture of benzoin or other tape adherent
Swabs for applying benzoin
Alcohol pads
One DuoDerm® pad
Tube of zinc oxide
Tube of 2 % Xylocaine jelly
Gauze pads (2”x 2” & 4”x 4”) for draining blisters
Toenail clippers
Nail or pedicure file
Callus file
Utility scissors for cutting tape and shoe
Betadine® for cleaning dirty wounds/blisters
Antiseptic ointment
Extra socks
Self-adhering wrap
Ace wrap
Ankle support
Pads for metatarsal, arch, or heel pain
Latex gloves
Antibacterial hand wipes
Small basin for soaking feet
Sponge for cleaning feet
Hand towel for drying feet
Plastic bags for garbage

Fixing a Blister

Despite an athlete’s best efforts, occasionally a blister will develop. Here are some tips to keep you on the trail.

  • Drain blisters that are more than 3/4-inch in diameter and/or in a weight-bearing area, or ones containing hazy fluid, indicating infection. Draining a blood-filled blister requires attention to prevent infection.
  • Clean the skin around the blister with an alcohol wipe. Your feet must be cleaned of all lubricant and oils for the patch to stick.
  • If appropriate, drain the blister by making a small V-shaped cut in the sides of the blister where foot pressure will allow further drainage. Use flame-sterilized nail clippers or small scissors (this allows better drainage than needle holes). Push excess fluid out of the blister and dry the skin.
  • Use your fingers to push fluid out of the blister and dry the skin.
  • Apply a small dab of antibiotic ointment or zinc oxide to the top of the blister.
  • Directly over the blister, apply a blister patch like Spenco® Sports Blister Pads, or a large duct tape patch with a piece of toilet paper in the middle to keep the tape from sticking to the roof of the blister.
  • After applying a patch, roll socks on and off to avoid disturbing the patch, and use a shoe horn to ease the heel into the shoe.

To Drain Or Not To Drain

Should you drain or not? Use these tips to manage your blisters.

  • Drain the blister if it is more than three-quarters of an inch in diameter and in a weight bearing area.
  • Drain the blister if the fluid appears hazy or cloudy; this indicates infection has set in.
  • With a flame-sterilized nail clippers or small scissors make a small “V” cut in the side of the blister. Make the cut at the edges of the blister where ongoing foot pressure will push out additional fluid. This allows better drainage than needle holes.
  • Do not drain the blister if it is blood-filled.


John Vonhof, EMT-P, works for Alameda County EMS as a Prehospital Care Coordinator in the San Francisco Bay Area. He is the author of Fixing Your Feet: Prevention and Treatments for Athletes, (4th edition, Wilderness Press, 2006). John has provided medical aid and foot care expertise for racers at adventure races and ultra-marathons around the world. He has taught many MDs, RNs, paramedics, and EMTs how to manage and patch feet in a field setting. His articles have appeared in magazines and on the Web, and his e-zine and blog have thousands of subscribers.